Jonas Jonas - 5 months ago 128
JSON Question

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": "jonas@example.com"}


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.

Answer

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).

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