public class Result<T> {

  private String msg;
  
  private List<T> module;

  public String getMsg() {
    return msg;
  }

  public void setMsg(String msg) {
    this.msg = msg;
  }

  public List<T> getModule() {
    return module;
  }

  public void setModule(List<T> module) {
    this.module = module;
  }  
}

public class User {

  private Long user_id;

  private String user_name;

  public User() {

  }

  public User(Long userId, String name) {
    this.user_id = userId;
    this.user_name = name;
  }

  public Long getUser_id() {
    return user_id;
  }

  public void setUser_id(Long user_id) {
    this.user_id = user_id;
  }

  public String getUser_name() {
    return user_name;
  }

  public void setUser_name(String user_name) {
    this.user_name = user_name;
  }
}

  public static void main(String[] args) {
    
    Result<User> r = new Result<User>();
    
    r.setMsg("msg");
    
    List<User> users = new ArrayList<>();
    users.add(new User(1L, "hehe"));
    users.add(new User(2L, "haha"));
    
    r.setModule(users);
    
    String js = JSON.toJSONString(r);
    
    System.out.println(js);
    
    Result<User> obj = (Result<User>)JSON.parseObject(js, Result.class);
    
    User user = obj.getModule().get(0);
    System.out.println(user);
  }

Result<User> obj = (Result<User>) JSON.parseObject(js, new TypeReference<Result<User>>(){});