Spring boot JPA: recursion on JSON-view in a self join relationShip - spring-boot

Question

For a project that I am trying to build, I need a manyToMany relationship From the User Class to the Class User.(Users have Friends and Friends are friends of Users).
When trying to get Json out of SpringBoot with JPA. I get a recursive loop on my JSON.(This happens only if two users are friends of each other)
I know what happens but cannot find the solution to the problem.
As you can see I'm using different views to 'filter' the view. The question is: How can I stop the recursion?
#Entity
public class User {
#Id
#GeneratedValue
#JsonView(JsonViews.UserView.class)
private long userId;
#JsonView(JsonViews.UserView.class)
private String username;
#JsonView(JsonViews.UserView.class)
private String password;
#JsonView(JsonViews.UserView.class)
private String email;
#JsonView(JsonViews.UserView.class)
private String location;
//private String avatar;
#JsonView(JsonViews.UserView.class)
private int ranking;
private String UserStatus;
//Friend Relation Many > Many
#JsonView({JsonViews.UserView.class, JsonViews.FriendWith.class})
#ManyToMany()
private List<User> friendsWith;
#ManyToMany(mappedBy = "friendsWith")
private List<User> friendOf;
#JsonView({JsonViews.UserView.class, JsonViews.Player.class})
#OneToMany(mappedBy = "user")
private List<Player> player;
public User() {
this.friendsWith = new ArrayList<>();
this.friendOf = new ArrayList<>();
}
public User(String username, String password, String email, String location, int ranking) {
this();
//this.userId = userId;
this.username = username;
this.password = password;
this.email = email;
this.location = location;
this.ranking = ranking;
}
// and th usual getters and setters
Stack:
2019-12-10 22:17:52.414 ERROR 1618 --- [nio-8084-exec-1] o.a.c.c.C.[.[.[/].[dispatcherServlet] : Servlet.service()
for servlet [dispatcherServlet] in context with path [] threw exception [Request processing failed; nested exception is org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: Infinite recursion (StackOverflowError); nested exception is com.fasterxml.jackson.databind.JsonMappingException: Infinite recursion (StackOverflowError) (through reference chain: org.hibernate.collection.internal.PersistentBag[0]->
nl.hva.ewa.stratego.backend.models.User["friendsWith"]->
org.hibernate.collection.internal.PersistentBag[0]->
nl.hva.ewa.stratego.backend.models.User["friendsWith"]->
org.hibernate.collection.internal.PersistentBag[0]->
nl.hva.ewa.stratego.backend.models.User["friendsWith"]->
org.hibernate.collection.internal.PersistentBag[0]->
nl.hva.ewa.stratego.backend.models.User["friendsWith"]->
ETC.... ETC.....
for completeness the JsonViews class:
public class JsonViews {
public class UserView { };
public class AComplete extends UserView { };
public class BComplete extends UserView { };
public class FriendsWith extends UserView {};
public class FriendsOf extends UserView {};
public class Player extends UserView {};
}

Answer 1

Try the #JsonIdentityInfo annotation, where you can tell Jackson what is the POJO id, so it does not repeat it:
#JsonIdentityInfo(generator = ObjectIdGenerators.PropertyGenerator.class, property = "userId")
public class User {
public long userId;
public String name;
public List<User> friends;
}
That will generate the following JSON:
{
"userId" : 11,
"name" : "A",
"friends" : [ {
"userId" : 22,
"name" : "B",
"friends" : [ 11, {
"userId" : 33,
"name" : "C",
"friends" : [ ]
} ]
} ]
}
Notice that in the inner user B, the friends list contains just the ID if the user has been already added to the JSON body [ 11, {.