无法从 json 字符串实例化类型的值

Question

我在用户和组之间有双向多对多关系。

    @Entity
    @Table(name = "UserTBL")
    public class User {

    @Id
    @SequenceGenerator(name = "user_generator", sequenceName = "user_seq", allocationSize = 1, initialValue = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "user_generator")
    private Long id;

    private String username;

    private String password;

    @ManyToMany
    private List<Group> groups;
}


@Entity
@Table(name = "GroupTBL")
public class Group {

    @Id
    @SequenceGenerator(name = "Group_generator", sequenceName = "group_seq", allocationSize = 1, initialValue = 1)
    @GeneratedValue(strategy = GenerationType.SEQUENCE, generator = "Group_generator")
    private Long id;

    private String name;

    private String description;

    @ManyToMany(mappedBy = "groups")
    private List<User> users;
}

我正在使用 java jax-rs web 服务开发 angular js 应用程序，我正在使用 jackson 处理 json 绑定，但我遇到了这个异常：

无法从 JSON String 实例化类型 [simple type, class com.blog.models.Group] 的值；没有单字符串构造函数/工厂方法（通过引用链：com.blog.models.User["groups"]）

这是我发送到服务器的 json 对象：

{  
   "groups":[  
      "{\"id\":1,\"name\":\"admin\",\"description\":null,\"users\":[]}",
      "{\"id\":2,\"name\":\"user\",\"description\":null,\"users\":[]}"
   ],
   "username":"hussien",
   "password":"ammar"
}

资源：

@Stateless
@Path("/users")
public class UserResource {

    @EJB
    private UserFacade userFacade;

    @POST
    @Consumes(MediaType.APPLICATION_JSON)
    public void createUser(User user) {
        userFacade.create(user);
    }
}

Answer 1

看起来在客户端您可能会将 JSON 对象字符串化两次（组）。你应该只调用一次。 JSON应该是这样过来的：

{"groups":[
{"id":1,"name":"admin","description":null,"users":[]},
{"id":2,"name":"user","description":null,"users":[]}
],
"username":"hussien",
"password":"ammar"
}