Adapts a Java type for custom marshaling.

Usage:

Some Java types do not map naturally to an XML representation, for example HashMap or other non JavaBean classes. Conversely, an XML representation may map to a Java type but an application may choose to access the XML representation using another Java type. For example, the schema to Java binding rules bind xs:DateTime by default to XmlGregorianCalendar. But an application may desire to bind xs:DateTime to a custom type, MyXmlGregorianCalendar, for example. In both cases, there is a mismatch between bound type , used by an application to access XML content and the value type, that is mapped to an XML representation.

This abstract class defines methods for adapting a bound type to a value type or vice versa. The methods are invoked by the JAXB binding framework during marshaling and unmarshalling:

• XmlAdapter.marshal(...): During marshalling, JAXB binding framework invokes XmlAdapter.marshal(..) to adapt a bound type to value type, which is then marshaled to XML representation.
• XmlAdapter.unmarshal(...): During unmarshalling, JAXB binding framework first unmarshals XML representation to a value type and then invokes XmlAdapter.unmarshal(..) to adapt the value type to a bound type.

Writing an adapter therefore involves the following steps:

• Write an adapter that implements this abstract class.
• Install the adapter using the annotation XmlJavaTypeAdapter

Example: Customized mapping of HashMap

The following example illustrates the use of @XmlAdapter and @XmlJavaTypeAdapter to customize the mapping of a HashMap.

Step 1: Determine the desired XML representation for HashMap.

<hashmap>
        <entry key="id123">this is a value</entry>
        <entry key="id312">this is another value</entry>
        ...
    </hashmap>

Step 2: Determine the schema definition that the desired XML representation shown above should follow.

<xs:complexType name="myHashMapType">
      <xs:sequence>
        <xs:element name="entry" type="myHashMapEntryType"
                       minOccurs = "0" maxOccurs="unbounded"/>
      </xs:sequence>
    </xs:complexType>

    <xs:complexType name="myHashMapEntryType">
      <xs:simpleContent>
        <xs:extension base="xs:string">
          <xs:attribute name="key" type="xs:int"/>
        </xs:extension>
      </xs:simpleContent>
    </xs:complexType>

Step 3: Write value types that can generate the above schema definition.

    public class MyHashMapType {
        List<MyHashMapEntryType> entry;
    }

    public class MyHashMapEntryType {
        @XmlAttribute
        public Integer key; 

        @XmlValue
        public String value;
    }

Step 4: Write the adapter that adapts the value type, MyHashMapType to a bound type, HashMap, used by the application.

public final class MyHashMapAdapter extends
                       XmlAdapter<MyHashMapType,HashMap> { ... }

Step 5: Use the adapter.

    public class Foo {
        @XmlJavaTypeAdapter(MyHashMapAdapter.class)
        HashMap hashmap;
        ...
    }

The above code fragment will map to the following schema:

<xs:complexType name="Foo">
      <xs:sequence>
        <xs:element name="hashmap" type="myHashMapType">
      </xs:sequence>
    </xs:complexType>