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

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{{EclipseLink_UserGuide
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'''[[Image:Elug_draft_icon.png|Warning]] This page is obsolete. Please see ''[http://www.eclipse.org/eclipselink/documentation/ Developing JAXB Applications Using EclipseLink MOXy]'' for current information.'''
|info=y
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|toc=y
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|eclipselink=y
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|eclipselinktype=MOXy
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|api=y
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|apis= * [http://www.eclipse.org/eclipselink/api/latest/javax/xml/bind/annotation/adapters/XmlAdapter.html XmlAdapter]
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* [http://www.eclipse.org/eclipselink/api/latest/javax/xml/bind/annotation/adapters/XmlJavaTypeAdapter.html XmlJavaTypeAdapter]
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}}
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= XmlAdapter =
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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.
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== Example - java.util.Currency ==
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Our first example will use the following domain class:
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<source lang="java">
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package example;
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import java.util.Currency;
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@XmlRootElement
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@XmlAccessorType(XmlAccessType.FIELD)
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public class PurchaseOrder {
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  private Double amount;
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  private Currency currency;
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  ...
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}
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</source>
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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''':
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<source lang="java">
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package example;
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public class CurrencyAdapter extends XmlAdapter<String, Currency> {
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  /*
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    * Java => XML
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    * Given the unmappable Java object, return the desired XML representation.
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    */
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  public String marshal(Currency val) throws Exception {
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      return val.toString();
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  }
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  /*
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    * XML => Java
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    * Given an XML string, use it to build an instance of the unmappable class.
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    */
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  public Currency unmarshal(String val) throws Exception {
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      return Currency.getInstance(val);
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  }
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}
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</source>
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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:
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<source lang="java">
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package example;
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import java.util.Currency;
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@XmlRootElement
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@XmlAccessorType(XmlAccessType.FIELD)
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public class PurchaseOrder {
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  private Double amount;
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  @XmlJavaTypeAdapter(CurrencyAdapter.class)
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  private Currency currency;
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  ...
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}
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</source>
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== Example - java.awt.Point ==
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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.
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<source lang="java">
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package example;
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public class MyPoint {
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  private int x, y;
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  public MyPoint() {
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      this(0, 0);
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  }
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  public MyPoint(int x, int y) {
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      this.x = x;
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      this.y = y;
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  }
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  public int getX() {
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      return x;
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  }
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  ...
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}
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</source>
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<source lang="java">
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package example;
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public class MyPointAdapter extends XmlAdapter<MyPoint, Point> {
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  /*
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    * Java => XML
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    */
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  public MyPoint marshal(Point val) throws Exception {
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      return new MyPoint((int) val.getX(), (int) val.getY());
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  }
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  /*
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    * XML => Java
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    */
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  public Point unmarshal(MyPoint val) throws Exception {
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      return new Point(val.getX(), val.getY());
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  }
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}
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</source>
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Finally, our '''Point''' properties are marked with '''@XmlJavaTypeAdapter''':
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<source lang="java">
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package example;
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import java.awt.Point;
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@XmlRootElement
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@XmlAccessorType(XmlAccessType.FIELD)
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public class Zone {
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  private String name;
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  @XmlJavaTypeAdapter(MyPointAdapter.class)
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  private Point startCoord;
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  @XmlJavaTypeAdapter(MyPointAdapter.class)
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  private Point endCoord;
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  ...
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}
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</source>
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Latest revision as of 13:19, 30 January 2013

Warning This page is obsolete. Please see Developing JAXB Applications Using EclipseLink MOXy for current information.