Spark Java IllegalArgumentException: class org.eclipse.jetty.server.handler.ErrorHandler declares multiple JSON fields named _listeners

Question

I'm developing a simple project using Java Spark, and at the moment I'm trying to implement a little RESTful service. I have this post method:

post("/users", (request, response) -> {
        response.type("application/json");
        User user = new Gson().fromJson(request.body(),User.class);
        userService.addUser(user);
        return new Gson().toJson(response);
    });

I'm using postman to specify the body of the request:

{   "id": "1012", 
"name": "Mac",
"lastname": "Mason1",
"email": "email"
}

And the User class is:

 public class User {

public int id;
public String name;
public String lastname;
public String email;

public User (int id, String name, String lastname, String email) {
    this.id = id;
    this.name = name;
    this.lastname = lastname;
    this.email = email;
}

Whenever I try to do a post request, the following error pops up:

ERROR spark.http.matching.GeneralError - 
java.lang.IllegalArgumentException: class 
org.eclipse.jetty.server.handler.ErrorHandler declares multiple JSON fields 
named _listeners

Answer 1

return new Gson().toJson(response);

This code is trying to serialize the response object as JSON. This is probably a typo (surely you meant return new Gson().toJson(user);, or something entirely different instead) and fails because of the way Gson tries to recursively serialize all fields in the class hierarchy of response.