JPA/Hibernate bidirectional many-to-one results in StackOverflowException

Question

I have entities User and GrantedRole that have a bidirectional one-to-many relation.

When I try to add a GrantedRole to the Set in User, there is no exception thrown, but when I debug the variables for the User and GrantedRole object have a description that reads

com.sun.jdi.InvocationException occurred invoking method.

The different fields for the variables can be read while debugging, but when I select the roles field in User or the user field in GrantedRole I get the same description as above. When I go into the Set of GrantedRole in user, I eventually find the following description:

Detail formatter error: An exception occurred: java.lang.StackOverflowError

My code:

public class User {
    private Set<GrantedRole> roles = new HashSet<GrantedRole>();

    public User() {
        super();
    }
    public User(String name, String password) {
        this.name = name;
        this.password = password;
    }

    @OneToMany(fetch = FetchType.EAGER, mappedBy = "user")
    public Set<GrantedRole> getRoles() {
        return roles;
    }

    public void setRoles(Set<GrantedRole> roles) {
        this.roles = roles;
    }

    // equals and hashCode are based on username
    // toString is based on all fields
}

public class GrantedRole {
    private user user;

    public GrantedRole() {
        super();
    }
    public GrantedRole(User user, Role role, Organization organization) {
        this.user = user;
        this.role = role;
        this.organization = organization;
    }

    @ManyToOne
    @NotNull
    public User getUser() {
        return user;
    }

    public void setUser(User user) {
        this.user = user;
    }

    // equals and hashCode are based on all fields
    // toString was based on all fields, I've changed it to not include User.
}

public class Test {
    public void testStuff() {
        User user = new User("name");
        GrantedRole role = new GrantedRole(user, "role"); //this sets the user field
        user.getRoles().add(role); //doesn't throw Exception, but debugging shows InvocationTargetException and StackOverflowException
        System.out.println(user.getRoles()); //throws StackOverflowException, as I would expect
    }
}

It was my understanding that I should be able to set up a bidirectional relation in this way. I read multiple tutorials, like this one, and I don´t see what I am doing differently to cause the .add(role) to go wrong.

I left out some trivial code, if it is needed I will gladly provide it. I haven't made code to ensure that a GrantedRole with a reference to a User is referenced by that user in return, but I think that is not relevant for the problem I'm having.

Answer 1

So, I was just being stupid and had a recursive call in the toString() methods. User.toString() tried to print the roles, and GrantedRole.toString() tried to print the user.

I fixed it by altering the GrantedRole.toString() to print user.getUsername(), thus breaking the cycle.