How to Properly Implement Custom Serializers with Jackson\'s `handledType()` and `serialize()`?

Using Custom Serializers with Jackson

In your code, you receive an IllegalArgumentException because JsonSerializer requires a handledType() method to specify the type it handles. Here's how to correctly implement it using handledType() and serialize():

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.getId());
    jgen.writeNumberField("itemNr", value.getItemNr());
    jgen.writeNumberField("createdBy", value.getCreatedBy().getId());
    jgen.writeEndObject();
  }

  @Override
  public Class<Item> handledType() {
    return Item.class;
  }
}

Integrating with Jackson:

After defining the custom serializer, you need to integrate it with Jackson. This can be achieved using a SimpleModule:

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

Now, you can use the customized mapper to serialize Item objects.

Serializing User Objects:

To serialize User objects with a custom format, you can define a separate serializer:

public class UserSerializer extends JsonSerializer<User> {

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

  @Override
  public Class<User> handledType() {
    return User.class;
  }
}

Don't forget to integrate it with Jackson:

simpleModule.addSerializer(new UserSerializer());

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