How to fluently build JSON in Java?

I'm thinking of something like:

String json = new JsonBuilder()
  .add("key1", "value1")
  .add("key2", "value2")
  .add("key3", new JsonBuilder()
    .add("innerKey1", "value3"))
  .toJson();

Which Java JSON library is best for this kind of fluent building?

Update: I wrapped GSON and got almost the desired result ... with one hitch.