final Item myItem = new Item(1, "theItem", new User(2, "theUser"));
final ObjectMapper mapper = new ObjectMapper();
final SimpleModule simpleModule = new SimpleModule();
simpleModule.addSerializer(Item.class, new ItemSerializer());
mapper.registerModule(simpleModule);
final String serialized = mapper.writeValueAsString(myItem);
System.out.println(serialized);
}