jersey (+ jackson) map field serialization - json

Question

I have a simple jersey web service and I'd like to consume / produce objects that contain map fields, like
#XmlElement
private Map<String,String> properties;
if this string goes into the web service,
{ properties: { key1: val1, key2: val2 )}
the properties field is deserialized as null with no errors. the same JSON goes in and out of GSON no problems, and in the short term I solved this by having jersey consume produce strings and using GSON to serialize / deserialize the JSON.
any ideas?

Answer 1

One option is to use annotated classes. So for instance a user might be represented by the following data.
import javax.xml.bind.annotation.XmlRootElement;
#XmlRootElement(name = "user")
public class User {
private int uid;
public int user_id;
public String user_name;
public String email;
public URI image_url;
public List<User> friends;
public boolean admin;
public User() {
...
}
public User(final int userid) {
// Find user by id
}
}
If you return the User object as in the following piece of code, then jaxb will automatically serialize the List as a JSON list etc etc....
#GET
#Path("/{userid}")
#Produces("application/json", "application/xml")
public User showUser(#PathParam("userid") final int userid) {
return new User(userid);
}

Answer 2

Jersey uses JAXB for serialization. JAXB can not serialize a Map as there is no XML type for Java type Map. Also, Map is an interface and JAXB does not like interfaces.
If you are using JAXBJackson bridge to marshal, you will run into issue.
You will need to create an adapter like below and annotate your Map property with
#XmlJavaTypeAdapter(MapAdapter.class)
private Map<String,String> properties;
#XmlSeeAlso({ Adapter.class, MapElement.class })
public class MapAdapter<K,V> extends XmlAdapter<Adapter<K,V>, Map<K,V>>{
#Override
public Adapter<K,V> marshal(Map<K,V> map) throws Exception {
if ( map == null )
return null;
return new Adapter<K,V>(map);
}
#Override
public Map<K,V> unmarshal(Adapter<K,V> adapter) throws Exception {
throw new UnsupportedOperationException("Unmarshalling a list into a map is not supported");
}
#XmlAccessorType(XmlAccessType.FIELD)
#XmlType(name="Adapter", namespace="MapAdapter")
public static final class Adapter<K,V>{
List<MapElement<K,V>> item;
public Adapter(){}
public Adapter(Map<K,V> map){
item = new ArrayList<MapElement<K,V>>(map.size());
for (Map.Entry<K, V> entry : map.entrySet()) {
item.add(new MapElement<K,V>(entry));
}
}
}
#XmlAccessorType(XmlAccessType.FIELD)
#XmlType(name="MapElement", namespace="MapAdapter")
public static final class MapElement<K,V>{
#XmlAnyElement
private K key;
#XmlAnyElement
private V value;
public MapElement(){};
public MapElement(K key, V value){
this.key = key;
this.value = value;
}
public MapElement(Map.Entry<K, V> entry){
key = entry.getKey();
value = entry.getValue();
}
public K getKey() {
return key;
}
public void setKey(K key) {
this.key = key;
}
public V getValue() {
return value;
}
public void setValue(V value) {
this.value = value;
}
}
}