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Copyright © 2004 W3C® (MIT, ERCIM, Keio), All Rights Reserved. W3C liability, trademark, document use and software licensing rules apply.
XML Schema Part 0: Primer is a non-normative document intended to provide an easily readable description of the XML Schema facilities, and is oriented towards quickly understanding how to create schemas using the XML Schema language. XML Schema Part 1: Structures and XML Schema Part 2: Datatypes provide the complete normative description of the XML Schema language. This primer describes the language features through numerous examples which are complemented by extensive references to the normative texts.
This section describes the status of this document at the time of its publication. Other documents may supersede this document. A list of current W3C publications and the latest revision of this technical report can be found in the W3C technical reports index at http://www.w3.org/TR/.
This is a W3C Proposed Edited Recommendation, intended to become the first part of the Second Edition of XML Schema. It is here made available for review by W3C members and other interested parties. Note that a Candidate Recommendation draft has not been deemed necessary by the Working Group, as there are no substantial implementation issues arising as a result of this edition, which aims only to incorporate the published corrigenda to the first edition.
Please send comments on this Proposed Edited Recommendation to www-xml-schema-comments@w3.org, including 2E PER
in the subject line, no later than 16 April 2004.
Publication as a Proposed Edited Recommendation does not imply endorsement by the W3C Membership. This is a draft document and may be updated, replaced or obsoleted by other documents at any time. It is inappropriate to cite this document as other than work in progress.
This document has been produced by the W3C XML Schema Working Group as part of the W3C XML Activity. The goals of the XML Schema language are discussed in the XML Schema Requirements document. The authors of this document are the members of the XML Schema Working Group. Different parts of this specification have different editors.
Documentation of intellectual property possibly relevant to this recommendation may be found at the Working Group's public IPR disclosure page.
The English version of this specification is the only normative version. Information about translations of this document is available at http://www.w3.org/2001/05/xmlschema-translations.
This second edition is not a new version, it merely incorporates the changes dictated by the corrections to errors found in the first edition as agreed by the XML Schema Working Group, as a convenience to readers. A separate list of all such corrections is available at http://www.w3.org/2001/05/xmlschema-errata. The errata list for this second edition is available at http://www.w3.org/2004/03/xmlschema-errata.
Please report errors in this document to www-xml-schema-comments@w3.org (archive).
1 Introduction
2 Basic Concepts: The Purchase Order
2.1 The Purchase Order Schema
2.2 Complex Type Definitions, Element & Attribute Declarations
2.3 Simple Types
2.4 Anonymous Type Definitions
2.5 Element Content
2.6 Annotations
2.7 Building Content Models
2.8 Attribute Groups
2.9 Nil Values
3 Advanced Concepts I: Namespaces, Schemas & Qualification
3.1 Target Namespaces & Unqualified Locals
3.2 Qualified Locals
3.3 Global vs. Local Declarations
3.4 Undeclared Target Namespaces
4 Advanced Concepts II: The International Purchase Order
4.1 A Schema in Multiple Documents
4.2 Deriving Types by Extension
4.3 Using Derived Types in Instance Documents
4.4 Deriving Complex Types by Restriction
4.5 Redefining Types & Groups
4.6 Substitution Groups
4.7 Abstract Elements and Types
4.8 Controlling the Creation & Use of Derived Types
5 Advanced Concepts III: The Quarterly Report
5.1 Specifying Uniqueness
5.2 Defining Keys & their References
5.3 XML Schema Constraints vs. XML 1.0 ID Attributes
5.4 Importing Types
5.5 Any Element, Any Attribute
5.6 schemaLocation
5.7 Conformance
A Acknowledgements
B Simple Types & their Facets
C Using Entities
D Regular Expressions
E Index
E.1 XML Schema Elements
E.2 XML Schema Attributes
This document, XML Schema Part 0: Primer, provides an easily approachable description of the XML Schema definition language, and should be used alongside the formal descriptions of the language contained in Parts 1 and 2 of the XML Schema specification. The intended audience of this document includes application developers whose programs read and write schema documents, and schema authors who need to know about the features of the language, especially features that provide functionality above and beyond what is provided by DTDs. The text assumes that you have a basic understanding of XML 1.0 and Namespaces in XML. Each major section of the primer introduces new features of the language, and describes those features in the context of concrete examples.
Basic Concepts: The Purchase Order (§2) covers the basic mechanisms of XML Schema. It describes how to declare the elements and attributes that appear in XML documents, the distinctions between simple and complex types, defining complex types, the use of simple types for element and attribute values, schema annotation, a simple mechanism for re-using element and attribute definitions, and nil values.
Advanced Concepts I: Namespaces, Schemas & Qualification (§3), the first advanced section in the primer, explains the basics of how namespaces are used in XML and schema documents. This section is important for understanding many of the topics that appear in the other advanced sections.
Advanced Concepts II: The International Purchase Order (§4), the second advanced section in the primer, describes mechanisms for deriving types from existing types, and for controlling these derivations. The section also describes mechanisms for merging together fragments of a schema from multiple sources, and for element substitution.
Advanced Concepts III: The Quarterly Report (§5) covers more advanced features, including a mechanism for specifying uniqueness among attributes and elements, a mechanism for using types across namespaces, a mechanism for extending types based on namespaces, and a description of how documents are checked for conformance.
In addition to the sections just described, the primer contains a number of appendices that provide detailed reference information on simple types and a regular expression language.
The primer is a non-normative document, which means that it does not provide a definitive (from the W3C's point of view) specification of the XML Schema language. The examples and other explanatory material in this document are provided to help you understand XML Schema, but they may not always provide definitive answers. In such cases, you will need to refer to the XML Schema specification, and to help you do this, we provide many links pointing to the relevant parts of the specification. More specifically, XML Schema items mentioned in the primer text are linked to an index [Index (§E)] of element names and attributes, and a summary table of datatypes, both in the primer. The table and the index contain links to the relevant sections of XML Schema parts 1 and 2.
The purpose of a schema is to define a class of XML documents, and so the term "instance document" is often used to describe an XML document that conforms to a particular schema. In fact, neither instances nor schemas need to exist as documents per se -- they may exist as streams of bytes sent between applications, as fields in a database record, or as collections of XML Infoset "Information Items" -- but to simplify the primer, we have chosen to always refer to instances and schemas as if they are documents and files.
Let us start by considering an instance document in a file
called po.xml
. It
describes a purchase order generated by a home products
ordering and billing application:
<?xml version="1.0"?> <purchaseOrder orderDate="1999-10-20"> <shipTo country="US"> <name>Alice Smith</name> <street>123 Maple Street</street> <city>Mill Valley</city> <state>CA</state> <zip>90952</zip> </shipTo> <billTo country="US"> <name>Robert Smith</name> <street>8 Oak Avenue</street> <city>Old Town</city> <state>PA</state> <zip>95819</zip> </billTo> <comment>Hurry, my lawn is going wild<!/comment> <items> <item partNum="872-AA"> <productName>Lawnmower</productName> <quantity>1</quantity> <USPrice>148.95</USPrice> <comment>Confirm this is electric</comment> </item> <item partNum="926-AA"> <productName>Baby Monitor</productName> <quantity>1</quantity> <USPrice>39.98</USPrice> <shipDate>1999-05-21</shipDate> </item> </items> </purchaseOrder>
The purchase order consists of a main element,
purchaseOrder
, and the subelements
shipTo
, billTo
, comment
,
and items
. These subelements (except
comment
) in turn contain other subelements, and so
on, until a subelement such as USPrice
contains a number rather than any subelements. Elements
that contain subelements or carry attributes are said to
have complex types, whereas elements that contain numbers
(and strings, and dates, etc.) but do not contain any
subelements are said to have simple types. Some elements
have attributes; attributes always have simple types.
The complex types in the instance document, and some of the simple types, are defined in the schema for purchase orders. The other simple types are defined as part of XML Schema's repertoire of built-in simple types.
Before going on to examine the purchase order schema, we digress briefly to mention the association between the instance document and the purchase order schema. As you can see by inspecting the instance document, the purchase order schema is not mentioned. An instance is not actually required to reference a schema, and although many will, we have chosen to keep this first section simple, and to assume that any processor of the instance document can obtain the purchase order schema without any information from the instance document. In later sections, we will introduce explicit mechanisms for associating instances and schemas.
The purchase order schema is contained in the file
po.xsd
:
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <xsd:annotation> <xsd:documentation xml:lang="en"> Purchase order schema for Example.com. Copyright 2000 Example.com. All rights reserved. </xsd:documentation> </xsd:annotation> <xsd:element name="purchaseOrder" type="PurchaseOrderType"/> <xsd:element name="comment" type="xsd:string"/> <xsd:complexType name="PurchaseOrderType"> <xsd:sequence> <xsd:element name="shipTo" type="USAddress"/> <xsd:element name="billTo" type="USAddress"/> <xsd:element ref="comment" minOccurs="0"/> <xsd:element name="items" type="Items"/> </xsd:sequence> <xsd:attribute name="orderDate" type="xsd:date"/> </xsd:complexType> <xsd:complexType name="USAddress"> <xsd:sequence> <xsd:element name="name" type="xsd:string"/> <xsd:element name="street" type="xsd:string"/> <xsd:element name="city" type="xsd:string"/> <xsd:element name="state" type="xsd:string"/> <xsd:element name="zip" type="xsd:decimal"/> </xsd:sequence> <xsd:attribute name="country" type="xsd:NMTOKEN" fixed="US"/> </xsd:complexType> <xsd:complexType name="Items"> <xsd:sequence> <xsd:element name="item" minOccurs="0" maxOccurs="unbounded"> <xsd:complexType> <xsd:sequence> <xsd:element name="productName" type="xsd:string"/> <xsd:element name="quantity"> <xsd:simpleType> <xsd:restriction base="xsd:positiveInteger"> <xsd:maxExclusive value="100"/> </xsd:restriction> </xsd:simpleType> </xsd:element> <xsd:element name="USPrice" type="xsd:decimal"/> <xsd:element ref="comment" minOccurs="0"/> <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/> </xsd:sequence> <xsd:attribute name="partNum" type="SKU" use="required"/> </xsd:complexType> </xsd:element> </xsd:sequence> </xsd:complexType> <!-- Stock Keeping Unit, a code for identifying products --> <xsd:simpleType name="SKU"> <xsd:restriction base="xsd:string"> <xsd:pattern value="\d{3}-[A-Z]{2}"/> </xsd:restriction> </xsd:simpleType> </xsd:schema>
The purchase order schema consists of a schema
element and a variety
of subelements, most notably element
, complexType
, and simpleType
which
determine the appearance of elements and their content in
instance documents.
Each of the elements in the schema has
a prefix xsd:
which is associated with the XML
Schema namespace through the declaration,
xmlns:xsd="http://www.w3.org/2001/XMLSchema"
,
that appears in the
schema
element. The prefix xsd:
is
used by convention to denote the XML Schema namespace,
although any prefix can be used. The same prefix, and hence
the same association, also appears on the names of built-in
simple types, e.g. xsd:string
. The purpose of the association
is to identify the elements and simple types as belonging
to the vocabulary of the XML Schema language rather than
the vocabulary of the schema author. For the sake of
clarity in the text, we just mention the names of elements
and simple types (e.g.
simpleType
), and omit the prefix.
In XML Schema, there is a basic difference between complex types which allow elements in their content and may carry attributes, and simple types which cannot have element content and cannot carry attributes. There is also a major distinction between definitions which create new types (both simple and complex), and declarations which enable elements and attributes with specific names and types (both simple and complex) to appear in document instances. In this section, we focus on defining complex types and declaring the elements and attributes that appear within them.
New complex types are defined using
the
complexType
element and such definitions
typically contain a set of element declarations, element
references, and attribute declarations. The declarations
are not themselves types, but rather an association between
a name and the constraints which govern the appearance of that
name in documents governed by the associated schema.
Elements are declared using the element
element, and
attributes are declared using the attribute
element. For
example, USAddress
is defined as a complex
type, and within the definition of USAddress
we see five element declarations and one attribute
declaration:
<xsd:complexType name="USAddress" > <xsd:sequence> <xsd:element name="name" type="xsd:string"/> <xsd:element name="street" type="xsd:string"/> <xsd:element name="city" type="xsd:string"/> <xsd:element name="state" type="xsd:string"/> <xsd:element name="zip" type="xsd:decimal"/> </xsd:sequence> <xsd:attribute name="country" type="xsd:NMTOKEN" fixed="US"/> </xsd:complexType>
The consequence of this definition is
that any element appearing in an instance whose type is
declared to be USAddress
(e.g.
shipTo
in
po.xml
) must consist of five elements and one
attribute. These elements must be called name
,
street
, city
, state
and zip
as specified by the values of the
declarations' name
attributes, and the
elements must appear in the same sequence (order) in which
they are declared. The first four of these elements will
each contain a string, and the fifth will contain a
number. The element whose type is declared to be
USAddress
may appear with an attribute called
country
which must contain the string
US
.
The USAddress
definition
contains only declarations involving the simple types: string
,
decimal
and
NMTOKEN
. In contrast, the
PurchaseOrderType
definition contains element
declarations involving complex types, e.g.
USAddress
, although note that both declarations use
the same type
attribute to identify the type, regardless of whether the
type is simple or complex.
<xsd:complexType name="PurchaseOrderType"> <xsd:sequence> <xsd:element name="shipTo" type="USAddress"/> <xsd:element name="billTo" type="USAddress"/> <xsd:element ref="comment" minOccurs="0"/> <xsd:element name="items" type="Items"/> </xsd:sequence> <xsd:attribute name="orderDate" type="xsd:date"/> </xsd:complexType>
In defining PurchaseOrderType
, two of the
element declarations, for shipTo
and
billTo
, associate different element names with the
same complex type, namely USAddress
. The
consequence of this definition is that any element
appearing in an instance document (e.g.
po.xml
) whose type is declared to be
PurchaseOrderType
must consist of elements named
shipTo
and billTo,
each
containing the five subelements (name
,
street
, city
, state
and
zip
) that were declared as part of
USAddress
. The shipTo
and
billTo
elements may also carry the
country
attribute that was declared as part of
USAddress
.
The PurchaseOrderType
definition contains an
orderDate
attribute declaration which, like
the country
attribute declaration, identifies
a simple type. In fact, all attribute declarations must
reference simple types because, unlike element
declarations, attributes cannot contain other elements or
other attributes.
The element declarations we have described so far have each associated a name with an existing type definition. Sometimes it is preferable to use an existing element rather than declare a new element, for example:
<xsd:element ref="comment" minOccurs="0"/>
This declaration references an existing element,
comment
, that was declared elsewhere in the purchase
order schema. In general, the value of the ref
attribute must reference a
global element, i.e. one that has been declared under
schema
rather
than as part of a complex type definition. The consequence
of this declaration is that an element called
comment
may appear in an instance document, and its
content must be consistent with that element's type, in
this case, string
.
The comment
element is
optional within PurchaseOrderType
because the
value of the
minOccurs
attribute in its declaration is 0. In
general, an element is required to appear when the value of
minOccurs
is 1 or more. The maximum number of times an element may
appear is determined by the value of a maxOccurs
attribute in
its declaration. This value may be a positive integer such
as 41, or the term unbounded
to indicate there
is no maximum number of occurrences. The default value for
both the
minOccurs
and the maxOccurs
attributes is
1. Thus, when an element such as comment
is
declared without a
maxOccurs
attribute, the element may not occur
more than once. Be sure that if you specify a value for
only the
minOccurs
attribute, it is less than or equal to
the default value of
maxOccurs
, i.e. it is 0 or 1. Similarly, if you
specify a value for only the maxOccurs
attribute, it
must be greater than or equal to the default value of
minOccurs
,
i.e. 1 or more. If both attributes are omitted, the element
must appear exactly once.
Attributes may appear once or not at
all, but no other number of times, and so the
syntax for specifying
occurrences of attributes is different than the syntax for
elements. In particular, attributes can be declared with a
use
attribute
to indicate whether the attribute is
required
(see for example, the partNum
attribute declaration in
po.xsd
), optional
, or even
prohibited
.
Default values of both attributes and elements are declared using the
default
attribute, although this attribute has a slightly
different consequence in each case. When an attribute is declared with a
default value, the value of the attribute is whatever value
appears as the attribute's value in an instance document; if
the attribute does not appear in the instance document, the schema
processor provides the attribute with a value equal to that of the
default
attribute.
Note that default values for attributes only make sense if the
attributes themselves are optional, and so it is an error to specify
both a default value and anything other than a value of
optional
for use
.
The schema processor treats defaulted elements slightly differently.
When an element is declared with a default value, the value of the
element is whatever value appears as the element's content in the
instance document; if the element appears without any content, the
schema processor provides the element with a value equal to that of the
default
attribute.
However, if the element does not appear in the instance document, the schema
processor does not provide the element at all. In summary, the
differences between element and attribute defaults can be stated as:
Default attribute values apply when attributes are missing, and default
element values apply when elements are empty.
The fixed
attribute is used in both attribute and element
declarations to ensure that the attributes and elements are
set to particular values. For example, po.xsd
contains a declaration for the country
attribute, which is declared with a fixed
value US
. This
declaration means that the appearance of a country
attribute in an instance document is optional (the default value of
use
is optional
),
although if the attribute does appear, its value must be US
,
and if the attribute does not appear, the schema processor will provide
a country
attribute with the value US
. Note
that the concepts of a fixed value and a default value are mutually
exclusive, and so it is an error for a declaration to contain both
fixed
and default
attributes.
The values of the attributes used in element and attribute declarations to constrain their occurrences are summarized in Table 1.
Table 1. Occurrence Constraints for Elements and Attributes | |||||||
---|---|---|---|---|---|---|---|
|
| Notes | |||||
(1, 1) -, - | required, -, - | element/attribute must appear once, it may have any value | |||||
(1, 1) 37, - | required, 37, - | element/attribute must appear once, its value must be 37 | |||||
(2, unbounded) 37, - | n/a | element must appear twice or more, its value must be 37; in general, minOccurs and maxOccurs values may be positive integers, and maxOccurs value may also be "unbounded" | |||||
(0, 1) -, - | optional, -, - | element/attribute may appear once, it may have any value | |||||
(0, 1) 37, - | n/a | element may appear once, if it does not appear it is not provided; if it does appear and it is empty, its value is 37; if it does appear and it is not empty, its value must be 37 | |||||
n/a | optional, 37, - | attribute may appear once, if it does appear its value must be 37, if it does not appear its value is 37 | |||||
(0, 1) -, 37 | n/a | element may appear once; if it does not appear it is not provided; if it does appear and it is empty, its value is 37; otherwise its value is that given | |||||
n/a | optional, -, 37 | attribute may appear once; if it does not appear its value is 37, otherwise its value is that given | |||||
(0, 2) -, 37 | n/a | element may appear once, twice, or not at all; if the element does not appear it is not provided; if it does appear and it is empty, its value is 37; otherwise its value is that given; in general, minOccurs and maxOccurs values may be positive integers, and maxOccurs value may also be "unbounded" | |||||
(0, 0) -, - | prohibited, -, - | element/attribute must not appear | |||||
Note that neither minOccurs, maxOccurs, nor use may appear in the declarations of global elements and attributes. |
Global elements, and global attributes, are created by
declarations that appear as the children of the schema
element. Once
declared, a global element or a global attribute can be
referenced in one or more declarations using the ref
attribute as described
above. A declaration that references a global element
enables the referenced element to appear in the instance
document in the context of the referencing declaration. So, for
example, the comment
element appears in
po.xml
at the same level as
the shipTo
, billTo
and
items
elements because the declaration that
references comment
appears in the complex type
definition at the same level as the declarations of the
other three elements.
The declaration of a global element also enables the
element to appear at the top-level of an instance document.
Hence purchaseOrder
, which is declared as a
global element in
po.xsd
, can appear as the top-level element in
po.xml
. Note that this
rationale will also allow a comment
element to
appear as the top-level element in a document like
po.xml
.
There are a number of caveats concerning the use of
global elements and attributes. One caveat is that global
declarations cannot contain references; global declarations
must identify simple and complex types directly. Put
concretely, global declarations cannot contain the ref
attribute, they must
use the type
attribute (or, as we describe shortly, be followed by an anonymous type definition). A
second caveat is that cardinality constraints cannot be
placed on global declarations, although they can be placed
on local declarations that reference global declarations.
In other words, global declarations cannot contain the
attributes minOccurs, maxOccurs, or use.
We have now described how to define new complex types
(e.g. PurchaseOrderType
), declare elements
(e.g. purchaseOrder
) and declare attributes
(e.g. orderDate
). These activities generally
involve naming, and so the question naturally arises: What
happens if we give two things the same name? The answer
depends upon the two things in question, although in
general the more similar are the two things, the more
likely there will be a conflict.
Here are some examples to illustrate when same names cause problems. If the two things are both types, say we define a complex type called USStates and a simple type called USStates, there is a conflict. If the two things are a type and an element or attribute, say we define a complex type called USAddress and we declare an element called USAddress, there is no conflict. If the two things are elements within different types (i.e. not global elements), say we declare one element called name as part of the USAddress type and a second element called name as part of the Item type, there is no conflict. (Such elements are sometimes called local element declarations.) Finally, if the two things are both types and you define one and XML Schema has defined the other, say you define a simple type called decimal, there is no conflict. The reason for the apparent contradiction in the last example is that the two types belong to different namespaces. We explore the use of namespaces in schema in a later section.
The purchase order schema declares several elements and
attributes that have simple types. Some of these simple
types, such as string
and decimal
, are built
in to XML Schema, while others are derived from the
built-in's. For example, the partNum
attribute
has a type called SKU
(Stock Keeping Unit)
that is derived from
string
. Both built-in simple types and their
derivations can be used in all element and attribute
declarations. Table 2 lists
all the simple types built in to XML Schema, along with
examples of the different types.
Table 2. Simple Types Built In to XML Schema | ||||
---|---|---|---|---|
Simple Type | Examples (delimited by commas) | Notes | ||
string | Confirm this is electric | |||
normalizedString | Confirm this is electric | see (3) | ||
token | Confirm this is electric | see (4) | ||
base64Binary | GpM7 | |||
hexBinary | 0FB7 | |||
integer | ...-1, 0, 1, ... | see (2) | ||
positiveInteger | 1, 2, ... | see (2) | ||
negativeInteger | ... -2, -1 | see (2) | ||
nonNegativeInteger | 0, 1, 2, ... | see (2) | ||
nonPositiveInteger | ... -2, -1, 0 | see (2) | ||
long | -9223372036854775808, ... -1, 0, 1, ... 9223372036854775807 | see (2) | ||
unsignedLong | 0, 1, ... 18446744073709551615 | see (2) | ||
int | -2147483648, ... -1, 0, 1, ... 2147483647 | see (2) | ||
unsignedInt | 0, 1, ...4294967295 | see (2) | ||
short | -32768, ... -1, 0, 1, ... 32767 | see (2) | ||
unsignedShort | 0, 1, ... 65535 | see (2) | ||
byte | -128, ...-1, 0, 1, ... 127 | see (2) | ||
unsignedByte | 0, 1, ... 255 | see (2) | ||
decimal | -1.23, 0, 123.4, 1000.00 | see (2) | ||
float | -INF, -1E4, -0, 0, 12.78E-2, 12, INF, NaN | equivalent to single-precision 32-bit floating point, NaN is "not a number", see (2) | ||
double | -INF, -1E4, -0, 0, 12.78E-2, 12, INF, NaN | equivalent to double-precision 64-bit floating point, see (2) | ||
boolean | true, false, 1, 0 | |||
duration | P1Y2M3DT10H30M12.3S | 1 year, 2 months, 3 days, 10 hours, 30 minutes, and 12.3 seconds | ||
dateTime | 1999-05-31T13:20:00.000-05:00 | May 31st 1999 at 1.20pm Eastern Standard Time which is 5 hours behind Co-Ordinated Universal Time, see (2) | ||
date | 1999-05-31 | see (2) | ||
time | 13:20:00.000, 13:20:00.000-05:00 | see (2) | ||
gYear | 1999 | 1999, see (2) (5) | ||
gYearMonth | 1999-02 | the month of February 1999, regardless of the number of days, see (2) (5) | ||
gMonth | --05 | May, see (2) (5) | ||
gMonthDay | --05-31 | every May 31st, see (2) (5) | ||
gDay | ---31 | the 31st day, see (2) (5) | ||
Name | shipTo | XML 1.0 Name type | ||
QName | po:USAddress | XML Namespace QName | ||
NCName | USAddress | XML Namespace NCName, i.e. a QName without the prefix and colon | ||
anyURI |
| |||
language | en-GB, en-US, fr | valid values for xml:lang as defined in XML 1.0 | ||
ID | XML 1.0 ID attribute type, see (1) | |||
IDREF | XML 1.0 IDREF attribute type, see (1) | |||
IDREFS | XML 1.0 IDREFS attribute type, see (1) | |||
ENTITY | XML 1.0 ENTITY attribute type, see (1) | |||
ENTITIES | XML 1.0 ENTITIES attribute type, see (1) | |||
NOTATION | XML 1.0 NOTATION attribute type, see (1) | |||
NMTOKEN |
| XML 1.0 NMTOKEN attribute type, see (1) | ||
NMTOKENS |
| XML 1.0 NMTOKENS attribute type, i.e. a whitespace separated list of NMTOKEN's, see (1) | ||
Notes: (1) To retain compatibility between XML Schema and XML 1.0 DTDs, the simple types ID, IDREF, IDREFS, ENTITY, ENTITIES, NOTATION, NMTOKEN, NMTOKENS should only be used in attributes. (2) A value of this type can be represented by more than one lexical format, e.g. 100 and 1.0E2 are both valid float formats representing "one hundred". However, rules have been established for this type that define a canonical lexical format, see XML Schema Part 2. (3) Newline, tab and carriage-return characters in a normalizedString type are converted to space characters before schema processing. (4) As normalizedString, and adjacent space characters are collapsed to a single space character, and leading and trailing spaces are removed. (5) The "g" prefix signals time periods in the Gregorian calendar. |
New simple types are defined by
deriving them from existing simple types (built-in's and
derived). In particular, we can derive a new simple type by
restricting an existing simple type, in other words, the
legal range of values for the new type are a subset of the
existing type's range of values. We use the simpleType
element to
define and name the new simple type. We use the restriction
element
to indicate the existing (base) type, and to identify the
"facets" that constrain the range of values. A complete
list of facets is provided in
Appendix B.
Suppose we wish to create a new type
of integer called myInteger
whose range of
values is between 10000 and 99999 (inclusive). We base our
definition on the built-in simple type integer
, whose range of values also
includes integers less than 10000 and greater than 99999.
To define myInteger
, we restrict the range of
the integer
base type
by employing two facets called minInclusive
and
maxInclusive
:
<xsd:simpleType name="myInteger"> <xsd:restriction base="xsd:integer"> <xsd:minInclusive value="10000"/> <xsd:maxInclusive value="99999"/> </xsd:restriction> </xsd:simpleType>
The example shows one particular combination of a base
type and two facets used to define myInteger
,
but a look at the list of built-in simple types and their
facets (Appendix B) should
suggest other viable combinations.
The purchase order schema contains
another, more elaborate, example of a simple type
definition. A new simple type called SKU
is
derived (by restriction) from the simple type string
. Furthermore, we
constrain the values of SKU
using a facet
called pattern
in conjunction with the regular expression
"\d{3}-[A-Z]{2}
" that is read "three digits
followed by a hyphen followed by two upper-case ASCII
letters":
<xsd:simpleType name="SKU"> <xsd:restriction base="xsd:string"> <xsd:pattern value="\d{3}-[A-Z]{2}"/> </xsd:restriction> </xsd:simpleType>
This regular expression language is described more fully in Appendix D.
XML Schema defines twelve facets
which are listed in Appendix
B. Among these, the enumeration
facet is
particularly useful and it can be used to constrain the
values of almost every simple type, except the boolean
type. The enumeration
facet limits
a simple type to a set of distinct values. For example, we
can use the
enumeration
facet to define a new simple type
called USState
, derived from string
, whose value must be one of the
standard US state abbreviations:
<xsd:simpleType name="USState"> <xsd:restriction base="xsd:string"> <xsd:enumeration value="AK"/> <xsd:enumeration value="AL"/> <xsd:enumeration value="AR"/> <!-- and so on ... --> </xsd:restriction> </xsd:simpleType>
USState
would be a good replacement for the
string
type currently
used in the state
element declaration. By
making this replacement, the legal values of a
state
element, i.e. the state
subelements of billTo
and
shipTo
, would be limited to one of AK
,
AL
, AR
, etc. Note that the
enumeration values specified for a particular type must be
unique.
XML Schema has the concept of a list type, in addition to
the so-called atomic types that constitute most of the
types listed in Table 2.
(Atomic types, list types, and the union types described in
the next section are collectively called simple types.) The
value of an atomic type is indivisible from XML Schema's
perspective. For example, the
NMTOKEN
value US
is indivisible in
the sense that no part of US
, such as the
character "S", has any meaning by itself. In contrast, list
types are comprised of sequences of atomic types and
consequently the parts of a sequence (the "atoms")
themselves are meaningful. For example, NMTOKENS
is a list type, and an
element of this type would be a white-space delimited list
of NMTOKEN
's, such as
"US UK FR". XML Schema has three built-in list types, they
are NMTOKENS
,
IDREFS
, and ENTITIES
.
In addition to using the built-in
list types, you can create new list types by derivation
from existing atomic types. (You cannot create list types
from existing list types, nor from complex types.) For
example, to create a list of myInteger
's:
<xsd:simpleType name="listOfMyIntType"> <xsd:list itemType="myInteger"/> </xsd:simpleType>
And an element in an instance document whose content
conforms to listOfMyIntType
is:
<listOfMyInt>20003 15037 95977 95945</listOfMyInt>
Several facets can be applied to list
types: length
,
minLength
,
maxLength
, pattern
,
and
enumeration
. For example, to define a list of
exactly six US states (SixUSStates
), we first
define a new list type called USStateList
from
USState
, and then we derive
SixUSStates
by restricting USStateList
to only six items:
<xsd:simpleType name="USStateList"> <xsd:list itemType="USState"/> </xsd:simpleType> <xsd:simpleType name="SixUSStates"> <xsd:restriction base="USStateList"> <xsd:length value="6"/> </xsd:restriction> </xsd:simpleType>
Elements whose type is SixUSStates
must have
six items, and each of the six items must be one of the
(atomic) values of the enumerated type
USState
, for example:
<sixStates>PA NY CA NY LA AK</sixStates>
Note that it is possible to derive a list type from the
atomic type string
.
However, a string
may
contain white space, and white space delimits the items in
a list type, so you should be careful using
list types whose base type is
string
. For example, suppose we have defined a
list type with a
length
facet equal to 3, and base type string
, then the following 3 item
list is legal:
Asie Europe Afrique
But the following 3 "item" list is illegal:
Asie Europe Amérique Latine
Even though "Amérique Latine" may exist as a single string outside of the list, when it is included in the list, the whitespace between Amérique and Latine effectively creates a fourth item, and so the latter example will not conform to the 3-item list type.
Atomic types and list types enable an
element or an attribute value to be one or more instances
of one atomic type. In contrast, a union type enables an
element or attribute value to be one or more instances of
one type drawn from the union of multiple atomic and list
types. To illustrate, we create a union type for
representing American states as singleton letter
abbreviations or lists of numeric codes. The
zipUnion
union type is built from one atomic type and
one list type:
<xsd:simpleType name="zipUnion"> <xsd:union memberTypes="USState listOfMyIntType"/> </xsd:simpleType>
When we define a union type, the memberTypes
attribute value is a list of all the types in the union.
Now, assuming we have declared an element called
zips
of type zipUnion
, valid instances
of the element are:
<zips>CA</zips> <zips>95630 95977 95945</zips> <zips>AK</zips>
Two facets,
pattern
and enumeration
, can be
applied to a union type.
Schemas can be constructed by defining sets of named types
such as PurchaseOrderType
and then declaring
elements such as purchaseOrder
that reference
the types using the
type=
construction. This style of schema
construction is straightforward but it can be unwieldy,
especially if you define many types that are referenced
only once and contain very few constraints. In these cases,
a type can be more succinctly defined as an anonymous type
which saves the overhead of having to be named and
explicitly referenced.
The definition of the type Items
in po.xsd
contains two element
declarations that use anonymous types (item
and quantity
). In general, you can identify
anonymous types by the lack of a type=
in an element (or
attribute) declaration, and by the presence of an un-named
(simple or complex) type definition:
<xsd:complexType name="Items"> <xsd:sequence> <xsd:element name="item" minOccurs="0" maxOccurs="unbounded"> <xsd:complexType> <xsd:sequence> <xsd:element name="productName" type="xsd:string"/> <xsd:element name="quantity"> <xsd:simpleType> <xsd:restriction base="xsd:positiveInteger"> <xsd:maxExclusive value="100"/> </xsd:restriction> </xsd:simpleType> </xsd:element> <xsd:element name="USPrice" type="xsd:decimal"/> <xsd:element ref="comment" minOccurs="0"/> <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/> </xsd:sequence> <xsd:attribute name="partNum" type="SKU" use="required"/> </xsd:complexType> </xsd:element> </xsd:sequence> </xsd:complexType>
In the case of the item
element, it has an
anonymous complex type consisting of the elements
productName
, quantity
,
USPrice
, comment
, and
shipDate
, and an attribute called
partNum
. In the case of the quantity
element, it has an anonymous simple type derived from
positiveInteger
whose value
ranges between 1 and 99.
The purchase order schema has many examples of elements
containing other elements (e.g. items
),
elements having attributes and containing other elements
(e.g. shipTo
), and elements containing only a
simple type of value (e.g. USPrice
). However,
we have not seen an element having attributes but
containing only a simple type of value, nor have we seen an
element that contains other elements mixed with character
content, nor have we seen an element that has no content at
all. In this section we'll examine these variations in the
content models of elements.
Let us first consider how to declare an element that has an attribute and contains a simple value. In an instance document, such an element might appear as:
<internationalPrice currency="EUR">423.46</internationalPrice>
The purchase order schema declares a USPrice
element that is a starting point:
<xsd:element name="USPrice" type="decimal"/>
Now, how do we add an attribute to
this element? As we have said before, simple types cannot
have attributes, and
decimal
is a simple type. Therefore, we must
define a complex type to carry the attribute declaration.
We also want the content to be simple type decimal
. So our original question
becomes: How do we define a complex type that is based on
the simple type
decimal
? The answer is to derive a new
complex type from the simple type
decimal
:
<xsd:element name="internationalPrice"> <xsd:complexType> <xsd:simpleContent> <xsd:extension base="xsd:decimal"> <xsd:attribute name="currency" type="xsd:string"/> </xsd:extension> </xsd:simpleContent> </xsd:complexType> </xsd:element>
We use the
complexType
element to start the definition of a
new (anonymous) type. To indicate that the content model of
the new type contains only character data and no elements,
we use a
simpleContent
element. Finally, we derive the
new type by extending the simple
decimal
type. The extension consists of adding a
currency
attribute using a standard attribute
declaration. (We cover type derivation in detail in Advanced Concepts II: The International Purchase Order (§4).) The
internationalPrice
element declared in this way will
appear in an instance as shown in the example at the
beginning of this section.
The construction of the purchase order schema may be characterized as elements containing subelements, and the deepest subelements contain character data. XML Schema also provides for the construction of schemas where character data can appear alongside subelements, and character data is not confined to the deepest subelements.
To illustrate, consider the following snippet from a customer letter that uses some of the same elements as the purchase order:
<letterBody> <salutation>Dear Mr.<name>Robert Smith</name>.</salutation> Your order of <quantity>1</quantity> <productName>Baby Monitor</productName> shipped from our warehouse on <shipDate>1999-05-21</shipDate>. .... </letterBody>
Notice the text appearing between elements and their child
elements. Specifically, text appears between the elements
salutation
, quantity
,
productName
and shipDate
which are all
children of letterBody
, and text appears
around the element name
which is the child of a child of
letterBody
. The following snippet of a schema
declares letterBody
:
<xsd:element name="letterBody"> <xsd:complexType mixed="true"> <xsd:sequence> <xsd:element name="salutation"> <xsd:complexType mixed="true"> <xsd:sequence> <xsd:element name="name" type="xsd:string"/> </xsd:sequence> </xsd:complexType> </xsd:element> <xsd:element name="quantity" type="xsd:positiveInteger"/> <xsd:element name="productName" type="xsd:string"/> <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/> <!-- etc. --> </xsd:sequence> </xsd:complexType> </xsd:element>
The elements appearing in the customer letter are
declared, and their types are defined using the element
and complexType
element
constructions we have seen before. To enable character data
to appear between the child-elements of
letterBody
, the
mixed
attribute on the type definition is set to
true.
Note that the mixed
model in XML Schema
differs fundamentally from the
mixed model in XML 1.0. Under the XML Schema
mixed model, the order and number of child elements
appearing in an instance must agree with the order and
number of child elements specified in the model. In
contrast, under the XML 1.0 mixed model, the order and
number of child elements appearing in an instance cannot be
constrained. In summary, XML Schema provides full validation of
mixed models in contrast to the partial schema validation
provided by XML 1.0.
Now suppose that we want the
internationalPrice
element to convey both the unit
of currency and the price as attribute values rather than
as separate attribute and content values. For example:
<internationalPrice currency="EUR" value="423.46"/>
Such an element has no content at all; its content model is empty. To define a type whose content is empty, we essentially define a type that allows only elements in its content, but we do not actually declare any elements and so the type's content model is empty:
<xsd:element name="internationalPrice"> <xsd:complexType> <xsd:complexContent> <xsd:restriction base="xsd:anyType"> <xsd:attribute name="currency" type="xsd:string"/> <xsd:attribute name="value" type="xsd:decimal"/> </xsd:restriction> </xsd:complexContent> </xsd:complexType> </xsd:element>
In this example, we define an (anonymous) type having
complexContent
, i.e. only elements. The
complexContent
element signals that we intend to
restrict or extend the content model of a complex type, and the
restriction
of anyType
declares
two attributes but does not introduce any element content
(see Deriving Complex Types by Restriction (§4.4) for more
details on restriction). The
internationalPrice
element declared in this way may
legitimately appear in an instance as shown in the example
above.
The preceding syntax for an empty-content element is
relatively verbose, and it is possible to declare the
internationalPrice
element more compactly:
<xsd:element name="internationalPrice"> <xsd:complexType> <xsd:attribute name="currency" type="xsd:string"/> <xsd:attribute name="value" type="xsd:decimal"/> </xsd:complexType> </xsd:element>
This compact syntax works because a complex type defined
without any simpleContent
or
complexContent
is interpreted as shorthand for
complex content that restricts anyType
.
The anyType
represents an abstraction called
the
ur-type which is the base type from which all
simple and complex types are derived. An
anyType
type does not constrain its content in any
way. It is possible to use anyType
like other
types, for example:
<xsd:element name="anything" type="xsd:anyType"/>
The content of the element declared in this way is
unconstrained, so the element value may be 423.46, but it
may be any other sequence of characters as well, or indeed
a mixture of characters and elements. In fact,
anyType
is the default type when none is specified,
so the above could also be written as follows:
<xsd:element name="anything"/>
If unconstrained element content is needed, for example in
the case of elements containing prose which requires embedded
markup to support internationalization, then the default
declaration or a slightly restricted form of it may be suitable.
The text
type described in Any Element, Any Attribute (§5.5)
is an example of such a type that is suitable for such
purposes.
XML Schema provides three elements
for annotating schemas for the benefit of both human
readers and applications. In the purchase order schema, we
put a basic schema description and copyright information
inside the
documentation
element, which is the recommended
location for human readable material. We recommend you use
the xml:lang
attribute with any
documentation
elements to indicate the language of the information. Alternatively,
you may indicate the language of all information in a schema by placing
an xml:lang
attribute on the schema
element.
The appinfo
element, which we did not use in the
purchase order schema, can be used to provide information
for tools, stylesheets and other applications. An
interesting example using appinfo
is a
schema
that describes
the simple types in XML Schema Part 2: Datatypes.
Information describing this schema, e.g. which facets are
applicable to particular simple types, is represented
inside appinfo
elements, and this information was used by an application to automatically
generate text for the XML Schema Part 2 document.
Both documentation
and
appinfo
appear
as subelements of
annotation
, which may itself appear at the
beginning of most schema constructions. To illustrate, the
following example shows annotation
elements
appearing at the beginning of an element declaration and a
complex type definition:
<xsd:element name="internationalPrice"> <xsd:annotation> <xsd:documentation xml:lang="en"> element declared with anonymous type </xsd:documentation> </xsd:annotation> <xsd:complexType> <xsd:annotation> <xsd:documentation xml:lang="en"> empty anonymous type with 2 attributes </xsd:documentation> </xsd:annotation> <xsd:complexContent> <xsd:restriction base="xsd:anyType"> <xsd:attribute name="currency" type="xsd:string"/> <xsd:attribute name="value" type="xsd:decimal"/> </xsd:restriction> </xsd:complexContent> </xsd:complexType> </xsd:element>
The
annotation
element may also appear at the
beginning of other schema constructions such as those
indicated by the elements
schema
,
simpleType
, and attribute
.
The definitions of complex types in the purchase order
schema all declare sequences of elements that must appear
in the instance document. The occurrence of individual
elements declared in the so-called content models of these
types may be optional, as indicated by a 0 value for the
attribute
minOccurs
(e.g. in comment
), or be
otherwise constrained depending upon the values of minOccurs
and
maxOccurs
. XML
Schema also provides constraints that apply to groups of
elements appearing in a content model. These constraints
mirror those available in XML 1.0 plus some additional
constraints. Note that the constraints do not apply to
attributes.
XML Schema enables groups of elements to be defined and named, so that the elements can be used to build up the content models of complex types (thus mimicking common usage of parameter entities in XML 1.0). Un-named groups of elements can also be defined, and along with elements in named groups, they can be constrained to appear in the same order (sequence) as they are declared. Alternatively, they can be constrained so that only one of the elements may appear in an instance.
To illustrate, we introduce two groups
into the PurchaseOrderType
definition from the
purchase order schema so that purchase orders may contain
either separate shipping and billing addresses, or a single
address for those cases in which the shippee and billee are
co-located:
<xsd:complexType name="PurchaseOrderType"> <xsd:sequence> <xsd:choice> <xsd:group ref="shipAndBill"/> <xsd:element name="singleUSAddress" type="USAddress"/> </xsd:choice> <xsd:element ref="comment" minOccurs="0"/> <xsd:element name="items" type="Items"/> </xsd:sequence> <xsd:attribute name="orderDate" type="xsd:date"/> </xsd:complexType> <xsd:group id="shipAndBill"> <xsd:sequence> <xsd:element name="shipTo" type="USAddress"/> <xsd:element name="billTo" type="USAddress"/> </xsd:sequence> </xsd:group>
The choice
group element allows only one of its children to appear in
an instance. One child is an inner group
element that references
the named group shipAndBill
consisting of the
element sequence shipTo
, billTo
,
and the second child is a singleUSAddress
.
Hence, in an instance document, the
purchaseOrder
element must contain either a
shipTo
element followed by a billTo
element or a singleUSAddress
element. The
choice
group is
followed by the comment
and items
element declarations, and both the choice
group and the element
declarations are children of a sequence
group. The effect
of these various groups is that the address element(s) must
be followed by comment
and items
elements in that order.
There exists a third option for
constraining elements in a group: All the elements in the
group may appear once or not at all, and they may appear in
any order. The all
group (which provides a simplified version of the SGML
&-Connector) is limited to the top-level of any content
model. Moreover, the group's children must all be
individual elements (no groups), and no element in the
content model may appear more than once, i.e. the
permissible values of
minOccurs
and maxOccurs
are 0 and 1.
For example, to allow the child elements of
purchaseOrder
to appear in any order, we could
redefine PurchaseOrderType
as:
<xsd:complexType name="PurchaseOrderType"> <xsd:all> <xsd:element name="shipTo" type="USAddress"/> <xsd:element name="billTo" type="USAddress"/> <xsd:element ref="comment" minOccurs="0"/> <xsd:element name="items" type="Items"/> </xsd:all> <xsd:attribute name="orderDate" type="xsd:date"/> </xsd:complexType>
By this definition, a comment
element may
optionally appear within purchaseOrder
, and it
may appear before or after any shipTo
,
billTo
and items
elements, but it can
appear only once. Moreover, the stipulations of an all
group do not allow us to
declare an element such as comment
outside the
group as a means of enabling it to appear more than once.
XML Schema stipulates that an
all
group must appear as the sole child at the
top of a content model. In other words, the following is
illegal:
<xsd:complexType name="PurchaseOrderType"> <xsd:sequence> <xsd:all> <xsd:element name="shipTo" type="USAddress"/> <xsd:element name="billTo" type="USAddress"/> <xsd:element name="items" type="Items"/> </xsd:all> <xsd:sequence> <xsd:element ref="comment" minOccurs="0" maxOccurs="unbounded"/> </xsd:sequence> </xsd:sequence> <xsd:attribute name="orderDate" type="xsd:date"/> </xsd:complexType>
Finally, named and un-named groups that appear in content
models (represented by
group
and
choice
,
sequence
,
all
respectively) may carry minOccurs
and maxOccurs
attributes. By combining and nesting the various groups
provided by XML Schema, and by setting the values of
minOccurs
and
maxOccurs
,
it is possible to represent any content model expressible
with an XML 1.0 DTD. Furthermore, the all
group provides additional
expressive power.
Suppose we want to provide more information about each
item in a purchase order, for example, each item's weight
and preferred shipping method. We can accomplish this by
adding weightKg
and shipBy
attribute declarations to the item
element's
(anonymous) type definition:
<xsd:element name="item" minOccurs="0" maxOccurs="unbounded"> <xsd:complexType> <xsd:sequence> <xsd:element name="productName" type="xsd:string"/> <xsd:element name="quantity"> <xsd:simpleType> <xsd:restriction base="xsd:positiveInteger"> <xsd:maxExclusive value="100"/> </xsd:restriction> </xsd:simpleType> </xsd:element> <xsd:element name="USPrice" type="xsd:decimal"/> <xsd:element ref="comment" minOccurs="0"/> <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/> </xsd:sequence> <xsd:attribute name="partNum" type="SKU" use="required"/> <!-- add weightKg and shipBy attributes --> <xsd:attribute name="weightKg" type="xsd:decimal"/> <xsd:attribute name="shipBy"> <xsd:simpleType> <xsd:restriction base="xsd:string"> <xsd:enumeration value="air"/> <xsd:enumeration value="land"/> <xsd:enumeration value="any"/> </xsd:restriction> </xsd:simpleType> </xsd:attribute> </xsd:complexType> </xsd:element>
Alternatively, we can create a named
attribute group containing all the desired attributes of an
item
element, and reference this group by name
in the item
element declaration:
<xsd:element name="item" minOccurs="0" maxOccurs="unbounded"> <xsd:complexType> <xsd:sequence> <xsd:element name="productName" type="xsd:string"/> <xsd:element name="quantity"> <xsd:simpleType> <xsd:restriction base="xsd:positiveInteger"> <xsd:maxExclusive value="100"/> </xsd:restriction> </xsd:simpleType> </xsd:element> <xsd:element name="USPrice" type="xsd:decimal"/> <xsd:element ref="comment" minOccurs="0"/> <xsd:element name="shipDate" type="xsd:date" minOccurs="0"/> </xsd:sequence> <!-- attributeGroup replaces individual declarations --> <xsd:attributeGroup ref="ItemDelivery"/> </xsd:complexType> </xsd:element> <xsd:attributeGroup id="ItemDelivery"> <xsd:attribute name="partNum" type="SKU" use="required"/> <xsd:attribute name="weightKg" type="xsd:decimal"/> <xsd:attribute name="shipBy"> <xsd:simpleType> <xsd:restriction base="xsd:string"> <xsd:enumeration value="air"/> <xsd:enumeration value="land"/> <xsd:enumeration value="any"/> </xsd:restriction> </xsd:simpleType> </xsd:attribute> </xsd:attributeGroup>
Using an attribute group in this way can improve the readability of schemas, and facilitates updating schemas because an attribute group can be defined and edited in one place and referenced in multiple definitions and declarations. These characteristics of attribute groups make them similar to parameter entities in XML 1.0. Note that an attribute group may contain other attribute groups. Note also that both attribute declarations and attribute group references must appear at the end of complex type definitions.
One of the purchase order items listed in po.xml
, the Lawnmower
,
does not have a shipDate
element. Within the
context of our scenario, the schema author may have
intended such absences to indicate item
s not
yet shipped. But in general, the absence of an element does
not have any particular meaning: It may indicate that the
information is unknown, or not applicable, or the element
may be absent for some other reason. Sometimes it is
desirable to represent an unshipped item
,
unknown information, or inapplicable information
explicitly with an element, rather than by an absent
element. For example, it may be desirable to represent a
"null" value being sent to or from a relational database
with an element that is present. Such cases can be
represented using XML Schema's nil mechanism which enables
an element to appear with or without a non-nil value.
XML Schema's nil mechanism involves
an "out of band" nil signal. In other words, there is no
actual nil value that appears as element content, instead
there is an attribute to indicate that the element content
is nil. To illustrate, we modify the
shipDate
element declaration so that nils can be
signalled:
<xsd:element name="shipDate" type="xsd:date" nillable="true"/>
And to explicitly represent that
shipDate
has a nil value in the instance
document, we set the nil
attribute (from the XML Schema
namespace for instances) to true:
<shipDate xsi:nil="true"></shipDate>
The nil
attribute is defined as part of the XML Schema namespace
for instances,
http://www.w3.org/2001/XMLSchema-instance
,
and so it must appear in the instance document with a
prefix (such as xsi:
) associated with that namespace.
(As with the xsd:
prefix, the
xsi:
prefix is used by convention only.) Note that
the nil mechanism applies only to element values, and not
to attribute values. An element with xsi:nil="true"
may not
have any element content but it may still carry attributes.
A schema can be viewed as a
collection (vocabulary) of type definitions and element
declarations whose names belong to a particular namespace
called a target namespace. Target namespaces enable us
to distinguish between definitions and declarations from
different vocabularies. For example, target namespaces
would enable us to distinguish between the declaration for
element
in the
XML Schema language vocabulary, and a declaration for
element
in a hypothetical chemistry language
vocabulary. The former is part of the
http://www.w3.org/2001/XMLSchema
target
namespace, and the latter is part of another target
namespace.
When we want to check that an instance document conforms to one or more schemas (through a process called schema validation), we need to identify which element and attribute declarations and type definitions in the schemas should be used to check which elements and attributes in the instance document. The target namespace plays an important role in the identification process. We examine the role of the target namespace in the next section.
The schema author also has several options that affect how the identities of elements and attributes are represented in instance documents. More specifically, the author can decide whether or not the appearance of locally declared elements and attributes in an instance must be qualified by a namespace, using either an explicit prefix or implicitly by default. The schema author's choice regarding qualification of local elements and attributes has a number of implications regarding the structures of schemas and instance documents, and we examine some of these implications in the following sections.
In a new version of the purchase order schema, po1.xsd
, we explicitly declare
a target namespace, and specify that both locally defined
elements and locally defined attributes must be
unqualified. The target namespace in po1.xsd
is
http://www.example.com/PO1
, as indicated by the
value of the
targetNamespace
attribute.
Qualification of local elements and attributes can be
globally specified by a pair of attributes,
elementFormDefault
and
attributeFormDefault
, on the schema
element, or can be
specified separately for each local declaration using the
form
attribute.
All such attributes' values may each be set to
unqualified
or qualified
, to indicate
whether or not locally declared elements and attributes
must be unqualified.
In po1.xsd
we globally
specify the qualification of elements and attributes by
setting the values of both
elementFormDefault
and
attributeFormDefault
to
unqualified
. Strictly speaking, these settings are
unnecessary because the values are the defaults for the two
attributes; we make them here to highlight the contrast between
this case and other cases we describe later.
<schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:po="http://www.example.com/PO1" targetNamespace="http://www.example.com/PO1" elementFormDefault="unqualified" attributeFormDefault="unqualified"> <element name="purchaseOrder" type="po:PurchaseOrderType"/> <element name="comment" type="string"/> <complexType name="PurchaseOrderType"> <sequence> <element name="shipTo" type="po:USAddress"/> <element name="billTo" type="po:USAddress"/> <element ref="po:comment" minOccurs="0"/> <!-- etc. --> </sequence> <!-- etc. --> </complexType> <complexType name="USAddress"> <sequence> <element name="name" type="string"/> <element name="street" type="string"/> <!-- etc. --> </sequence> </complexType> <!-- etc. --> </schema>
To see how the target namespace of this schema is
populated, we examine in turn each of the type
definitions and element declarations. Starting from the end
of the schema, we first define a type called
USAddress
that consists of the elements
name
, street
, etc. One consequence of
this type definition is that the USAddress
type is included in the schema's target namespace. We next
define a type called PurchaseOrderType
that
consists of the elements shipTo
,
billTo
, comment
, etc.
PurchaseOrderType
is also included in the schema's
target namespace. Notice that the type references in the
three element declarations are prefixed, i.e.
po:USAddress
, po:USAddress
and
po:comment
, and the prefix is associated with the
namespace http://www.example.com/PO1
. This is
the same namespace as the schema's target namespace, and so
a processor of this schema will know to look within this
schema for the definition of the type
USAddress
and the declaration of the element
comment
. It is also possible to refer to types in
another schema with a different target namespace, hence
enabling re-use of definitions and declarations between
schemas.
At the beginning of the schema
po1.xsd
, we declare the elements
purchaseOrder
and comment
. They are
included in the schema's target namespace. The
purchaseOrder
element's type is prefixed, for the
same reason that USAddress
is prefixed. In
contrast, the comment
element's type, string
, is not prefixed. The
po1.xsd
schema
contains a default namespace declaration, and so unprefixed
types such as string
and unprefixed elements such as element
and complexType
are
associated with the default namespace
http://www.w3.org/2001/XMLSchema
. In fact, this
is the target namespace of XML Schema itself, and so a
processor of po1.xsd
will know to look within the schema of XML Schema
-- otherwise known as the "schema for schemas" -- for the
definition of the type
string
and the declaration of the element called
element
.
Let us now examine how the target namespace of the schema affects a conforming instance document:
<?xml version="1.0"?> <apo:purchaseOrder xmlns:apo="http://www.example.com/PO1" orderDate="1999-10-20"> <shipTo country="US"> <name>Alice Smith</name> <street>123 Maple Street</street> <!-- etc. --> </shipTo> <billTo country="US"> <name>Robert Smith</name> <street>8 Oak Avenue</street> <!-- etc. --> </billTo> <apo:comment>Hurry, my lawn is going wild<!/apo:comment> <!-- etc. --> </apo:purchaseOrder>
The instance document declares one namespace,
http://www.example.com/PO1
, and associates it with
the prefix apo:
. This prefix is used to
qualify two elements in the document, namely
purchaseOrder
and comment
. The
namespace is the same as the target namespace of the schema
in po1.xsd
, and so a
processor of the instance document will know to look in
that schema for the declarations of
purchaseOrder
and comment
. In fact,
target namespaces are so named because of the sense in
which there exists a target namespace for the elements
purchaseOrder
and comment
. Target
namespaces in the schema therefore control the validation
of corresponding namespaces in the instance.
The prefix apo:
is applied to the global
elements purchaseOrder
and
comment
elements. Furthermore,
elementFormDefault
and
attributeFormDefault
require that the prefix is
not applied to any of the locally declared
elements such as shipTo
, billTo
,
name
and street
, and it is
not applied to any of the attributes (which were all
declared locally). The purchaseOrder
and
comment
are global elements because they are
declared in the context of the schema as a whole rather
than within the context of a particular type. For example,
the declaration of purchaseOrder
appears as a
child of the
schema
element in
po1.xsd
, whereas the declaration of
shipTo
appears as a child of the complexType
element that
defines PurchaseOrderType
.
When local elements and attributes are not required to be
qualified, an instance author may require more or less
knowledge about the details of the schema to create schema
valid instance documents. More specifically, if the author
can be sure that only the root element (such as
purchaseOrder
) is global, then it is a simple matter
to qualify only the root element. Alternatively, the author
may know that all the elements are declared globally, and
so all the elements in the instance document can be
prefixed, perhaps taking advantage of a default namespace
declaration. (We examine this approach in Global vs. Local Declarations (§3.3).) On the other hand, if
there is no uniform pattern of global and local
declarations, the author will need detailed knowledge of
the schema to correctly prefix global elements and
attributes.
Elements and attributes can be independently required to
be qualified, although we start by describing the
qualification of local elements. To specify that all
locally declared elements in a schema must be qualified, we
set the value of
elementFormDefault
to qualified
:
<schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:po="http://www.example.com/PO1" targetNamespace="http://www.example.com/PO1" elementFormDefault="qualified" attributeFormDefault="unqualified"> <element name="purchaseOrder" type="po:PurchaseOrderType"/> <element name="comment" type="string"/> <complexType name="PurchaseOrderType"> <!-- etc. --> </complexType> <!-- etc. --> </schema>
And in this conforming instance document, we qualify all the elements explicitly:
<?xml version="1.0"?> <apo:purchaseOrder xmlns:apo="http://www.example.com/PO1" orderDate="1999-10-20"> <apo:shipTo country="US"> <apo:name>Alice Smith</apo:name> <apo:street>123 Maple Street</apo:street> <!-- etc. --> </apo:shipTo> <apo:billTo country="US"> <apo:name>Robert Smith</apo:name> <apo:street>8 Oak Avenue</apo:street> <!-- etc. --> </apo:billTo> <apo:comment>Hurry, my lawn is going wild<!/apo:comment> <!-- etc. --> </apo:purchaseOrder>
Alternatively, we can replace the explicit qualification
of every element with implicit qualification provided by a
default namespace, as shown here in
po2.xml
:
<?xml version="1.0"?> <purchaseOrder xmlns="http://www.example.com/PO1" orderDate="1999-10-20"> <shipTo country="US"> <name>Alice Smith</name> <street>123 Maple Street</street> <!-- etc. --> </shipTo> <billTo country="US"> <name>Robert Smith</name> <street>8 Oak Avenue</street> <!-- etc. --> </billTo> <comment>Hurry, my lawn is going wild<!/comment> <!-- etc. --> </purchaseOrder>
In po2.xml
, all the elements in the
instance belong to the same namespace, and the namespace
statement declares a default namespace that applies to all
the elements in the instance. Hence, it is unnecessary to
explicitly prefix any of the elements. As another
illustration of using qualified elements, the schemas in Advanced Concepts III: The Quarterly Report (§5) all require qualified
elements.
Qualification of attributes is very similar to the
qualification of elements. Attributes that must be
qualified, either because they are declared globally or
because the
attributeFormDefault
attribute is set to
qualified
, appear prefixed in instance documents.
One example of a qualified attribute is the xsi:nil
attribute that
was introduced in Nil Values (§2.9). In
fact, attributes that are required to be qualified must be
explicitly prefixed because the Namespaces in XML
specification does not provide a mechanism for defaulting
the namespaces of attributes. Attributes that are not
required to be qualified appear in instance documents
without prefixes, which is the typical case.
The qualification mechanism we have
described so far has controlled all local element and
attribute declarations within a particular target
namespace. It is also possible to control qualification on
a declaration by declaration basis using the form
attribute. For example,
to require that the locally declared attribute
publicKey
is qualified in instances, we declare it
in the following way:
<schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:po="http://www.example.com/PO1" targetNamespace="http://www.example.com/PO1" elementFormDefault="qualified" attributeFormDefault="unqualified"> <!-- etc. --> <element name="secure"> <complexType> <sequence> <!-- element declarations --> </sequence> <attribute name="publicKey" type="base64Binary" form="qualified"/> </complexType> </element> </schema>
Notice that the value of the form
attribute overrides the
value of the
attributeFormDefault
attribute for the
publicKey
attribute only. Also, the form
attribute can be applied
to an element declaration in the same manner. An instance
document that conforms to the schema is:
Another authoring style, applicable when all element names are
unique within a namespace, is to create schemas in which
all elements are global. This is similar in effect to the
use of <!ELEMENT> in a DTD. In the example below, we
have modified the original po1.xsd
such that all the elements are declared globally. Notice
that we have omitted the
elementFormDefault
and
attributeFormDefault
attributes in this example
to emphasize that their values are irrelevant when there
are only global element and attribute declarations.
<schema xmlns="http://www.w3.org/2001/XMLSchema" xmlns:po="http://www.example.com/PO1" targetNamespace="http://www.example.com/PO1"> <element name="purchaseOrder" type="po:PurchaseOrderType"/> <element name="shipTo" type="po:USAddress"/> <element name="billTo" type="po:USAddress"/> <element name="comment" type="string"/> <element name="name" type="string"/> <element name="street" type="string"/> <complexType name="PurchaseOrderType"> <sequence> <element ref="po:shipTo"/> <element ref="po:billTo"/> <element ref="po:comment" minOccurs="0"/> <!-- etc. --> </sequence> </complexType> <complexType name="USAddress"> <sequence> <element ref="po:name"/> <element ref="po:street"/> <!-- etc. --> </sequence> </complexType> <!-- etc. --> </schema>
This "global" version of po1.xsd
will validate the instance document
po2.xml
which, as we described previously, is also
schema valid against the "qualified" version of po1.xsd
. In other words, both schema
approaches can validate the same, namespace defaulted,
document. Thus, in one respect the two schema approaches
are similar, although in another important respect the two
schema approaches are very different. Specifically, when
all elements are declared globally, it is not possible to
take advantage of local names. For example, you can only
declare one global element called "title". However, you can
locally declare one element called "title" that has a
string type, and is a subelement of "book". Within the same
schema (target namespace) you can declare a second element
also called "title" that is an enumeration of the values
"Mr Mrs Ms".
In Basic Concepts: The Purchase Order (§2) we explained the basics of XML Schema using a schema that did not declare a target namespace and an instance document that did not declare a namespace. So the question naturally arises: What is the target namespace in these examples and how is it referenced?
In the purchase order schema,
po.xsd
, we did not declare a target namespace
for the schema, nor did we declare a prefix (like
po
: above) associated with the schema's target
namespace with which we could refer to types and elements
defined and declared within the schema. The consequence of
not declaring a target namespace in a schema is that the
definitions and declarations from that schema, such as
USAddress
and purchaseOrder
, are
referenced without namespace qualification. In other words
there is no explicit namespace prefix applied to the
references nor is there any implicit namespace applied to
the reference by default. So for example, the
purchaseOrder
element is declared using the type
reference PurchaseOrderType
. In contrast, all
the XML Schema elements and types used in po.xsd
are explicitly qualified with
the prefix xsd:
that is associated with the
XML Schema namespace.
In cases where a schema is designed without a target namespace, it is
strongly recommended that all XML Schema elements and types are explicitly
qualified with a prefix such as xsd:
that is associated
with the XML Schema namespace (as in po.xsd
). The rationale for this recommendation is
that if XML Schema elements and types are associated with the XML Schema
namespace by default, i.e. without prefixes, then references to XML
Schema types may not be distinguishable from references to user-defined
types.
Element declarations from a schema with no target
namespace validate unqualified elements in the instance
document. That is, they validate elements for which no
namespace qualification is provided by either an explicit
prefix or by default (xmlns:
). So, to validate
a traditional XML 1.0 document which does not use
namespaces at all, you must provide a schema with no target
namespace. Of course, there are many XML 1.0 documents that
do not use namespaces, so there will be many schema
documents written without target namespaces; you must be
sure to give to your processor a schema document that
corresponds to the vocabulary you wish to validate.
The purchase order schema described in Basic Concepts: The Purchase Order (§2) was contained in a single document, and most of the schema constructions-- such as element declarations and type definitions-- were constructed from scratch. In reality, schema authors will want to compose schemas from constructions located in multiple documents, and to create new types based on existing types. In this section, we examine mechanisms that enable such compositions and creations.
As schemas become larger, it is often desirable to divide
their content among several schema documents for purposes
such as ease of maintenance, access control, and
readability. For these reasons, we have taken the schema
constructs concerning addresses out of po.xsd
, and put them in a new file
called address.xsd
.
The modified purchase order schema file is called ipo.xsd
:
<schema targetNamespace="http://www.example.com/IPO" xmlns="http://www.w3.org/2001/XMLSchema" xmlns:ipo="http://www.example.com/IPO"> <annotation> <documentation xml:lang="en"> International Purchase order schema for Example.com Copyright 2000 Example.com. All rights reserved. </documentation> </annotation> <!-- include address constructs --> <include schemaLocation="http://www.example.com/schemas/address.xsd"/> <element name="purchaseOrder" type="ipo:PurchaseOrderType"/> <element name="comment" type="string"/> <complexType name="PurchaseOrderType"> <sequence> <element name="shipTo" type="ipo:Address"/> <element name="billTo" type="ipo:Address"/> <element ref="ipo:comment" minOccurs="0"/> <element name="items" type="ipo:Items"/> </sequence> <attribute name="orderDate" type="date"/> </complexType> <complexType name="Items"> <sequence> <element name="item" minOccurs="0" maxOccurs="unbounded"> <complexType> <sequence> <element name="productName" type="string"/> <element name="quantity"> <simpleType> <restriction base="positiveInteger"> <maxExclusive value="100"/> </restriction> </simpleType> </element> <element name="USPrice" type="decimal"/> <element ref="ipo:comment" minOccurs="0"/> <element name="shipDate" type="date" minOccurs="0"/> </sequence> <attribute name="partNum" type="ipo:SKU" use="required"/> </complexType> </element> </sequence> </complexType> <simpleType name="SKU"> <restriction base="string"> <pattern value="\d{3}-[A-Z]{2}"/> </restriction> </simpleType> </schema>
The file containing the address constructs is:
<schema targetNamespace="http://www.example.com/IPO" xmlns="http://www.w3.org/2001/XMLSchema" xmlns:ipo="http://www.example.com/IPO"> <annotation> <documentation xml:lang="en"> Addresses for International Purchase order schema Copyright 2000 Example.com. All rights reserved. </documentation> </annotation> <complexType name="Address"> <sequence> <element name="name" type="string"/> <element name="street" type="string"/> <element name="city" type="string"/> </sequence> </complexType> <complexType name="USAddress"> <complexContent> <extension base="ipo:Address"> <sequence> <element name="state" type="ipo:USState"/> <element name="zip" type="positiveInteger"/> </sequence> </extension> </complexContent> </complexType> <complexType name="UKAddress"> <complexContent> <extension base="ipo:Address"> <sequence> <element name="postcode" type="ipo:UKPostcode"/> </sequence> <attribute name="exportCode" type="positiveInteger" fixed="1"/> </extension> </complexContent> </complexType> <!-- other Address derivations for more countries --> <simpleType name="USState"> <restriction base="string"> <enumeration value="AK"/> <enumeration value="AL"/> <enumeration value="AR"/> <!-- and so on ... --> </restriction> </simpleType> <!-- simple type definition for UKPostcode --> </schema>
The various purchase order and
address constructions are now contained in two schema
files, ipo.xsd
and
address.xsd
. To
include these constructions as part of the international
purchase order schema, in other words to include them in
the international purchase order's namespace, ipo.xsd
contains the include
element:
<include schemaLocation="http://www.example.com/schemas/address.xsd"/>
The effect of this
include
element is to bring in the definitions
and declarations contained in
address.xsd
, and make them available as part of
the international purchase order schema target namespace.
The one important caveat to using include
is that the target
namespace of the included components must be the same as
the target namespace of the including schema, in this case
http://www.example.com/IPO
. Bringing in
definitions and declarations using the include
mechanism
effectively adds these components to the existing target
namespace. In Redefining Types & Groups (§4.5), we
describe a similar mechanism that enables you to modify
certain components when they are brought in.
In our example, we have shown only one including document
and one included document. In practice it is possible to
include more than one document using multiple include
elements, and
documents can include documents that themselves include
other documents. However, nesting documents in this manner
is legal only if all the
included parts of the schema are declared with the same
target namespace.
Instance documents that conform to schema whose
definitions span multiple schema documents need only
reference the 'topmost' document and the common namespace,
and it is the responsibility of the processor to gather
together all the definitions specified in the various
included documents. In our example above, the instance
document ipo.xml
(see
Using Derived Types in Instance Documents (§4.3)) references
only the common target namespace,
http://www.example.com/IPO
, and (by implication) the one schema file
http://www.example.com/schemas/ipo.xsd
. The
processor is responsible for obtaining the schema file
address.xsd
.
In Importing Types (§5.4) we describe how schemas can be used to validate content from more than one namespace.
To create our address constructs, we start by creating a
complex type called Address
in the usual way
(see address.xsd
).
The Address
type contains the basic elements
of an address: a name, a street and a city. (Such a
definition will not work for all countries, but it
serves the purpose of our example.) From this starting
point we derive two new complex types that contain all the
elements of the original type plus additional elements that
are specific to addresses in the US and the UK. The
technique we use here to derive new (complex) address types
by extending an existing type is the same technique we used
in Complex Types from Simple Types (§2.5.1),
except that our base type here is a complex type whereas
our base type in the previous section was a simple type.
We define the two new complex types,
USAddress
and UKAddress
, using the
complexType
element. In addition, we indicate that the content models
of the new types are complex, i.e. contain elements, by
using the
complexContent
element, and we indicate that we
are extending the base type Address
by the
value of the
base
attribute on the extension
element.
When a complex type is derived by extension, its effective
content model is the content model of the base type plus
the content model specified in the type derivation.
Furthermore, the two content models are treated as two
children of a sequential group. In the case of
UKAddress
, the content model of
UKAddress
is the content model of
Address
plus the declarations for a
postcode
element and an exportCode
attribute. This is like defining the UKAddress
from scratch as follows:
<complexType name="UKAddress"> <sequence> <!-- content model of Address --> <element name="name" type="string"/> <element name="street" type="string"/> <element name="city" type="string"/> <!-- appended element declaration --> <element name="postcode" type="ipo:UKPostcode"/> </sequence> <!-- appended attribute declaration --> <attribute name="exportCode" type="positiveInteger" fixed="1"/> </complexType>
In our example scenario, purchase orders are generated in
response to customer orders which may involve shipping and
billing addresses in different countries. The international
purchase order, ipo.xml
below, illustrates one such case where goods are shipped to
the UK and the bill is sent to a US address. Clearly it is
better if the schema for international purchase orders
does not have to spell out every possible combination of
international addresses for billing and shipping, and even
more so if we can add new complex types of international
address simply by creating new derivations of
Address
.
XML Schema allows us to define the
billTo
and shipTo
elements as
Address
types (see
ipo.xsd
) but to use instances of international
addresses in place of instances of Address
.
In other words, an instance document whose content conforms
to the UKAddress
type will be valid if that
content appears within the document at a location where an
Address
is expected (assuming the
UKAddress
content itself is valid). To make this
feature of XML Schema work, and to identify exactly which
derived type is intended, the derived type must be
identified in the instance document. The type is identified
using the
xsi:type
attribute which is part of the XML
Schema instance namespace. In the example, ipo.xml
, use of the
UKAddress
and USAddress
derived types
is identified through the values assigned to the xsi:type
attributes.
<?xml version="1.0"?> <ipo:purchaseOrder xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ipo="http://www.example.com/IPO" orderDate="1999-12-01"> <shipTo exportCode="1" xsi:type="ipo:UKAddress"> <name>Helen Zoe</name> <street>47 Eden Street</street> <city>Cambridge</city> <postcode>CB1 1JR</postcode> </shipTo> <billTo xsi:type="ipo:USAddress"> <name>Robert Smith</name> <street>8 Oak Avenue</street> <city>Old Town</city> <state>PA</state> <zip>95819</zip> </billTo> <items> <item partNum="833-AA"> <productName>Lapis necklace</productName> <quantity>1</quantity> <USPrice>99.95</USPrice> <ipo:comment>Want this for the holidays<!/ipo:comment> <shipDate>1999-12-05</shipDate> </item> </items> </ipo:purchaseOrder>
In Controlling the Creation & Use of Derived Types (§4.8) we describe how to prevent derived types from being used in this sort of substitution.
In addition to deriving new complex types by extending content models, it is possible to derive new types by restricting the content models of existing types. Restriction of complex types is conceptually the same as restriction of simple types, except that the restriction of complex types involves a type's declarations rather than the acceptable range of a simple type's values. A complex type derived by restriction is very similar to its base type, except that its declarations are more limited than the corresponding declarations in the base type. In fact, the values represented by the new type are a subset of the values represented by the base type (as is the case with restriction of simple types). In other words, an application prepared for the values of the base type would not be surprised by the values of the restricted type.
For example, suppose we want to update our definition of a purchase
order so that it must contain a comment
;
the schema shown in ipo.xsd
allows a purchaseOrder
element to appear without any child
comment
elements. To create our new
RestrictedPurchaseOrderType
type, we define the new type in the usual way, indicate
that it is derived by restriction from the base type
PurchaseOrderType
, and provide a new (more restrictive) value for
the minimum number of
comment
element
occurrences. Notice that types derived by restriction must
repeat all the particle components
(element declarations, model groups, and wildcards) of the base type definition that
are to be included in the derived type. However,
attribute declarations do not need to be repeated in the derived type definition; in this example, RestrictedPurchaseOrderType
will inherit the
orderDate
attribute declaration from PurchaseOrderType
.
<complexType name="RestrictedPurchaseOrderType"> <complexContent> <restriction base="ipo:PurchaseOrderType"> <sequence> <element name="shipTo" type="ipo:Address"/> <element name="billTo" type="ipo:Address"/> <element ref="ipo:comment" minOccurs="1"/> <element name="items" type="ipo:Items"/> </sequence> </restriction> </complexContent> </complexType>
This change narrows the
allowable number of
comment
elements from a minimum of 0 to a
minimum of 1. Note that all
RestrictedPurchaseOrderType
type elements will also be acceptable as
PurchaseOrderType
type elements.
To further illustrate restriction, Table 3 shows several examples of how element and attribute declarations within type definitions may be restricted (the table shows element syntax although the first three examples are equally valid attribute restrictions).
Table 3. Restriction Examples | ||||||
---|---|---|---|---|---|---|
Base | Restriction(s) | Notes | ||||
default="1" | setting a default value where none was previously given | |||||
fixed="100" | setting a fixed value where none was previously given | |||||
type="string" | specifying a type where none was previously given | |||||
(minOccurs, maxOccurs) | (minOccurs, maxOccurs) | |||||
(0, 1) | (0, 0) | exclusion of an optional component; this may also be accomplished by omitting the component's declaration from the restricted type definition | ||||
(0, 1) | (1, 1) | making an optional component required | ||||
(0, unbounded) |
| |||||
(1, 9) |
| |||||
(1, unbounded) |
| |||||
(1, 1) | (1, 1) | cannot further restrict minOccurs or maxOccurs |
In A Schema in Multiple Documents (§4.1) we
described how to include definitions and declarations
obtained from external schema files having the same target
namespace. The
include
mechanism enables you to use externally
created schema components "as-is", that is, without any
modification. We have just described how to derive new
types by extension and by restriction, and the redefine
mechanism we
describe here enables you to redefine simple and
complex types, groups, and attribute groups that are
obtained from external schema files. Like the include
mechanism,
redefine
requires the external components to be in the same target
namespace as the redefining schema, although external
components from schemas that have no namespace can also be
redefined. In the latter cases, the redefined components
become part of the redefining schema's target namespace.
To illustrate the
redefine
mechanism, we use it instead of
the include
mechanism in the International Purchase Order schema,
ipo.xsd
, and we
use it to modify the definition of the complex type
Address
contained in
address.xsd
:
<schema targetNamespace="http://www.example.com/IPO" xmlns="http://www.w3.org/2001/XMLSchema" xmlns:ipo="http://www.example.com/IPO"> <!-- bring in address constructs --> <redefine schemaLocation="http://www.example.com/schemas/address.xsd"> <!-- redefinition of Address --> <complexType name="Address"> <complexContent> <extension base="ipo:Address"> <sequence> <element name="country" type="string"/> </sequence> </extension> </complexContent> </complexType> </redefine> <!-- etc. --> </schema>
The redefine
element acts very much like the include
element as it includes
all the declarations and definitions from the address.xsd
file. The
complex type definition of Address
uses the
familiar extension syntax to add a country
element to the definition of Address
. However,
note that the base type is also Address
.
Outside of the
redefine
element, any such attempt to define a
complex type with the same name (and in the same namespace)
as the base from which it is being derived would cause an
error. But in this case, there is no error, and the
extended definition of Address
becomes the
only definition of Address
.
Now that Address
has been redefined, the
extension applies to all schema components that make use of
Address
. For example,
address.xsd
contains definitions of international
address types that are derived from Address
.
These derivations reflect the redefined
Address
type, as shown in the following snippet:
.... <shipTo exportCode="1" xsi:type="ipo:UKAddress"> <name>Helen Zoe</name> <street>47 Eden Street</street> <city>Cambridge</city> <!-- country was added to Address which is base type of UKAddress --> <country>United Kingdom</country> <!-- postcode was added as part of UKAddress --> <postcode>CB1 1JR</postcode> </shipTo> ....
Our example has been carefully constructed so that the
redefined Address
type does not conflict in
any way with the types that are derived from the original
Address
definition. But note that it would be
very easy to create a conflict. For example, if the
international address type derivations had extended
Address
by adding a country
element,
then the redefinition of Address
would be
adding an element of the same name to the content model of
Address
. It is illegal to have two elements of
the same name (and in the same target namespace) but
different types in a content model, and so the attempt to
redefine Address
would cause an error. In
general,
redefine
does not protect you from such errors,
and it should be used cautiously.
XML Schema provides a mechanism,
called substitution groups, that allows elements to be
substituted for other elements. More specifically, elements
can be assigned to a special group of elements that are
said to be substitutable for a particular named element
called the head element. (Note that the head element
as well as the substitutable
elementsmust
be declared as global
elements.) To illustrate, we
declare two elements called customerComment
and shipComment
and assign them to a
substitution group whose head element is
comment
, and so customerComment
and
shipComment
can be used anyplace that we are
able to use comment
. Elements in a
substitution group must have the same type as the head
element, or they can have a type that has been derived from
the head element's type. To declare these two new elements,
and to make them substitutable for the
comment
element, we use the following syntax:
<element name="shipComment" type="string" substitutionGroup="ipo:comment"/> <element name="customerComment" type="string" substitutionGroup="ipo:comment"/>
When these declarations are added to the international
purchase order schema, shipComment
and
customerComment
can be substituted for
comment
in the instance document, for example:
.... <items> <item partNum="833-AA"> <productName>Lapis necklace</productName> <quantity>1</quantity> <USPrice>99.95</USPrice> <ipo:shipComment> Use gold wrap if possible </ipo:shipComment> <ipo:customerComment> Want this for the holidays! </ipo:customerComment> <shipDate>1999-12-05</shipDate> </item> </items> ....
Note that when an instance document contains element
substitutions whose types are derived from those of their
head elements, it is not necessary to identify the
derived types using the xsi:type
construction that
we described in Using Derived Types in Instance Documents (§4.3).
The existence of a substitution group does not require any of the elements in that class to be used, nor does it preclude use of the head element. It simply provides a mechanism for allowing elements to be used interchangeably.
XML Schema provides a mechanism to
force substitution for a particular element or type. When
an element or type is declared to be "abstract", it cannot
be used in an instance document. When an element is
declared to be abstract, a member of that element's
substitution group must appear in the instance document.
When an element's corresponding type definition is declared
as abstract, all instances of that element must use
xsi:type
to
indicate a derived type that is not abstract.
In the substitution group example we described in Substitution Groups (§4.6), it would be useful to
specifically disallow use of the comment
element so that instances must make use of the
customerComment
and shipComment
elements. To declare the comment
element
abstract, we modify its original declaration in the
international purchase order schema, ipo.xsd
, as follows:
<element name="comment" type="string" abstract="true"/>
With comment
declared as abstract, instances
of international purchase orders are now only valid if they
contain customerComment
and
shipComment
elements.
Declaring an element as abstract requires the use of a
substitution group. Declaring a type as abstract simply
requires the use of a type derived from it (and identified
by the
xsi:type
attribute) in the instance document.
Consider the following schema definition:
<schema xmlns="http://www.w3.org/2001/XMLSchema" targetNamespace="http://cars.example.com/schema" xmlns:target="http://cars.example.com/schema"> <complexType name="Vehicle" abstract="true"/> <complexType name="Car"> <complexContent> <extension base="target:Vehicle"/> </complexContent> </complexType> <complexType name="Plane"> <complexContent> <extension base="target:Vehicle"/> </complexContent> </complexType> <element name="transport" type="target:Vehicle"/> </schema>
The transport
element is not abstract,
therefore it can appear in instance documents. However,
because its type definition is abstract, it may never
appear in an instance document without an xsi:type
attribute that
refers to a derived type. That means the following is not
schema-valid:
<transport xmlns="http://cars.example.com/schema"/>
because the transport
element's type is
abstract. However, the following is schema-valid:
<transport xmlns="http://cars.example.com/schema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:type="Car"/>
because it uses a non-abstract type that is substitutable
for Vehicle
.
So far, we have been able to derive new types and use them in instance documents without any restraints. In reality, schema authors will sometimes want to control derivations of particular types, and the use of derived types in instances.
XML Schema provides a couple of mechanisms that control
the derivation of types. One of these mechanisms allows the
schema author to specify that for a particular complex
type, new types may not be derived from it, either (a) by
restriction, (b) by extension, or (c) at all. To
illustrate, suppose we want to prevent any derivation of
the Address
type by restriction because we
intend for it only to be used as the base for extended
types such as USAddress
and
UKAddress
. To prevent any such derivations, we
slightly modify the original definition of
Address
as follows:
<complexType name="Address" final="restriction"> <sequence> <element name="name" type="string"/> <element name="street" type="string"/> <element name="city" type="string"/> </sequence> </complexType>
The restriction
value of
the final
attribute prevents derivations by restriction. Preventing
derivations at all, or by extension, are indicated by the
values #all
and extension
respectively. Moreover, there exists an optional finalDefault
attribute
on the schema
element whose value can be one of the values allowed for
the final
attribute. The effect of specifying the finalDefault
attribute
is equivalent to specifying a final
attribute on every type
definition and element declaration in the schema.
Another type-derivation mechanism
controls which facets can be applied in the derivation of a
new simple type. When a simple type is defined, the
fixed
attribute
may be applied to any of its facets to prevent a derivation
of that type from modifying the value of the fixed facets.
For example, we can define a Postcode
simple
type as:
<simpleType name="Postcode"> <restriction base="string"> <length value="7" fixed="true"/> </restriction> </simpleType>
Once this simple type has been defined, we can derive a new postal code type in which we apply a facet not fixed in the base definition, for example:
<simpleType name="UKPostcode"> <restriction base="ipo:Postcode"> <pattern value="[A-Z]{2}\d\s\d[A-Z]{2}"/> </restriction> </simpleType>
However, we cannot derive a new postal code in which we re-apply any facet that was fixed in the base definition:
<simpleType name="UKPostcode"> <restriction base="ipo:Postcode"> <pattern value="[A-Z]{2}\d\d[A-Z]{2}"/> <!-- illegal attempt to modify facet fixed in base type --> <length value="6" fixed="true"/> </restriction> </simpleType>
In addition to the mechanisms that
control type derivations, XML Schema provides a mechanism
that controls which derivations and substitution groups may
be used in instance documents. In Using Derived Types in Instance Documents (§4.3), we described how the
derived types, USAddress
and
UKAddress
, could be used by the shipTo
and billTo
elements in instance documents.
These derived types can replace the content model provided
by the Address
type because they are derived
from the Address
type. However, replacement by
derived types can be controlled using the block
attribute in a type
definition. For example, if we want to block any
derivation-by-restriction from being used in place of
Address
(perhaps for the same reason we
defined Address
with final="restriction"
), we can
modify the original definition of Address
as
follows:
<complexType name="Address" block="restriction"> <sequence> <element name="name" type="string"/> <element name="street" type="string"/> <element name="city" type="string"/> </sequence> </complexType>
The restriction
value on
the block
attribute prevents derivations-by-restriction from
replacing Address
in an instance. However, it
would not prevent UKAddress
and
USAddress
from replacing Address
because they were derived by extension. Preventing
replacement by derivations at all, or by
derivations-by-extension, are indicated by the values
#all
and extension
respectively.
As with final
,
there exists an optional blockDefault
attribute
on the schema
element whose value can be one of the values allowed for
the block
attribute. The effect of specifying the blockDefault
attribute
is equivalent to specifying a block
attribute on every type
definition and element declaration in the schema.
The home-products ordering and billing application can
generate ad-hoc reports that summarize how many of which
types of products have been billed on a per region basis.
An example of such a report, one that covers the fourth
quarter of 1999, is shown in
4Q99.xml
.
Notice that in this section we use qualified elements in the schema, and default namespaces where possible in the instances.
<purchaseReport xmlns="http://www.example.com/Report" period="P3M" periodEnding="1999-12-31"> <regions> <zip code="95819"> <part number="872-AA" quantity="1"/> <part number="926-AA" quantity="1"/> <part number="833-AA" quantity="1"/> <part number="455-BX" quantity="1"/> </zip> <zip code="63143"> <part number="455-BX" quantity="4"/> </zip> </regions> <parts> <part number="872-AA">Lawnmower</part> <part number="926-AA">Baby Monitor</part> <part number="833-AA">Lapis Necklace</part> <part number="455-BX">Sturdy Shelves</part> </parts> </purchaseReport>
The report lists, by number and quantity, the parts billed
to various zip codes, and it provides a description of each
part mentioned. In summarizing the billing data, the
intention of the report is clear and the data is
unambiguous because a number of constraints are in effect.
For example, each zip code appears only once (uniqueness
constraint). Similarly, the description of every billed
part appears only once although parts may be billed to
several zip codes (referential constraint), see for example
part number 455-BX
. In the following sections, we'll see
how to specify these constraints using XML Schema.
<schema targetNamespace="http://www.example.com/Report" xmlns="http://www.w3.org/2001/XMLSchema" xmlns:r="http://www.example.com/Report" xmlns:xipo="http://www.example.com/IPO" elementFormDefault="qualified"> <!-- for SKU --> <import namespace="http://www.example.com/IPO"/> <annotation> <documentation xml:lang="en"> Report schema for Example.com Copyright 2000 Example.com. All rights reserved. </documentation> </annotation> <element name="purchaseReport"> <complexType> <sequence> <element name="regions" type="r:RegionsType"/> <element name="parts" type="r:PartsType"/> </sequence> <attribute name="period" type="duration"/> <attribute name="periodEnding" type="date"/> </complexType> <unique name="dummy1"> <selector xpath="r:regions/r:zip"/> <field xpath="@code"/> </unique> <key name="pNumKey"> <selector xpath="r:parts/r:part"/> <field xpath="@number"/> </key> <keyref name="dummy2" refer="r:pNumKey"> <selector xpath="r:regions/r:zip/r:part"/> <field xpath="@number"/> </keyref> </element> <complexType name="RegionsType"> <sequence> <element name="zip" maxOccurs="unbounded"> <complexType> <sequence> <element name="part" maxOccurs="unbounded"> <complexType> <complexContent> <restriction base="anyType"> <attribute name="number" type="xipo:SKU"/> <attribute name="quantity" type="positiveInteger"/> </restriction> </complexContent> </complexType> </element> </sequence> <attribute name="code" type="positiveInteger"/> </complexType> </element> </sequence> </complexType> <complexType name="PartsType"> <sequence> <element name="part" maxOccurs="unbounded"> <complexType> <simpleContent> <extension base="string"> <attribute name="number" type="xipo:SKU"/> </extension> </simpleContent> </complexType> </element> </sequence> </complexType> </schema>
XML Schema enables us to indicate
that any attribute or element value must be unique within a
certain scope. To indicate that one particular attribute or
element value is unique, we use the unique
element first to
"select" a set of elements, and then to identify the
attribute or element "field" relative to each selected
element that has to be unique within the scope of the set
of selected elements. In the case of our report schema,
report.xsd
, the
selector
element's xpath
attribute contains an XPath expression,
r:
regions/
r:
zip
,
that selects a list of all the
zip
elements in a report instance. Likewise, the
field
element's
xpath
attribute
contains a second XPath expression, @code
,
that specifies that the code
attribute values
of those elements must be unique. Note that the XPath
expressions limit the scope of what must be unique. The
report might contain another code
attribute,
but its value does not have to be unique because it lies
outside the scope defined by the XPath expressions. Also
note that the XPath expressions you can use in the
xpath
attribute
are limited to a subset
of the full XPath language defined in XML Path Language 1.0.
We can also indicate combinations of fields that
must be unique. Going
back to our purchase order example, suppose we want each item to have a
unique combination of part number and product name. We could achieve
such a constraint by specifying that for each item
element,
the combined values of its partNum
attribute
and its productName
child must be unique.
To define combinations of values, we simply
use multiple field
elements to identify
all the values involved:
In the 1999 quarterly report, the
description of every billed part appears only once. We
could enforce this constraint using unique
, however, we also want
to ensure that every part-quantity element listed under a
zip code has a corresponding part description. We enforce
the constraint using the
key
and
keyref
elements. The report schema, report.xsd
, shows that the
key
and keyref
constructions are
applied using almost the same syntax as unique
. The key element
applies to the number
attribute value of
part
elements that are children of the
parts
element. This declaration of
number
as a key means that its value must be unique
and cannot be set to nil (i.e. is not nillable), and the
name that is associated with the
key, pNumKey
, makes the key referenceable from
elsewhere.
To ensure that the part-quantity elements have
corresponding part descriptions, we say that the
number
attribute (
<field xpath="@number"/>
) of those
elements
(
<selector xpath="r:regions/r:zip/r:part"/>
)
must reference the pNumKey
key. This
declaration of number
as a keyref
does not
mean that its value must be unique, but it does mean there
must exist a pNumKey
with the same value.
As you may have figured out by analogy with unique
, it is possible to
define combinations of
key
and
keyref
values. Using this mechanism, we could go
beyond simply requiring the product numbers to be equal,
and define a combination of values that must be equal. Such
values may involve combinations of multiple value types
(string
, integer
,
date
, etc.), provided that the order and type of
the field
element
references is the same in both the key
and keyref
definitions.
XML 1.0
provides a mechanism for ensuring uniqueness using
the ID attribute and its associated attributes IDREF and
IDREFS. This mechanism is also provided in XML Schema
through the ID
, IDREF
, and IDREFS
simple types which can be used
for declaring XML 1.0-style attributes. XML Schema also
introduces new mechanisms that are more flexible and
powerful. For example, XML Schema's mechanisms can be
applied to any element and attribute content, regardless of
its type. In contrast, ID is a type of attribute
and so it cannot be applied to attributes, elements or
their content. Furthermore, Schema enables you to specify
the scope within which uniqueness applies whereas the scope
of an ID is fixed to be the whole document. Finally, Schema
enables you to create
key
s or a
keyref
from combinations of element and
attribute content whereas ID has no such facility.
The report schema, report.xsd
, makes use of the
simple type xipo:SKU
that is defined in
another schema, and in another target
namespace. Recall that we used include
so that the schema in
ipo.xsd
could make use
of definitions and declarations from address.xsd
. We cannot use
include
here because
it can only pull in definitions and declarations from a
schema whose target namespace is the same as the including
schema's target namespace. Hence, the include
element does not
identify a namespace (although it does require a
schemaLocation
). The import mechanism that we
describe in this section is an important mechanism that
enables schema components from different target namespaces
to be used together, and hence enables the schema
validation of instance content defined across multiple
namespaces.
To import the type SKU
and use it in the report schema, we identify the namespace
in which SKU
is defined, and associate that
namespace with a prefix for use in the report schema.
Concretely, we use the
import
element to identify SKU
's
target namespace, http://www.example.com/IPO
,
and we associate the namespace with the prefix
xipo
using a standard namespace declaration. The
simple type SKU
, defined in the namespace
http://www.example.com/IPO
, may then be
referenced as xipo:SKU
in any of the report
schema's definitions and declarations.
In our example, we imported one simple type from one
external namespace, and used it for declaring attributes.
XML Schema in fact permits multiple schema components to be
imported, from multiple namespaces, and they can be
referred to in both definitions and declarations. For
example in
report.xsd
we could additionally reuse the
comment
element declared in ipo.xsd
by referencing that element
in a declaration:
<element ref="xipo:comment"/>
Note however, that we cannot reuse the shipTo
element from ipo.xsd
, and
the following is not legal because only global schema
components can be imported:
<element ref="xipo:shipTo"/>
In ipo.xsd
,
comment
is declared as a global element, in other
words it is declared as an element of the schema
. In contrast,
shipTo
is declared locally, in other words it is an
element declared inside a complex type definition,
specifically the PurchaseOrderType
type.
Complex types can also be imported, and they can be used
as the base types for deriving new types. Only named
complex types can be imported; local, anonymously defined
types cannot. Suppose we want to include in our reports the
name of an analyst, along with contact information. We can
reuse the (globally defined) complex type
USAddress
from
address.xsd
, and extend it to define a new type
called Analyst
in the report
schema by adding the new elements
phone
and email
:
<complexType name="Analyst"> <complexContent> <extension base="xipo:USAddress"> <sequence> <element name="phone" type="string"/> <element name="email" type="string"/> </sequence> </extension> </complexContent> </complexType>
Using this new type we declare an element called
analyst
as part of the purchaseReport
element declaration (declarations not shown) in the report
schema. Then, the following instance document would conform
to the modified report schema:
<r:purchaseReport xmlns:r="http://www.example.com/Report" period="P3M" periodEnding="1999-12-31"> <!-- regions and parts elements omitted --> <r:analyst> <name>Wendy Uhro</name> <street>10 Corporate Towers</street> <city>San Jose</city> <state>CA</state> <zip>95113</zip> <r:phone>408-271-3366</r:phone> <r:email>uhro@example.com</r:email> </r:analyst> </r:purchaseReport>
Note that the report now has both qualified and unqualified elements. This
is because some of the elements (name
, street
, city
,
state
and zip
) are locally declared in ipo.xsd
, whose
elementFormDefault
is unqualified
(by default). The other elements in the
example are declared in report.xsd
, whose elementFormDefault
is set to qualified
.
When schema components are imported from multiple
namespaces, each namespace must be identified with a
separate import
element. The
import
elements themselves must appear as the
first children of the
schema
element. Furthermore, each namespace must
be associated with a prefix, using a standard namespace
declaration, and that prefix is used to qualify references
to any schema components belonging to that namespace.
Finally, import
elements optionally contain a schemaLocation
attribute to help locate resources associated with the
namespaces. We discuss the schemaLocation
attribute in more detail in a later section.
As XML schemas become more widespread, schema authors will want to create simple and complex types that can be shared and used as building blocks for creating new schemas. XML Schemas already provides types that play this role, in particular, the types described in the Simple Types appendix and in an introductory type library.
Schema authors will undoubtedly want to create their own libraries of types to represent currency, units of measurement, business addresses, and so on. Each library might consist of a schema containing one or more definitions, for example, a schema containing a currency type:
<schema targetNamespace="http://www.example.com/Currency" xmlns:c="http://www.example.com/Currency" xmlns="http://www.w3.org/2001/XMLSchema"> <annotation> <documentation xml:lang="en"> Definition of Currency type based on ISO 4217 </documentation> </annotation> <complexType name="Currency"> <simpleContent> <extension base="decimal"> <attribute name="name"> <simpleType> <restriction base="string"> <enumeration value="AED"> <annotation> <documentation xml:lang="en"> United Arab Emirates: Dirham (1 Dirham = 100 Fils) </documentation> </annotation> </enumeration> <enumeration value="AFA"> <annotation> <documentation xml:lang="en"> Afghanistan: Afghani (1 Afghani = 100 Puls) </documentation> </annotation> </enumeration> <enumeration value="ALL"> <annotation> <documentation xml:lang="en"> Albania, Lek (1 Lek = 100 Qindarka) </documentation> </annotation> </enumeration> <!-- and other currencies --> </restriction> </simpleType> </attribute> </extension> </simpleContent> </complexType> </schema>
An example of an element appearing in an instance and having this type:
<convertFrom name="AFA">199.37</convertFrom>
Once we have defined the currency type, we can make it
available for re-use in other schemas through the import
mechanism just
described.
In previous sections we have seen several mechanisms for extending the content models of complex types. For example, a mixed content model can contain arbitrary character data in addition to elements, and for example, a content model can contain elements whose types are imported from external namespaces. However, these mechanisms provide very broad and very narrow controls respectively. The purpose of this section is to describe a flexible mechanism that enables content models to be extended by any elements and attributes belonging to specified namespaces.
To illustrate, consider a version of the quarterly report,
4Q99html.xml
, in
which we have embedded an XHTML representation of the XML
parts data. The XHTML content appears as the content of the
element htmlExample
, and the default
namespace is changed on the outermost XHTML element
(table
) so that all the XHTML elements belong
to the XHTML namespace,
http://www.w3.org/1999/xhtml
:
<purchaseReport xmlns="http://www.example.com/Report" period="P3M" periodEnding="1999-12-31"> <regions> <!-- part sales listed by zip code, data from 4Q99.xml --> </regions> <parts> <!-- part descriptions from 4Q99.xml --> </parts> <htmlExample> <table xmlns="http://www.w3.org/1999/xhtml" border="0" width="100%"> <tr> <th align="left">Zip Code</th> <th align="left">Part Number</th> <th align="left">Quantity</th> </tr> <tr><td>95819</td><td> </td><td> </td></tr> <tr><td> </td><td>872-AA</td><td>1</td></tr> <tr><td> </td><td>926-AA</td><td>1</td></tr> <tr><td> </td><td>833-AA</td><td>1</td></tr> <tr><td> </td><td>455-BX</td><td>1</td></tr> <tr><td>63143</td><td> </td><td> </td></tr> <tr><td> </td><td>455-BX</td><td>4</td></tr> </table> </htmlExample> </purchaseReport>
To permit the appearance of XHTML in
the instance document we modify the report schema by
declaring a new element htmlExample
whose
content is defined by the
any
element. In general, an any
element specifies that any
well-formed XML is permissible in a type's content model.
In the example, we require the XML to belong to the
namespace http://www.w3.org/1999/xhtml
, in
other words, it should be XHTML. The example also requires
there to be at least one element present from this
namespace, as indicated by the values of minOccurs
and maxOccurs
:
<element name="purchaseReport"> <complexType> <sequence> <element name="regions" type="r:RegionsType"/> <element name="parts" type="r:PartsType"/> <element name="htmlExample"> <complexType> <sequence> <any namespace="http://www.w3.org/1999/xhtml" minOccurs="1" maxOccurs="unbounded" processContents="skip"/> </sequence> </complexType> </element> </sequence> <attribute name="period" type="duration"/> <attribute name="periodEnding" type="date"/> </complexType> </element>
The modification permits some
well-formed XML belonging to the namespace
http://www.w3.org/1999/xhtml
to appear inside the
htmlExample
element. Therefore 4Q99html.xml
is permissible
because there is one element which (with its children) is
well-formed, the element appears inside the appropriate
element (htmlExample
), and the instance
document asserts that the element and its content belongs
to the required namespace. However, the XHTML may not
actually be valid because nothing in 4Q99html.xml
by itself can
provide that guarantee. If such a guarantee is required,
the value of the processContents
attribute should be set to strict
(the default value). In this case, an XML processor is
obliged to obtain the schema associated with the required
namespace, and validate the XHTML appearing within the
htmlExample
element.
In another example, we define a text
type which is
similar to the text type defined in XML Schema's introductory
type library (see also Type Libraries (§5.4.1)), and
is suitable for internationalized human-readable text. The
text type allows an unrestricted mixture of character content
and element content from any namespace, for example Ruby annotations, along with
an optional xml:lang
attribute. The lax
value of the
processContents
attribute instructs an XML processor to validate the element
content on a can-do basis: It will validate elements and attributes for
which it can obtain schema information, but it will not signal errors
for those it cannot obtain any schema information.
<xsd:complexType name="text"> <xsd:complexContent mixed="true"> <xsd:restriction base="xsd:anyType"> <xsd:sequence> <xsd:any processContents="lax" minOccurs="0" maxOccurs="unbounded"/> </xsd:sequence> <xsd:attribute ref="xml:lang"/> </xsd:restriction> </xsd:complexContent> </xsd:complexType>
Namespaces may be used to permit and
forbid element content in various ways depending upon the
value of the
namespace
attribute, as shown in Table 4:
Table 4. Namespace Attribute in Any | |
---|---|
Value of Namespace Attribute | Allowable Element Content |
##any | Any well-formed XML from any namespace (default) |
##local | Any well-formed XML that is not qualified, i.e. not declared to be in a namespace |
##other | Any well-formed XML that is from a namespace other than the target namespace of the type being defined (unqualified elements are not allowed) |
"http://www.w3.org/1999/xhtml ##targetNamespace" | Any well-formed XML belonging to any namespace in the (whitespace separated) list; ##targetNamespace is shorthand for the target namespace of the type being defined |
In addition to the any
element which enables element
content according to namespaces, there is a corresponding
anyAttribute
element which enables attributes to
appear in elements. For example, we can permit any XHTML
attribute to appear as part of the htmlExample
element by adding
anyAttribute
to its declaration:
<element name="htmlExample"> <complexType> <sequence> <any namespace="http://www.w3.org/1999/xhtml" minOccurs="1" maxOccurs="unbounded" processContents="skip"/> </sequence> <anyAttribute namespace="http://www.w3.org/1999/xhtml"/> </complexType> </element>
This declaration permits an XHTML attribute, say
href
, to appear in the htmlExample
element. For example:
.... <htmlExample xmlns:h="http://www.w3.org/1999/xhtml" h:href="http://www.example.com/reports/4Q99.html"> <!-- XHTML markup here --> </htmlExample> ....
The
namespace
attribute in an anyAttribute
element can
be set to any of the values listed in
Table 4 for the
any
element, and anyAttribute
can be
specified with a processContents
attribute. In contrast to an
any
element, anyAttribute
cannot
constrain the number of attributes that may appear in an
element.
XML Schema uses the schemaLocation
and
xsi:schemaLocation
attributes in three
circumstances.
1. In an instance document, the
attribute
xsi:schemaLocation
provides hints from the
author to a processor regarding the location of schema
documents. The author warrants that these schema documents
are relevant to checking the validity of the document
content, on a namespace by namespace basis. For example, we
can indicate the location of the Report schema to a
processor of the Quarterly Report:
<purchaseReport xmlns="http://www.example.com/Report" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://www.example.com/Report http://www.example.com/Report.xsd" period="P3M" periodEnding="1999-12-31"> <!-- etc. --> </purchaseReport>
The schemaLocation
attribute value consists of one or more pairs of URI references,
separated by white space. The first member of each pair is a namespace
name, and the second member of the pair is a hint describing where to
find an appropriate schema document for that namespace.
The presence of
these hints does not require the processor to obtain or use
the cited schema documents, and the processor is free to
use other schemas obtained by any suitable means, or to use
no schema at all.
A schema is not required to have a namespace (see Undeclared Target Namespaces (§3.4)) and so there is a
noNamespaceSchemaLocation
attribute which is
used to provide hints for the locations of schema documents
that do not have target namespaces.
2. In a schema, the
include
element has a required schemaLocation
attribute, and it contains a URI reference which must
identify a schema document. The effect is to compose a
final effective schema by merging the declarations and
definitions of the including and the included schemas. For
example, in Advanced Concepts II: The International Purchase Order (§4), the type
definitions of Address
,
USAddress
, UKAddress
,
USState
(along with their attribute and local
element declarations) from
address.xsd
were added to the element
declarations of purchaseOrder
and
comment
, and the type definitions of
PurchaseOrderType
, Items
and
SKU
(along with their attribute and local element
declarations) from
ipo.xsd
to create a single schema.
3. Also in a schema, the
import
element has optional namespace
and schemaLocation
attributes. If present, the schemaLocation
attribute is understood in a way which parallels the
interpretation of
xsi:schemaLocation
in (1). Specifically, it
provides a hint from the author to a processor regarding
the location of a schema document that the author warrants
supplies the required components for the namespace
identified by the
namespace
attribute. To import components that
are not in any target namespace, the import
element is used without
a namespace
attribute (and with or without a schemaLocation
attribute). References to components imported in this
manner are unqualified.
Note that the
schemaLocation
is only a hint and some
processors and applications will have reasons to not use
it. For example, an XHTML editor may have a built-in XHTML
schema.
An instance document may be processed against a schema to verify whether the rules specified in the schema are honored in the instance. Typically, such processing actually does two things, (1) it checks for conformance to the rules, a process called schema validation, and (2) it adds supplementary information that is not immediately present in the instance, such as types and default values, called infoset contributions.
The author of an instance document, such as a particular
purchase order, may claim, in the instance itself, that it
conforms to the rules in a particular schema. The author
does this using the schemaLocation
attribute discussed above. But regardless of whether a
schemaLocation
attribute is present, an
application is free to process the document against any
schema. For example, a purchasing application may have the
policy of always using a certain purchase order schema,
regardless of any schemaLocation
values.
Conformance checking can be thought of as proceeding in steps, first checking that the root element of the document instance has the right contents, then checking that each subelement conforms to its description in a schema, and so on until the entire document is verified. Processors are required to report what checking has been carried out.
To check an element for conformance, the processor first
locates the declaration for the element in a schema, and
then checks that the targetNamespace
attribute in the schema matches the actual namespace URI of
the element. Alternatively, it may determine that the schema does not
have a
targetNamespace
attribute and the instance
element is not namespace-qualified.
Supposing the namespaces match, the processor then
examines the type of the element, either as given by the
declaration in the schema, or by an xsi:type
attribute in the
instance. If the latter, the instance type must be an
allowed substitution for the type given in the schema; what
is allowed is controlled by the block
attribute in the
element declaration. At this same time, default values and
other infoset contributions are applied.
Next the processor checks the immediate attributes and
contents of the element, comparing these against the
attributes and contents permitted by the element's type.
For example, considering a shipTo
element such
as the one in The Purchase Order Schema (§2.1), the
processor checks what is permitted for an
Address
, because that is the shipTo
element's type.
If the element has a simple type, the processor verifies that the element has no attributes or contained elements, and that its character content matches the rules for the simple type. This sometimes involves checking the character sequence against regular expressions or enumerations, and sometimes it involves checking that the character sequence represents a value in a permitted range.
If the element has a complex type, then the processor checks that any required attributes are present and that their values conform to the requirements of their simple types. It also checks that all required subelements are present, and that the sequence of subelements (and any mixed text) matches the content model declared for the complex type. Regarding subelements, schemas can either require exact name matching, permit substitution by an equivalent element or permit substitution by any element allowed by an 'any' particle.
Unless a schema indicates otherwise (as it can for 'any' particles) conformance checking then proceeds one level more deeply by looking at each subelement in turn, repeating the process described above.
Many people have contributed ideas, material and feedback that has improved this document. In particular, the editor acknowledges contributions from David Beech, Paul Biron, Don Box, Allen Brown, David Cleary, Dan Connolly, Roger Costello, Martin Dürst, Martin Gudgin, Dave Hollander, Joe Kesselman, John McCarthy, Andrew Layman, Eve Maler, Ashok Malhotra, Noah Mendelsohn, Michael Sperberg-McQueen, Henry Thompson, Misha Wolf, and Priscilla Walmsley for validating the examples.
The legal values for each simple type can be constrained through the application of one or more facets. Tables B1.a and B1.b list all of XML Schema's built-in simple types and the facets applicable to each type. The names of the simple types and the facets are linked from the tables to the corresponding descriptions in XML Schema Part 2: Datatypes.
Table B1.a. Simple Types & Applicable Facets | ||||||
---|---|---|---|---|---|---|
Simple Types | Facets | |||||
length | minLength | maxLength | pattern | enumeration | whiteSpace | |
string | y | y | y | y | y | y |
normalizedString | y | y | y | y | y | y |
token | y | y | y | y | y | see (1) |
base64Binary | y | y | y | y | y | see (1) |
hexBinary | y | y | y | y | y | see (1) |
integer | y | y | see (1) | |||
positiveInteger | y | y | see (1) | |||
negativeInteger | y | y | see (1) | |||
nonNegativeInteger | y | y | see (1) | |||
nonPositiveInteger | y | y | see (1) | |||
long | y | y | see (1) | |||
unsignedLong | y | y | see (1) | |||
int | y | y | see (1) | |||
unsignedInt | y | y | see (1) | |||
short | y | y | see (1) | |||
unsignedShort | y | y | see (1) | |||
byte | y | y | see (1) | |||
unsignedByte | y | y | see (1) | |||
decimal | y | y | see (1) | |||
float | y | y | see (1) | |||
double | y | y | see (1) | |||
boolean | y | see (1) | ||||
duration | y | y | see (1) | |||
dateTime | y | y | see (1) | |||
date | y | y | see (1) | |||
time | y | y | see (1) | |||
gYear | y | y | see (1) | |||
gYearMonth | y | y | see (1) | |||
gMonth | y | y | see (1) | |||
gMonthDay | y | y | see (1) | |||
gDay | y | y | see (1) | |||
Name | y | y | y | y | y | see (1) |
QName | y | y | y | y | y | see (1) |
NCName | y | y | y | y | y | see (1) |
anyURI | y | y | y | y | y | see (1) |
language | y | y | y | y | y | see (1) |
ID | y | y | y | y | y | see (1) |
IDREF | y | y | y | y | y | see (1) |
IDREFS | y | y | y | y | y | see (1) |
ENTITY | y | y | y | y | y | see (1) |
ENTITIES | y | y | y | y | y | see (1) |
NOTATION | y | y | y | y | y | see (1) |
NMTOKEN | y | y | y | y | y | see (1) |
NMTOKENS | y | y | y | y | y | see (1) |
Note: (1) Although the whiteSpace facet is
applicable to this type, the only value that can be specified is collapse . |
The facets listed in Table B1.b apply only to simple types which are ordered. Not all simple types are ordered and so B1.b does not list all of the simple types.
Table B1.b. Simple Types & Applicable Facets | ||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Simple Types | Facets | |||||||||||||||||
|
|
|
|
|
| |||||||||||||
integer | y | y | y | y | y | see (1) | ||||||||||||
positiveInteger | y | y | y | y | y | see (1) | ||||||||||||
negativeInteger | y | y | y | y | y | see (1) | ||||||||||||
nonNegativeInteger | y | y | y | y | y | see (1) | ||||||||||||
nonPositiveInteger | y | y | y | y | y | see (1) | ||||||||||||
long | y | y | y | y | y | see (1) | ||||||||||||
unsignedLong | y | y | y | y | y | see (1) | ||||||||||||
int | y | y | y | y | y | see (1) | ||||||||||||
unsignedInt | y | y | y | y | y | see (1) | ||||||||||||
short | y | y | y | y | y | see (1) | ||||||||||||
unsignedShort | y | y | y | y | y | see (1) | ||||||||||||
byte | y | y | y | y | y | see (1) | ||||||||||||
unsignedByte | y | y | y | y | y | see (1) | ||||||||||||
decimal | y | y | y | y | y | y | ||||||||||||
float | y | y | y | y | ||||||||||||||
double | y | y | y | y | ||||||||||||||
duration | y | y | y | y | ||||||||||||||
dateTime | y | y | y | y | ||||||||||||||
date | y | y | y | y | ||||||||||||||
time | y | y | y | y | ||||||||||||||
gYear | y | y | y | y | ||||||||||||||
gYearMonth | y | y | y | y | ||||||||||||||
gMonth | y | y | y | y | ||||||||||||||
gMonthDay | y | y | y | y | ||||||||||||||
gDay | y | y | y | y | ||||||||||||||
Note: (1) Although the fractionDigits facet is
applicable to this type, the only value that can be specified is zero. |
XML 1.0 provides various types of entities which are named fragments of content that can be used in the construction of both DTD's (parameter entities) and instance documents. In Building Content Models (§2.7), we noted how named groups mimic parameter entities. In this section we show how entities can be declared in instance documents, and how the functional equivalents of entities can be declared in schemas.
Suppose we want to declare and use an entity in an instance document, and that document is also constrained by a schema. For example:
<?xml version="1.0" ?> <!DOCTYPE purchaseOrder [ <!ENTITY eacute "é"> ]> <purchaseOrder xmlns="http://www.example.com/PO1" orderDate="1999-10-20"> <!-- etc. --> <city>Montréal</city> <!-- etc. --> </purchaseOrder>
Here, we declare an entity called eacute
as
part of an internal (DTD) subset, and we reference this
entity in the content of the city
element.
Note that when this instance document is processed, the
entity will be
resolved
before schema validation takes
place. In other words, a schema processor will determine
the validity of the city
element using
Montréal
as the element's value.
We can achieve a similar but not identical outcome by declaring an element in a schema, and by setting the element's content appropriately:
<xsd:element name="eacute" type="xsd:token" fixed="é"/>
And this element can be used in an instance document:
<?xml version="1.0" ?> <purchaseOrder xmlns="http://www.example.com/PO1" xmlns:c="http://www.example.com/characterElements" orderDate="1999-10-20"> <!-- etc. --> <city>Montr<c:eacute/>al</city> <!-- etc. --> </purchaseOrder>
In this case, a schema processor will process two
elements, a city
element, and an
eacute
element for the contents of which the
processor will supply the single character
é
. Note that the extra element will
complicate string matching; the two forms of the name
"Montréal" given in the two examples above will not
match each other using normal string-comparison techniques.
XML Schema's
pattern
facet uses a regular expression language
that supports
Unicode. It is fully described in
XML Schema Part 2. The language is similar to the
regular expression language used in the Perl
Programming language, although expressions are matched
against entire lexical representations rather than
user-scoped lexical representations such as line and
paragraph. For this reason, the expression language does
not contain the metacharacters ^ and $, although ^ is used
to express exception, e.g. [^0-9]x.
Table D1. Examples of Regular Expressions | |
---|---|
Expression | Match(es) |
Chapter \d | Chapter 0, Chapter 1, Chapter 2 .... |
Chapter\s\d | Chapter followed by a single whitespace character (space, tab, newline, etc.), followed by a single digit |
Chapter\s\w | Chapter followed by a single whitespace character (space, tab, newline, etc.), followed by a word character (XML 1.0 Letter or Digit) |
Española | Española |
\p{Lu} | any uppercase character, the value of \p{} (e.g. "Lu") is defined by Unicode |
\p{IsGreek} | any Greek character, the 'Is' construction may be applied to any block name (e.g. "Greek") as defined by Unicode |
\P{IsGreek} | any non-Greek character, the 'Is' construction may be applied to any block name (e.g. "Greek") as defined by Unicode |
a*x | x, ax, aax, aaax .... |
a?x | ax, x |
a+x | ax, aax, aaax .... |
(a|b)+x | ax, bx, aax, abx, bax, bbx, aaax, aabx, abax, abbx, baax, babx, bbax, bbbx, aaaax .... |
[abcde]x | ax, bx, cx, dx, ex |
[a-e]x | ax, bx, cx, dx, ex |
[\-ae]x | -x, ax, ex |
[ae\-]x | ax, ex, -x |
[^0-9]x | any non-digit character followed by the character x |
\Dx | any non-digit character followed by the character x |
.x | any character followed by the character x |
.*abc.* | 1x2abc, abc1x2, z3456abchooray .... |
ab{2}x | abbx |
ab{2,4}x | abbx, abbbx, abbbbx |
ab{2,}x | abbx, abbbx, abbbbx .... |
(ab){2}x | ababx |
Each element name is followed by one or more links to examples (identified by section number) in the Primer , plus a link to a formal XML description in either the Structures or Datatypes parts of the XML Schema specification.
Each attribute name is followed by one or more pairs of references. Each pair of references consists of a link to an example in the Primer, plus a link to a formal XML description in either the Structures or Datatypes parts of the XML Schema specification.
XML Schema's simple types are described in Table 2.