Interface representing a custom serializer for JSON. You should write a custom serializer, if you are not happy with the default serialization done by Gson. You will also need to register this serializer through com.google.gson.GsonBuilder#registerTypeAdapter(Type, Object).

Let us look at example where defining a serializer will be useful. The Id class defined below has two fields: clazz and value.

public class Id<T> {
  private final Class<T> clazz;
  private final long value;

  public Id(Class<T> clazz, long value) {
    this.clazz = clazz;
    this.value = value;
  }

  public long getValue() {
    return value;
  }
}

The default serialization of Id(com.foo.MyObject.class, 20L) will be {"clazz":"com.foo.MyObject","value":20}. Suppose, you just want the output to be the value instead, which is 20 in this case. You can achieve that by writing a custom serializer:

class IdSerializer implements JsonSerializer<Id> {
  public JsonElement serialize(Id id, Type typeOfId, JsonSerializationContext context) {
    return new JsonPrimitive(id.getValue());
  }
}

You will also need to register IdSerializer with Gson as follows:

Gson gson = new GsonBuilder().registerTypeAdapter(Id.class, new IdSerializer()).create();

Serializers should be stateless and thread-safe, otherwise the thread-safety guarantees of Gson might not apply.

New applications should prefer TypeAdapter, whose streaming API is more efficient than this interface's tree API.