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A schema describes data types, with the intent of defining one or more hierarchical types defining XML documents. Type information deals with the definition of structure, i.e., the composition of document nodes, and the classification of scalar (or list) values for content and attributes. The XML Schema Language offers a rich set of features to achieve the required structuring and typing. A good introduction to the XML Schema language is the XML Schema Part 0: Primer, provided by the World Wide Web Consortium (W3C).
Although all data in XML is text, it should generally not be
defined as String in the Java classes derived from a
schema. The XML Schema language provides the elementary data types for
numbers, booleans, strings, URIs, dates and times, and for references
and other XML constructs. Only by using these features will you receive
the full benefit of JAXB, which will not only take care of all the
necessary conversions to or from the textual representation in XML but
also with the transformation of XML structures to Java data patterns,
such as lists or maps.
User-defined data types can be derived from any elementary type by adding one or more restrictions. This is done by adding so-called facets, which are available to set lower or upper bounds for values or string lengths, to limit the precision, to enumerate all legal values, and to define a pattern for a string type. JAXB uses enumerations restricting strings to define an enum type. Other facets, however, are ignored by the Schmema Compiler. (Section Validation explains how to enable facet checking.)
Data structuring concepts are expressed by using the complex type construct
of the schema language. An XML element of some complex type may have
child elements or attributes or both. For child elements, the schema's
element grouping facilites <xsd:all>,
<xsd:union>, <xsd:choice> and
<xsd:sequence> in combination with repetition limit
attributes let you define XML structures that are the equivalent of the
classic concepts of array, list, structure (or record) and union.
We'll discuss schema structuring and typing concepts in more detail in the next section, where examples of XML Schema language constructs are related to the Java code that the JAXB compiler uses for their representation.
Please notice that the tags of schema elements are presented in
the qualified form, with xsd as the namespace identifier.
This (or some other name, e.g., xs) identifier must be bound
to the URI identifying the XML Schema language. If used, also the
jxb namespace prefix must be bound. Both is done in the
schema element of the XML schema:
<?xml version="1.0" encoding="UTF-8"?>
<xsd:schema xmlns:xsd="http://www.w3.org/2001/XMLSchema"
xmlns:jxb="http://java.sun.com/xml/ns/jaxb"
jxb:version="2.0">
...
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