JsonMappingException: Infinite recursion in Cloud Endpoints - google-app-engine

Question

I created an app server using Google Endpoints, it is a backend for an instant messaging app. Every user has a list of friends.
When I create a new friend, I am adding users to each other's friend list using the method below. However, it is giving me following error when I add a friend, because of circular dependency.
I looked at other questions and solutions posted. Most of them are structured differently and they didn't solve my problem.
One answer in this website recommends adding #JSONIgnore, but I don't have any field to add that. I tried to put #JsonManagedReference but I couldn't figure out where to put #JSONBackReference. Other examples on this website, usually have another field that refers to parent, but I don't have it.
Thanks for your help in advance.
Error 500 com.google.appengine.repackaged.org.codehaus.jackson.map.JsonMappingException:
Infinite recursion (StackOverflowError) (through reference chain: com.google.common.collect.TransformingRandomAccessList[0]-
>com.net.myapplication.backend.model.User[\"friends\"]->com.google.common.collect.TransformingRandomAccessList[0]-
>com.net.myapplication.backend.model.User[\"friends\"]-
addFriend method
#ApiMethod(name = "addFriend", httpMethod = "post")
public User addFriend(#Named("regId") String regId, #Named("email") String email) {
User user = findRecord(regId);
User friend = findRecordByEmail(email);
if (user == null){
log.info("User " + regId + " is not registered.");
} else{
if (friend == null){
log.info("User " + email + " is not registered.");
} else{
if (hasFriend(user, friend)){
log.info("User " + email + " is already a friend.");
} else {
user.getFriendsRef().add(Ref.create(friend));
friend.getFriendsRef().add(Ref.create(user));
ofy().save().entity(friend).now();
ofy().save().entity(user).now();
return friend;
}
}
}
return null;
}
User model
#Entity
public class User {
#Id
Long id;
#Index
private String regId;
private String firstName;
private String lastName;
#Index
private String email;
private Language language;
#Load
private List<Ref<User>> friends = new ArrayList<>();
public User() {}
public User(Long id, String regId, String firstName, String lastName, String email, Language language, List<Ref<User>> friends) {
this.id = id;
this.regId = regId;
this.firstName = firstName;
this.lastName = lastName;
this.email = email;
this.language = language;
this.friends = friends;
}
public Long getId() {
return id;
}
public void setId(Long id) {
this.id = id;
}
public String getRegId() {
return regId;
}
public void setRegId(String regId) {
this.regId = regId;
}
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
public String getLastName() {
return lastName;
}
public void setLastName(String lastName) {
this.lastName = lastName;
}
public String getEmail() {
return email;
}
public void setEmail(String email) {
this.email = email;
}
public Language getLanguage() {
return language;
}
public void setLanguage(Language language) {
this.language = language;
}
public List<User> getFriends() {
return Deref.deref(friends);
}
#ApiResourceProperty(ignored = AnnotationBoolean.TRUE)
public List<Ref<User>> getFriendsRef(){
return friends;
}
public void setFriends(ArrayList<Ref<User>> friends) {
this.friends = friends;
}
}

Answer 1

It sounds like this question is: "How do I return data structures with cycles through Cloud Endpoints?"
If it serializes data with vanilla Jackson (Jackson 1.x, from the look of that stacktrace), you can't. Jackson 2 has support for JSOG, but that would mean abandoning Cloud Endpoints:
https://github.com/jsog/jsog
https://github.com/jsog/jsog-jackson