Using an XmlAdapter

Some Java classes may not be well suited for use with JAXB and at first glance may seem "unmappable" – for example, classes that do not have a default no-arg constructor, or classes for which an XML representation cannot be automatically determined. Using JAXB's XmlAdapter, you can define custom code to convert the unmappable class into something that JAXB can handle. Then, you can use the @XmlJavaTypeAdapter annotation to indicate that your adapter should be used when working with the unmappable class.

XmlAdapter uses the following terminology:

ValueType – The type that JAXB knows how to handle out of the box.
BoundType – The type that JAXB doesn't know how to handle. An adapter is written to allow this type to be used as an in-memory representation through the ValueType.

The outline of an XmlAdapter class is as follows:

Example 8-32 XmlAdapter Class Outline

public class AdapterName extends XmlAdapter<ValueType, BoundType> {
 
   public BoundType unmarshal(ValueType value) throws Exception {
      ...
   }
 
   public ValueType marshal(BoundType value) throws Exception {
      ...
   }
 
}

Using java.util.Currency

Our first example will use the following domain class:

Example 8-33 Sample Domain Class

package example;
 
import java.util.Currency;
 
import javax.xml.bind.annotation.*;
 
@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class PurchaseOrder {
 
   private Double amount;
 
   private Currency currency;
 
   ...
}

Here, the Currency cannot be automatically mapped with JAXB because it does not contain a no-argument constructor. However, we can write an adapter that will convert the Currency into something that JAXB does know how to handle – a simple String. Luckily, in this case the Currency's toString() method returns the currency code, which can also be used to create a new Currency:

Example 8-34 Using an Adapter

package example;
 
import java.util.Currency;
 
import javax.xml.bind.annotation.adapters.XmlAdapter;
 
public class CurrencyAdapter extends XmlAdapter<String, Currency> {
 
   /*
    * Java => XML
    * Given the unmappable Java object, return the desired XML representation.
    */
   public String marshal(Currency val) throws Exception {
      return val.toString();
   }
 
   /*
    * XML => Java
    * Given an XML string, use it to build an instance of the unmappable class.
    */
   public Currency unmarshal(String val) throws Exception {
      return Currency.getInstance(val);
   }
 
}

To indicate that our adapter should be used for the Currency property, we annotate it with @XmlJavaTypeAdapter and provide the class name of our adapter:

Example 8-35 Using the @XmlJavaTypeAdapter Annotation

package example;
 
import java.util.Currency;
 
import javax.xml.bind.annotation.*;
 
@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class PurchaseOrder {
 
   private Double amount;
 
   @XmlJavaTypeAdapter(CurrencyAdapter.class)
   private Currency currency;
 
   ...
}

Using java.awt.Point

Sometimes the best way to handle an unmappable class is to write a "stand-in" class which can be mapped with JAXB, and convert between the two classes in the XmlAdapter. In this example, we want to use the Point class. Because of that class' getLocation() method (which JAXB will pickup automatically and map), an infinite loop will occur during marshalling. Because we cannot change the Point class, we will write a new class, MyPoint, and use it in the adapter.

Example 8-36 Using java.awt.Point

package example;
 
public class MyPoint {
 
   private int x, y;
 
   public MyPoint() {
      this(0, 0);
   }
 
   public MyPoint(int x, int y) {
      this.x = x;
      this.y = y;
   }
 
   public int getX() {
      return x;
   }
 
   ...
}
 
package example;
 
import java.awt.Point;
 
import javax.xml.bind.annotation.adapters.XmlAdapter;
 
public class MyPointAdapter extends XmlAdapter<MyPoint, Point> {
 
   /*
    * Java => XML
    */
   public MyPoint marshal(Point val) throws Exception {
      return new MyPoint((int) val.getX(), (int) val.getY());
   }
 
   /*
    * XML => Java
    */
   public Point unmarshal(MyPoint val) throws Exception {
      return new Point(val.getX(), val.getY());
   }
 
}

Finally, our Point properties are marked with @XmlJavaTypeAdapter:

Example 8-37 Using the @XmlJavaTypeAdapter Annotation

package example;
 
import java.awt.Point;
 
import javax.xml.bind.annotation.*;
 
@XmlRootElement
@XmlAccessorType(XmlAccessType.FIELD)
public class Zone {
 
   private String name;
 
   @XmlJavaTypeAdapter(MyPointAdapter.class)
   private Point startCoord;
 
   @XmlJavaTypeAdapter(MyPointAdapter.class)
   private Point endCoord;
 
   ...
}

Specifying Package-Level Adapters

In Example 8-37, we annotated both Point properties with the @XmlJavaTypeAdapter annotation. If you have many of these types of properties – for example, in other domain classes – it can be more convenient to specify the @XmlJavaTypeAdapters at the package level.

We could define both of the adapter classes in package-info.java, and would no longer have to annotate any further Currency or Point properties:

@XmlJavaTypeAdapters({
   @XmlJavaTypeAdapter(value=CurrencyAdapter.class,type=Currency.class),
   @XmlJavaTypeAdapter(value=MyPointAdapter.class,type=Point.class)
})
package example;

Specifying Class-Level @XmlJavaTypeAdapters

If you have a Java class and you would like to always use an XmlAdapter during marshalling and unmarshalling, then you can specify the @XmlJavaTypeAdapter directly at the class level:

package example;
 
import javax.xml.bind.annotation.adapters.XmlJavaTypeAdapter;
 
@XmlJavaTypeAdapter(DataStructureAdapter.class)
public class DataStructure {
 
   ...
 
}

Now, any object that has a DataStructure property will automatically use the DataStructureAdapter, without the need for an annotation on the property itself.