Maps a JavaBean property to an XML element derived from property's type.
Usage
@XmlElementRef
annotation can be used with a
JavaBean property or from within XmlElementRefs
This annotation dynamically associates an XML element name with the JavaBean
property. When a JavaBean property is annotated with XmlElement
, the XML element name is statically derived from the
JavaBean property name. However, when this annotation is used, the
XML element name is derived from the instance of the type of the
JavaBean property at runtime.
XML Schema substitution group support
XML Schema allows an XML document author to use XML element names that were not statically specified in the content model of a schema using substitution groups. Schema derived code provides support for substitution groups using an element property, (section 5.5.5, "Element Property" of Jakarta XML Binding specification). An element property method signature is of the form: public void setTerm(JAXBElement<? extends Operator>);
public JAXBElement<? extends Operator> getTerm();
An element factory method annotated with XmlElementDecl
is
used to create a JAXBElement
instance, containing an XML
element name. The presence of @XmlElementRef
annotation on an
element property indicates that the element name from JAXBElement
instance be used instead of deriving an XML element name from the
JavaBean property name.
The usage is subject to the following constraints:
- If the collection item type (for collection property) or
property type (for single valued property) is
JAXBElement
, then@XmlElementRef.name()
and@XmlElementRef.namespace()
must point an element factory method with an @XmlElementDecl annotation in a class annotated with @XmlRegistry (usually ObjectFactory class generated by the schema compiler) :- @XmlElementDecl.name() must equal @XmlElementRef.name()
- @XmlElementDecl.namespace() must equal @XmlElementRef.namespace().
- If the collection item type (for collection property) or
property type (for single valued property) is not
JAXBElement
, then the type referenced by the property or field must be annotated withXmlRootElement
. - This annotation can be used with the following annotations:
XmlElementWrapper
,XmlJavaTypeAdapter
.
See "Package Specification" in jakarta.xml.bind.package javadoc for additional common information.
Example 1: Ant Task Example
The following Java class hierarchy models an Ant build script. An Ant task corresponds to a class in the class hierarchy. The XML element name of an Ant task is indicated by the XmlRootElement annotation on its corresponding class. @XmlRootElement(name="target")
class Target {
// The presence of @XmlElementRef indicates that the XML
// element name will be derived from the @XmlRootElement
// annotation on the type (for e.g. "jar" for JarTask).
@XmlElementRef
List<Task> tasks;
}
abstract class Task {
}
@XmlRootElement(name="jar")
class JarTask extends Task {
...
}
@XmlRootElement(name="javac")
class JavacTask extends Task {
...
}
<!-- XML Schema fragment -->
<xs:element name="target" type="Target">
<xs:complexType name="Target">
<xs:sequence>
<xs:choice maxOccurs="unbounded">
<xs:element ref="jar"/>
<xs:element ref="javac"/>
</xs:choice>
</xs:sequence>
</xs:complexType>
</xs:element>
Thus the following code fragment:
Target target = new Target();
target.tasks.add(new JarTask());
target.tasks.add(new JavacTask());
marshal(target);
<target>
<jar>
....
</jar>
<javac>
....
</javac>
</target>
It is not an error to have a class that extends Task
that doesn't have XmlRootElement
. But they can't show up in an
XML instance (because they don't have XML element names).
Example 2: XML Schema Substitution group support
The following example shows the annotations for XML Schema substitution groups. The annotations and the ObjectFactory are derived from the schema.
@XmlElement
class Math {
// The value of type()
is
// JAXBElement.class , which indicates the XML
// element name ObjectFactory - in general a class marked
// with @XmlRegistry. (See ObjectFactory below)
//
// The name()
is "operator", a pointer to a
// factory method annotated with a
// XmlElementDecl
with the name "operator". Since
// "operator" is the head of a substitution group that
// contains elements "add" and "sub" elements, "operator"
// element can be substituted in an instance document by
// elements "add" or "sub". At runtime, JAXBElement
// instance contains the element name that has been
// substituted in the XML document.
//
@XmlElementRef(type=JAXBElement.class,name="operator")
JAXBElement<? extends Operator> term;
}
@XmlRegistry
class ObjectFactory {
@XmlElementDecl(name="operator")
JAXBElement<Operator> createOperator(Operator o) {...}
@XmlElementDecl(name="add",substitutionHeadName="operator")
JAXBElement<Operator> createAdd(Operator o) {...}
@XmlElementDecl(name="sub",substitutionHeadName="operator")
JAXBElement<Operator> createSub(Operator o) {...}
}
class Operator {
...
}
Thus, the following code fragment
Math m = new Math();
m.term = new ObjectFactory().createAdd(new Operator());
marshal(m);
<math>
<add>...</add>
</math>
- Author:
- Kohsuke Kawaguchi, Sun Microsystems,Inc.
- Sekhar Vajjhala, Sun Microsystems, Inc.
- See Also:
-
Nested Class Summary
Modifier and TypeClassDescriptionstatic final class
Used intype()
to signal that the type be inferred from the signature of the property. -
Optional Element Summary
-
Element Details
-
type
Class<?> typeThe Java type being referenced.If the value is DEFAULT.class, the type is inferred from the type of the JavaBean property.
- Default:
jakarta.xml.bind.annotation.XmlElementRef.DEFAULT.class
-
namespace
String namespaceThis parameter andname()
are used to determine the XML element for the JavaBean property.If
type()
isJAXBElement.class
, thennamespace()
andname()
point to a factory method withXmlElementDecl
. The XML element name is the element name from the factory method'sXmlElementDecl
annotation or if an element from its substitution group (of which it is a head element) has been substituted in the XML document, then the element name is from theXmlElementDecl
on the substituted element.If
type()
is notJAXBElement.class
, then the XML element name is the XML element name statically associated with the type using the annotationXmlRootElement
on the type. If the type is not annotated with anXmlElementDecl
, then it is an error.If
type()
is notJAXBElement.class
, then this value must be "".- Default:
""
-
name
String name- See Also:
- Default:
"##default"
-
required
boolean requiredCustomize the element declaration to be required.If required() is true, then Javabean property is mapped to an XML schema element declaration with minOccurs="1". maxOccurs is "1" for a single valued property and "unbounded" for a multivalued property.
If required() is false, then the Javabean property is mapped to XML Schema element declaration with minOccurs="0". maxOccurs is "1" for a single valued property and "unbounded" for a multivalued property.
For compatibility with Jakarta XML Binding, this property defaults to
true
, despite the fact thatXmlElement.required()
defaults to false.- Default:
true
-