JAXB，公共字段映射行为与带有@XmlElement批注的私有字段不同

Question

Should @XmlElement annotation force the mapping of a field as it was a public field?

That is, the field:

public HashMap<String, String> myMap = new HashMap<String, String>();

get mapped ok.
Regardless the things that are said around the web about mapping Map s, when i Marshall my class this produces a nice:

<myMap>
    <entry>
        <key>cat</key> <value>meows</value>
    </entry>
    <entry>
        <key>dog</key> <value>barks</value>
    </entry>
</myMap>

but if i add a second identical field, but for the fact that is private and so, in order to be mapped, annotated with @XmlElement :

@XmlElement
private HashMap<String, String> myMap2 = new HashMap<String, String>();

it doesn't work any more, that is it produces an empty element:

<myMap2/>

So @XmlElement doesn't just "turn on" mapping . Well, it does for simple type fields, even for Lists but NOT for Maps.

The question is: is it a wrong (naive) interpretation of the @XmlElement? Is there a way to accomplish just what JAXB already does for public Map fields without developing adapters?

This should be enough, just in case I've pasted the complete, yet minimal, source code here:

@XmlRootElement
public class K {

    public HashMap<String, String> myMap = new HashMap<String, String>();

    @XmlElement
    private HashMap<String, String> myMap2 = new HashMap<String, String>();

    //An Array list that yet gets correctly mapped
    @XmlElement
    private ArrayList<String> myList = new ArrayList<String>();

public static void main (String [] args) throws JAXBException {

    K k = new K();

    k.myMap.put("dog", "barks");
    k.myMap.put("cat", "meows");

    k.myMap2.put("bird", "tweets");
    k.myMap2.put("snake", "rattles");

    k.myList.add("first");
    k.myList.add("second");

    k.save();

}

public void save() throws JAXBException {

    JAXBContext jaxbContext = JAXBContext.newInstance(K.class);
    Marshaller marshaller = jaxbContext.createMarshaller();
    marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, Boolean.TRUE);
    Unmarshaller unmarshaller = jaxbContext.createUnmarshaller();

    marshaller.marshal(this, new File("K.xml"));

}
}

The XML produced, i've added empty lines around to show it.

<k>
<myMap>
    <entry>
        <key>cat</key>
        <value>meows</value>
    </entry>
    <entry>
        <key>dog</key>
        <value>barks</value>

   </entry>
</myMap>


<myMap2/>


<myList>first</myList>
<myList>second</myList>

Answer 1

Note: I'm the EclipseLink JAXB (MOXy) lead and a member of the JAXB (JSR-222) expert group.

Your interpretation of the JAXB spec is correct. I have confirmed the bug you are seeing with the JAXB implementation included in JDK 1.7.0_21 for the Mac. Could you enter an issue using the following link:

• https://java.net/jira/browse/JAXB/

If you use EclipseLink MOXy as your JAXB provider (see: http://blog.bdoughan.com/2011/05/specifying-eclipselink-moxy-as-your.html ) then you get the correct output:

<?xml version="1.0" encoding="UTF-8"?>
<k>
   <myMap>
      <entry>
         <key>cat</key>
         <value>meows</value>
      </entry>
      <entry>
         <key>dog</key>
         <value>barks</value>
      </entry>
   </myMap>
   <myMap2>
      <entry>
         <key>bird</key>
         <value>tweets</value>
      </entry>
      <entry>
         <key>snake</key>
         <value>rattles</value>
      </entry>
   </myMap2>
   <myList>first</myList>
   <myList>second</myList>
</k>