JAXB: How should I marshal complex nested data structures?

Question

I have several complex data structures like

Map< A, Set< B > > Set< Map< A, B > > Set< Map< A, Set< B > > > Map< A, Map< B, Set< C > > > and so on (more complex data structures) 

Note: In my case it doesn't really matter if I use Set or List.

Now I know that JAXB let me define XmlAdapter's, that's fine, but I don't want to define an XmlAdapter for every of the given data structures (it would be just too much copy-and-paste code).

I tried to achieve my goal by declaring two generalizing XmlAdapters:

• one for Map: MapAdapter<K,V>
• one for Set: SetAdapter<V>

The problem:
JAXB complains as following:

javax.xml.bind.JAXBException: class java.util.Collections$UnmodifiableMap nor any of its   super class is known to this context. 

Here is my adapter class:

import java.util.*; import javax.xml.bind.annotation.*; import javax.xml.bind.annotation.adapters.*;  public class Adapters {  public final static class MapAdapter<K, V>         extends XmlAdapter<MapAdapter.Adapter<K, V>, Map<K, V>> {      @XmlType     @XmlRootElement     public final static class Adapter<K, V> {          @XmlElement         protected List<MyEntry<K, V>> key = new LinkedList<MyEntry<K, V>>();          private Adapter() {         }          public Adapter(Map<K, V> original) {             for (Map.Entry<K, V> entry : original.entrySet()) {                 key.add(new MyEntry<K, V>(entry));             }         }      }      @XmlType     @XmlRootElement     public final static class MyEntry<K, V> {          @XmlElement         protected K key;          @XmlElement         protected V value;          private MyEntry() {         }          public MyEntry(Map.Entry<K, V> original) {             key = original.getKey();             value = original.getValue();         }      }      @Override     public Adapter<K, V> marshal(Map<K, V> obj) {         return new Adapter<K, V>(obj);     }      @Override     public Map<K, V> unmarshal(Adapter<K, V> obj) {         throw new UnsupportedOperationException("unmarshalling is never performed");     }  }  } 

Here is my JUnit test case:

import java.io.*; import java.util.*; import javax.xml.bind.*; import javax.xml.bind.annotation.*; import javax.xml.bind.annotation.adapters.*; import org.junit.*; import static java.lang.System.*;  public class SomeTest {  @Test public void _map2()         throws Exception {      Map<String, Map<String, String>> dataStructure =             new HashMap<String, Map<String, String>>();      Map<String, String> inner1 = new HashMap<String, String>();     Map<String, String> inner2 = new HashMap<String, String>();      dataStructure.put("a", inner1);     dataStructure.put("b", inner1);      inner1.put("a1", "1");     inner1.put("a2", "2");     inner2.put("b1", "1");     inner2.put("b2", "2");      JAXBContext context = JAXBContext.newInstance(Adapters.XMap.class,             Adapters.XCount.class, Adapters.XEntry.class);      Marshaller marshaller = context.createMarshaller();     marshaller.setProperty(Marshaller.JAXB_FRAGMENT, true);     marshaller.setProperty(Marshaller.JAXB_FORMATTED_OUTPUT, true);      marshaller.setAdapter(new Adapters.MapAdapter());      StringWriter sw = new StringWriter();      marshaller.marshal(dataStructure, sw);     out.println(sw.toString()); }  } 

Answer 1

I've solved the problem without XmlAdapter's.

I've written JAXB-annotated objects for Map, Map.Entry and Collection.
The main idea is inside the method xmlizeNestedStructure(...):

Take a look at the code:

public final class Adapters {  private Adapters() { }  public static Class<?>[] getXmlClasses() {     return new Class<?>[]{                 XMap.class, XEntry.class, XCollection.class, XCount.class             }; }  public static Object xmlizeNestedStructure(Object input) {     if (input instanceof Map<?, ?>) {         return xmlizeNestedMap((Map<?, ?>) input);     }     if (input instanceof Collection<?>) {         return xmlizeNestedCollection((Collection<?>) input);     }      return input; // non-special object, return as is }  public static XMap<?, ?> xmlizeNestedMap(Map<?, ?> input) {     XMap<Object, Object> ret = new XMap<Object, Object>();      for (Map.Entry<?, ?> e : input.entrySet()) {         ret.add(xmlizeNestedStructure(e.getKey()),                 xmlizeNestedStructure(e.getValue()));     }      return ret; }  public static XCollection<?> xmlizeNestedCollection(Collection<?> input) {     XCollection<Object> ret = new XCollection<Object>();      for (Object entry : input) {         ret.add(xmlizeNestedStructure(entry));     }      return ret; }  @XmlType @XmlRootElement public final static class XMap<K, V> {      @XmlElementWrapper(name = "map")     @XmlElement(name = "entry")     private List<XEntry<K, V>> list = new LinkedList<XEntry<K, V>>();      public XMap() {     }      public void add(K key, V value) {         list.add(new XEntry<K, V>(key, value));     }  }  @XmlType @XmlRootElement public final static class XEntry<K, V> {      @XmlElement     private K key;      @XmlElement     private V value;      private XEntry() {     }      public XEntry(K key, V value) {         this.key = key;         this.value = value;     }  }  @XmlType @XmlRootElement public final static class XCollection<V> {      @XmlElementWrapper(name = "list")     @XmlElement(name = "entry")     private List<V> list = new LinkedList<V>();      public XCollection() {     }      public void add(V obj) {         list.add(obj);     }  }  } 

It works!

Let's look at a demo output:

<xMap>     <map>         <entry>             <key xsi:type="xCount" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">                 <count>1</count>                 <content xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">a</content>             </key>             <value xsi:type="xCollection" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">                 <list>                     <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">a1</entry>                     <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">a2</entry>                     <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">a3</entry>                 </list>             </value>         </entry>         <entry>             <key xsi:type="xCount" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">                 <count>2</count>                 <content xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">b</content>             </key>             <value xsi:type="xCollection" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">                 <list>                     <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">b1</entry>                     <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">b3</entry>                     <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">b2</entry>                 </list>             </value>         </entry>         <entry>             <key xsi:type="xCount" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">                 <count>3</count>                 <content xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">c</content>             </key>             <value xsi:type="xCollection" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">                 <list>                     <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">c1</entry>                     <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">c2</entry>                     <entry xsi:type="xs:string" xmlns:xs="http://www.w3.org/2001/XMLSchema">c3</entry>                 </list>             </value>         </entry>     </map> </xMap> 

Sorry, the demo output uses also a data structure called "count" which is not mentioned in the Adapter's source code.

BTW: does anyone know how to remove all these annoying and (in my case) unnecessary xsi:type attributes?

Answer 2

I had the same requirement to use a Map< String,Map< String,Integer>>. I used the XMLAdapter and it worked fine. Using XMLAdaptor is the cleanest solution I think. Below is the code of the adaptor. This is the jaXb class code snippet.

    @XmlJavaTypeAdapter(MapAdapter.class)     Map<String, Map<String, Integer>> mapOfMap = new HashMap<String,Map<String, Integer>>(); 

MapType Class :

public class MapType {  public List<MapEntryType> host = new ArrayList<MapEntryType>();  } 

MapEntry Type Class:

public class MapEntryType {  @XmlAttribute public String ip;  @XmlElement public List<LinkCountMapType> request_limit = new ArrayList<LinkCountMapType>();  } 

LinkCountMapType Class:

public class LinkCountMapType { @XmlAttribute public String service;  @XmlValue public Integer count; } 

Finally the MapAdaptor Class:

    public final class MapAdapter extends XmlAdapter<MapType, Map<String, Map<String, Integer>>> {  @Override public Map<String, Map<String, Integer>> unmarshal(MapType v) throws Exception {     Map<String, Map<String, Integer>> mainMap = new HashMap<String, Map<String, Integer>>();      List<MapEntryType> myMapEntryTypes = v.host;     for (MapEntryType myMapEntryType : myMapEntryTypes) {         Map<String, Integer> linkCountMap = new HashMap<String, Integer>();         for (LinkCountMapType myLinkCountMapType : myMapEntryType.request_limit) {             linkCountMap.put(myLinkCountMapType.service, myLinkCountMapType.count);         }         mainMap.put(myMapEntryType.ip, linkCountMap);     }     return mainMap; }  @Override public MapType marshal(Map<String, Map<String, Integer>> v) throws Exception {     MapType myMapType = new MapType();      List<MapEntryType> entry = new ArrayList<MapEntryType>();      for (String ip : v.keySet()) {         MapEntryType myMapEntryType = new MapEntryType();         Map<String, Integer> linkCountMap = v.get(ip);         List<LinkCountMapType> linkCountList = new ArrayList<LinkCountMapType>();         for (String link : linkCountMap.keySet()) {             LinkCountMapType myLinkCountMapType = new LinkCountMapType();             Integer count = linkCountMap.get(link);             myLinkCountMapType.count = count;             myLinkCountMapType.service = link;             linkCountList.add(myLinkCountMapType);         }         myMapEntryType.ip = ip;         myMapEntryType.request_limit = linkCountList;         entry.add(myMapEntryType);     }     myMapType.host = entry;     return myMapType; } 

}

Marshalling a Jaxb Object will give the below XML

     <mapOfmap>     <host ip="127.0.0.1">         <request_limit service="service1">7</request_limit>         <request_limit service="service2">8</request_limit>     </host> </mapOfmap>