XmlAdapter (Java Platform SE 8 ) (original) (raw)

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>