Question

I am trying to use MOXy JAXB to serialize a class A which looks like:

@XmlAccessorType(XmlAccessType.NONE)
@XmlRootElement
public class A {
    private Map<Foo, Bar> fooBar = new HashMap<Foo, Bar>();
    private Set<Foo> foos = new HashSet<Foo>();

    @XmlJavaTypeAdapter(FooBarMapAdapter.class)
    public Map<Foo, Bar> getFooBar() {
        return fooBar;
    }

    public void setFooBar(Map<Foo, Bar> fooBar) {
        this.fooBar = fooBar;
    }

    @XmlElement
    public Set<Foo> getFoos() {
        return foos;
    }

    public void setFoos(Set<Foo> foos) {
        this.foos = foos;
    }
}

The point is that the Foo objects in the “foos” fields are a superset of those in the fooBar map. Therefore I would like to “link” the key elements for the “fooBar” map to the corresponding elements in the “foos” list. I have tried this using the XmlID and XmlIDREF annotations:

@XmlAccessorType(XmlAccessType.NONE)
public class Foo {
    private String xmlId;

    @XmlID
    @XmlAttribute
    public String getXmlId() {
        return xmlId;
    }

    public void setXmlId(String xmlId) {
        this.xmlId = xmlId;
    }
}


@XmlAccessorType(XmlAccessType.NONE)
public class Bar {
    // Some code...
}

Then in my XmlAdapter I have tried to use a XmlIDREF annotation on the adapted map entries’ foo object:

public class FooBarMapAdapter extends
        XmlAdapter<FooBarMapAdapter.FooBarMapType, Map<Foo, Bar>> {
    public static class FooBarMapType {
        public List<FooBarMapEntry> entries = new ArrayList<FooBarMapEntry>();
    }

    @XmlAccessorType(XmlAccessType.NONE)
    public static class FooBarMapEntry {
        private Foo foo;
        private Bar bar;

        @XmlIDREF
        @XmlAttribute
        public Foo getFoo() {
            return foo;
        }

        public void setFoo(Foo foo) {
            this.foo = foo;
        }

        @XmlElement
        public Bar getBar() {
            return bar;
        }

        public void setBar(Bar bar) {
            this.bar = bar;
        }
    }

    @Override
    public FooBarMapType marshal(Map<Foo, Bar> map) throws Exception {
        FooBarMapType fbmt = new FooBarMapType();
        for (Map.Entry<Foo, Bar> e : map.entrySet()) {
            FooBarMapEntry entry = new FooBarMapEntry();
            entry.setFoo(e.getKey());
            entry.setBar(e.getValue());
            fbmt.entries.add(entry);
        }
        return fbmt;
    }

    @Override
    public Map<Foo, Bar> unmarshal(FooBarMapType fbmt) throws Exception {
        Map<Foo, Bar> map = new HashMap<Foo, Bar>();
        for (FooBarMapEntry entry : fbmt.entries) {
            map.put(entry.getFoo(), entry.getBar());
        }
        return map;
    }
}

When marshaling the code above is working as expected and produces the following XML:

<?xml version="1.0" encoding="UTF-8"?>
<a>
   <fooBar>
      <entries foo="nr1">
         <bar/>
      </entries>
   </fooBar>
   <foos xmlId="nr1"/>
</a>

For testing unmarshal, I am using the following test-code:

public class Test {
    public static void main(String[] args) throws Exception {
        A a = new A();

        Map<Foo, Bar> map = new HashMap<Foo, Bar>();
        Foo foo = new Foo();
        foo.setXmlId("nr1");
        Bar bar = new Bar();
        map.put(foo, bar);
        a.setFooBar(map);
        a.setFoos(map.keySet());

        final File file = new File("test.xml");
        if (!file.exists())
            file.createNewFile();
        FileOutputStream fos = new FileOutputStream(file);

        JAXBContext jc = JAXBContext.newInstance(A.class);
        Marshaller m = jc.createMarshaller();
        m.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);
        m.marshal(a, fos);

        FileInputStream fis = new FileInputStream(file);
        Unmarshaller um = jc.createUnmarshaller();
        A newA = (A) um.unmarshal(fis);

        System.out.println(newA.getFooBar());
    }
}

This code produces the (for me) unexpected result:

{null=test.moxy.Bar@373c0b53}

That is, the Foo object used as key in the map is null. If I change the map adapter and marshal the Foo object twice, instead of using an ID reference, I do not get this null pointer.

I have been able to find some posts about this on google using the JAXB-RI, where the problem could be solved writing an IDResolver as described at http://weblogs.java.net/blog/2005/08/15/pluggable-ididref-handling-jaxb-20. Unfortunately I have not been able to find any information about such a class in the MOXy JAXB JavaDoc.

Suggestion for workaround
From Blaise Doughan answer, I have realized that this is a bug in the MOXy implementation of JAXB. I have been able to make a (ugly) workaround for this bug. The idea is that instead of using a XMLAdapter the map is “converted” inside its defining class. The class A now looks like:

@XmlAccessorType(XmlAccessType.NONE)
@XmlRootElement
public class A {
    private Map<Foo, Bar> fooBar = new HashMap<Foo, Bar>();
    private Set<Foo> foos = new HashSet<Foo>();

    // Due to a bug a XMLAdapter approch is not possible when using XmlIDREF.
    // The map is mapped by the wrapper method getXmlableFooBarMap.
    // @XmlJavaTypeAdapter(FooBarMapAdapter.class)
    public Map<Foo, Bar> getFooBar() {
        return fooBar;
    }

    public void setFooBar(Map<Foo, Bar> fooBar) {
        this.fooBar = fooBar;
    }

    @XmlElement
    public Set<Foo> getFoos() {
        return foos;
    }

    public void setFoos(Set<Foo> foos) {
        this.foos = foos;
    }

    // // WORKAROUND FOR JAXB BUG /////
    private List<FooBarMapEntry> mapEntries;

    @XmlElement(name = "entry")
    public List<FooBarMapEntry> getXmlableFooBarMap() {
        this.mapEntries = new LinkedList<FooBarMapEntry>();
        if (getFooBar() == null)
            return mapEntries;

        for (Map.Entry<Foo, Bar> e : getFooBar().entrySet()) {
            FooBarMapEntry entry = new FooBarMapEntry();
            entry.setFoo(e.getKey());
            entry.setBar(e.getValue());
            mapEntries.add(entry);
        }

        return mapEntries;
    }

    public void setXmlableFooBarMap(List<FooBarMapEntry> entries) {
        this.mapEntries = entries;
    }

    public void transferFromListToMap() {
        fooBar = new HashMap<Foo, Bar>();
        for (FooBarMapEntry entry : mapEntries) {
            fooBar.put(entry.getFoo(), entry.getBar());
        }
    }
}

After the unmarshal, the transferFromListToMap-method now needs to be called. So the following line should be added immediately after the reference to newA is obtained:

newA.transferFromListToMap();

Any suggestions for a nicer workaround / bug fix will be appreciated :).

Answer 1

Note: I’m the EclipseLink JAXB (MOXy) lead.

I have been able to confirm the issue that you are seeing:

• https://bugs.eclipse.org/353596

Why the Issue is Happening

The issue is due to MOXy processing the XmlAdapter logic before it has processed the @XmlIDREF logic. MOXy does a single pass of the XML document, and the @XmlIDREF relationships are processed at the end to ensure that all of the referenced objects have been built (as the reference may precede the referenced object, as in this case).

I will try to post a workaround to this issue, and you can track our progress on this issue using the above bug.