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Showing content from https://docs.oracle.com/javase/7/docs/api/javax/xml/bind/annotation/adapters/XmlAdapter.html below:

XmlAdapter (Java Platform SE 7 )

Adapts a Java type for custom marshaling.

Usage:

Some Java types do not map naturally to a XML representation, for example HashMap or other non JavaBean classes. Conversely, a XML repsentation may map to a Java type but an application may choose to accesss 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>
 

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