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Difference between revisions of "EclipseLink/UserGuide/MOXy/Mapping the Unmappable/Converters"

Line 22: Line 22:
  
 
import java.util.Currency;
 
import java.util.Currency;
 +
 +
import javax.xml.bind.annotation.*;
  
 
@XmlRootElement
 
@XmlRootElement
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<source lang="java">
 
<source lang="java">
 
package example;
 
package example;
 +
 +
import java.util.Currency;
 +
 +
import javax.xml.bind.annotation.adapters.XmlAdapter;
  
 
public class CurrencyAdapter extends XmlAdapter<String, Currency> {
 
public class CurrencyAdapter extends XmlAdapter<String, Currency> {
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import java.util.Currency;
 
import java.util.Currency;
 +
 +
import javax.xml.bind.annotation.*;
  
 
@XmlRootElement
 
@XmlRootElement
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<source lang="java">
 
<source lang="java">
 
package example;
 
package example;
 +
 +
import java.awt.Point;
 +
 +
import javax.xml.bind.annotation.adapters.XmlAdapter;
  
 
public class MyPointAdapter extends XmlAdapter<MyPoint, Point> {
 
public class MyPointAdapter extends XmlAdapter<MyPoint, Point> {
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import java.awt.Point;
 
import java.awt.Point;
 +
 +
import javax.xml.bind.annotation.*;
  
 
@XmlRootElement
 
@XmlRootElement

Revision as of 13:56, 7 June 2011

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XmlAdapter

Some Java classes are not 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 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.


Example - java.util.Currency

Our first example will use the following 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:

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:

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;
 
   ...
}


Example - 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 occurr during marshalling. Because we cannot change the Point class, we will write a new class, MyPoint, and use it in the adapter.

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:

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;
 
   ...
}