EclipseLink/UserGuide/MOXy/Mapping the Unmappable/Converters

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XmlAdapter

JAXB's XmlAdapter can be used to map classes that would normally be considered "unmappable" - such as classes that do not have a default no-arg constructor, or classes for which an XML representation cannot be automatically determined. In a custom subclass of 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

Our first example will use the following domain class:

package example;
 
import java.util.Currency;
 
@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;
 
public class CurrencyAdapter extends XmlAdapter<String, Currency> {
 
   /*
    * Object => XML
    * Given the unmappable Java object, return the desired XML representation.
    */
   public String marshal(Currency val) throws Exception {
      return val.toString();
   }
 
   /*
    * XML => Object
    * 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);
   }
 
}