Serialize a custom map with Jackson

Question

I want to serialize a custom Map to JSON.

The class with implements the map interface is the following:

public class MapImpl extends ForwardingMap<String, String> {
//ForwardingMap comes from Guava
    private String                  specialInfo;

    private HashMap<String, String> delegate;

    @Override
    protected Map<String, String> delegate() {
        return this.delegate;
    }
// some getters....
 }

If I call now

    ObjectMapper mapper = new ObjectMapper();
    mapper.writeValue(new File("/somePath/myJson.json"), objectOfMapImpl);

Jackson will serialize the map and ignores the variable specialInfo

I tried some things with a custom implementation of JsonSerializer but I ended up with this snippet:

    ObjectMapper mapper = new ObjectMapper();

    SimpleModule module = new SimpleModule("someModule");



       module.addSerializer(CheapestResponseDates.class, new JsonSerializer<MapImpl>() {

            @Override
            public void serialize(final MapImpl value, final JsonGenerator jgen, final SerializerProvider provider) throws IOException, JsonProcessingException {
CheapestResponseDurations.class);
// how to serialize the map here? maybe be in a data node...
                jgen.writeStartObject();
                jgen.writeObjectField("info", value.getInfo());
                jgen.writeEndObject();
            }
        });


    mapper.registerModule(module);

I am using JDK 1.7 and Jackson 2.3.1

Answer 1

You can leverage the @JsonAnySetter/ @JsonAnyGetter annotations, as described in this blog post . Since, as you mentioned, your custom map class must implement a Map interface, you could extract a separate "bean" interface and tell Jackson to use it instead when serializing via @JsonSerialize(as = ...) annotation.

I've slightly modified you example to illustrate how it could work. Note that if you want to deserialize the json string back to your map object, you may need to do some other tricks.

public class MapSerialize  {
public static interface MyInterface {
    String getSpecialInfo();

    @JsonAnyGetter
    Map<String, String> delegate();
}

@JsonSerialize(as = MyInterface.class)
public static class MyImpl extends ForwardingMap<String, String> implements MyInterface {

    private String                  specialInfo;
    private HashMap<String, String> delegate = new HashMap<String, String>();

    public Map<String, String> delegate() {
        return this.delegate;
    }

    @Override
    public String getSpecialInfo() {
        return specialInfo;
    }

    public void setSpecialInfo(String specialInfo) {
        this.specialInfo = specialInfo;
    }

    @Override
    public String put(String key, String value) {
        return delegate.put(key, value);
    }
}

public static void main(String[] args) throws IOException {
    ObjectMapper mapper = new ObjectMapper();
    MyImpl objectOfMapImpl = new MyImpl();
    objectOfMapImpl.setSpecialInfo("specialInfo");
    objectOfMapImpl.put("XXX", "YYY");
    String json = mapper.writeValueAsString(objectOfMapImpl);
    System.out.println(json);
}

}