// SERVER SIDE

@Path("/user")
public class UserService {

	private JsonParser parser = new JsonParser();

	@POST
	@Path("/register")
    @Consumes(MediaType.APPLICATION_JSON)
	public void createAccount(String userString) {
		JsonObject jsonObject = parser.parse(userString).getAsJsonObject();

		final GsonBuilder builder = new GsonBuilder();
        final Gson gson = builder.create();

        User user = gson.fromJson(jsonObject, User.class);
		User.addUserToDatabase(user.getNickname(), user.getEmail(), user.getPassword());
	}
}

// CLIENT SIDE AVEC LA MEME CLASSE USER

public class RegisterAsyncTask extends AsyncTask<String, Void, Void> {

    private Client owner;

    public RegisterAsyncTask(Client owner) {
        this.owner = owner;
    }

    protected Void doInBackground(String... strings) {
        final GsonBuilder builder = new GsonBuilder();
        final Gson gson = builder.create();

        User user = new User(strings[0], strings[1], strings[2]);
        owner.target("http://10.0.2.2:9029/")
                .path("user")
                .path("register")
                .request(MediaType.APPLICATION_JSON)
                .post(Entity.json(gson.toJson(user)));
        return null;
    }
}