StackOverflowError: null when doing a get request

Question

I'm having a problem when i try to do a get request to my /users endpoint, instead of returning what i want to, returns this error below:

2018-07-07 17:00:06.636 ERROR 294108 --- [nio-8080-exec-3] 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: io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"])] with root cause

java.lang.StackOverflowError: null
	at java.lang.ClassLoader.defineClass1(Native Method) ~[na:1.8.0_171]
	at java.lang.ClassLoader.defineClass(ClassLoader.java:763) ~

my endpoint method:

 @Override
    public ResponseEntity<List<M>> findAll() {
        List<M> models = repository.findAll();

        if(models == null || models.isEmpty()){
            return ResponseEntity.noContent().build();
        }

        return ResponseEntity.ok(models);
    }

User model:

@Entity
@Table(name = "UN_USERS")
public class User extends AbstractModel {

    @Column(name = "USERNAME", unique = true)
    private String username;

    @Column(name = "UUID")
    private String uuid;

    @Column(name = "COINS")
    private long coins;

    @Column(name = "TOKENS")
    private long tokens;

    @ManyToOne
    @JoinColumn(name = "GROUP_ID", nullable = true)
    private Group group;

    //getters & setters... 
}

Group model:

@Entity
@Table(name="UN_GROUPS")
public class Group extends AbstractModel{

    private String prefix;

    @OneToMany(mappedBy = "group")
    private Set<User> users;

    @ManyToMany(mappedBy = "groups")
    private Set<Permission> permissions;
}

What could possible give this error and how can i fix?

Answer 1

By analyzing the error message, this part specifically:

through reference chain: io.union.restapi.domain.Group["users"]->org.hibernate.collection.internal.PersistentSet[0]->io.union.restapi.domain.User["group"]->io.union.restapi.domain.Group["users"]

we got that the reference chain forms a cycle. You've run into the Jackson infinite recursion problem and there are several ways to solve it. You can find a good article that covers the ways to deal with it here