XmlAdapter (Java EE 5)

java.lang.Object
  javax.xml.bind.annotation.adapters.XmlAdapter<ValueType,BoundType>

修改 Java 类型以适应自定义编组。

使用：

一些 Java 类型不能自然映射到 XML 表示形式，例如，HashMap 或其他非 JavaBean 类。反之，XML 表示形式可以映射到 Java 类型，但是应用程序可能会选择使用另一种 Java 类型访问 XML 表示形式。例如，默认情况下，Java 绑定规则模式将 xs:DateTime 绑定到 XmlGregorianCalendar。但应用程序可能需要将 xs:DateTime 绑定到自定义类型，例如 MyXmlGregorianCalendar。在这两种情况下，应用程序用来访问 XML 内容的 bound 类型 与映射到 XML 表示形式的 value 类型 不匹配。

此抽象类定义将 bound 类型修改为 value 类型或将 value 类型修改为 bound 类型的方法。在编组或解组过程中，由 JAXB 绑定框架调用这些方法：

• XmlAdapter.marshal(...)：编组过程中，JAXB 绑定框架调用 XmlAdapter.marshal(..) 将 bound 类型修改为 value 类型，然后将 value 类型编组为 XML 表示形式。
• XmlAdapter.unmarshal(...)：解组过程中，JAXB 绑定框架首先将 XML 表示形式解组为 value 类型，然后调用 XmlAdapter.unmarshal(..) 将 value 类型修改为 bound 类型。

因此编写一个适配器涉及以下步骤：

• 编写实现此抽象类的适配器。
• 安装使用注释 XmlJavaTypeAdapter 的适配器。

示例：自定义 HashMap 的映射。

以下示例演示了如何使用 @XmlAdapter 和 @XmlJavaTypeAdapter 自定义 HashMap 的映射。

步骤 1：确定 HashMap 需要的 XML 表示形式。

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

步骤 2：确定上文所示的所需 XML 表示形式应当遵循的模式定义。

     
<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>

 

步骤 3：编写能够生成上述模式定义的 value 类型。

     public class MyHashMapType {
         List<MyHashMapEntryType> entry;
     }

     public class MyHashMapEntryType {
         @XmlAttribute
         public Integer key; 

         @XmlValue
         public String value;
     }
 

步骤 4：编写将 value 类型 (MyHashMapType) 修改为 bound 类型（应用程序使用的 HashMap）的适配器。

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

步骤 5：使用适配器。

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

上述代码片段将映射到以下模式：

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

BoundType	JAXB 不知道如何处理的一些类型。编写一个适配器，以便允许通过 ValueType 将此类型用作内存表示形式。
ValueType	JAXB 无需其他操作便知道如何处理的类型。
since	JAXB 2.0
See also	javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter

英文文档：

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<HashMap, MyHashMapType> { ... }
      
 

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>
 

Since:

JAXB 2.0

Author:

• Sekhar Vajjhala, Sun Microsystems Inc.
• Kohsuke Kawaguchi, Sun Microsystems Inc.

See Also:

XmlJavaTypeAdapter

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Constructor Summary
protected	XmlAdapter()           Do-nothing constructor for the derived classes.

 

Method Summary
abstract  ValueType	marshal(BoundType v)           Convert a bound type to a value type.
abstract  BoundType	unmarshal(ValueType v)           Convert a value type to a bound type.

 

Methods inherited from class java.lang.Object
clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

 

Constructor Detail

protected XmlAdapter()
派生类的构造方法，不执行任何操作。

英文文档：

XmlAdapter

protected XmlAdapter()

Do-nothing constructor for the derived classes.

Method Detail

英文文档：

unmarshal

public abstract BoundType unmarshal(ValueType v)
                             throws Exception

Convert a value type to a bound type.

Parameters:

v - The value to be converted. Can be null.

Throws:

Exception - if there's an error during the conversion. The caller is responsible for reporting the error to the user through ValidationEventHandler.

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英文文档：

marshal

public abstract ValueType marshal(BoundType v)
                           throws Exception

Convert a bound type to a value type.

Parameters:

v - The value to be convereted. Can be null.

Throws:

Exception - if there's an error during the conversion. The caller is responsible for reporting the error to the user through ValidationEventHandler.

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