How do I use a custom Serializer with Jackson?

I have two Java classes that I want to serialize to JSON using Jackson:

public class User {
    public final int id;
    public final String name;

    public User(int id, String name) {
        this.id = id;
        this.name = name;
    }
}

public class Item {
    public final int id;
    public final String itemNr;
    public final User createdBy;

    public Item(int id, String itemNr, User createdBy) {
        this.id = id;
        this.itemNr = itemNr;
        this.createdBy = createdBy;
    }
}

I want to serialize an Item to this JSON:

{"id":7, "itemNr":"TEST", "createdBy":3}

with User serialized to only include the id. I will also be able to serilize all user objects to JSON like:

{"id":3, "name": "Jonas", "email": "[email protected]"}

So I guess that I need to write a custom serializer for Item and tried with this:

public class ItemSerializer extends JsonSerializer<Item> {

@Override
public void serialize(Item value, JsonGenerator jgen,
        SerializerProvider provider) throws IOException,
        JsonProcessingException {
    jgen.writeStartObject();
    jgen.writeNumberField("id", value.id);
    jgen.writeNumberField("itemNr", value.itemNr);
    jgen.writeNumberField("createdBy", value.user.id);
    jgen.writeEndObject();
}

}

I serialize the JSON with this code from Jackson How-to: Custom Serializers:

ObjectMapper mapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule("SimpleModule", 
                                              new Version(1,0,0,null));
simpleModule.addSerializer(new ItemSerializer());
mapper.registerModule(simpleModule);
StringWriter writer = new StringWriter();
try {
    mapper.writeValue(writer, myItem);
} catch (JsonGenerationException e) {
    e.printStackTrace();
} catch (JsonMappingException e) {
    e.printStackTrace();
} catch (IOException e) {
    e.printStackTrace();
}

But I get this error:

Exception in thread "main" java.lang.IllegalArgumentException: JsonSerializer of type com.example.ItemSerializer does not define valid handledType() (use alternative registration method?)
    at org.codehaus.jackson.map.module.SimpleSerializers.addSerializer(SimpleSerializers.java:62)
    at org.codehaus.jackson.map.module.SimpleModule.addSerializer(SimpleModule.java:54)
    at com.example.JsonTest.main(JsonTest.java:54)

How can I use a custom Serializer with Jackson?

This is how I would do it with Gson:

public class UserAdapter implements JsonSerializer<User> {

    @Override 
    public JsonElement serialize(User src, java.lang.reflect.Type typeOfSrc,
            JsonSerializationContext context) {
        return new JsonPrimitive(src.id);
    }
}

    GsonBuilder builder = new GsonBuilder();
    builder.registerTypeAdapter(User.class, new UserAdapter());
    Gson gson = builder.create();
    String json = gson.toJson(myItem);
    System.out.println("JSON: "+json);

But I need to do it with Jackson now, since Gson doesn't have support for interfaces.

You can put @JsonSerialize(using = CustomDateSerializer.class) over any date field of object to be serialized.

public class CustomDateSerializer extends SerializerBase<Date> {

    public CustomDateSerializer() {
        super(Date.class, true);
    }

    @Override
    public void serialize(Date value, JsonGenerator jgen, SerializerProvider provider)
        throws IOException, JsonProcessingException {
        SimpleDateFormat formatter = new SimpleDateFormat("EEE MMM dd yyyy HH:mm:ss 'GMT'ZZZ (z)");
        String format = formatter.format(value);
        jgen.writeString(format);
    }

}

As mentioned, @JsonValue is a good way. But if you don't mind a custom serializer, there's no need to write one for Item but rather one for User -- if so, it'd be as simple as:

public void serialize(Item value, JsonGenerator jgen,
    SerializerProvider provider) throws IOException,
    JsonProcessingException {
  jgen.writeNumber(id);
}

Yet another possibility is to implement JsonSerializable, in which case no registration is needed.

As to error; that is weird -- you probably want to upgrade to a later version. But it is also safer to extend org.codehaus.jackson.map.ser.SerializerBase as it will have standard implementations of non-essential methods (i.e. everything but actual serialization call).

I tried doing this too, and there is a mistake in the example code on the Jackson web page that fails to include the type (.class) in the call to addSerializer() method, which should read like this:

simpleModule.addSerializer(Item.class, new ItemSerializer());

In other words, these are the lines that instantiate the simpleModule and add the serializer (with the prior incorrect line commented out):

ObjectMapper mapper = new ObjectMapper();
SimpleModule simpleModule = new SimpleModule("SimpleModule", 
                                          new Version(1,0,0,null));
// simpleModule.addSerializer(new ItemSerializer());
simpleModule.addSerializer(Item.class, new ItemSerializer());
mapper.registerModule(simpleModule);

FYI: Here is the reference for the correct example code: http://wiki.fasterxml.com/JacksonFeatureModules

Use @JsonValue:

public class User {
    int id;
    String name;

    @JsonValue
    public int getId() {
        return id;
    }
}

@JsonValue only works on methods so you must add the getId method. You should be able to skip your custom serializer altogether.

How do I use a custom Serializer with Jackson?

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