JPA with Hibernate 3 - ManyToMany-Stack overflow and Multiple bag errors

Question

I am facing issues while retrieving data for entities having bi-directional many-to-many relationship. If I use List for storing entities, I get unable to fetch multiple bags simultaneously error. If i change my code to use Set, I get stackoverflow error.

Details :

• Spring 3.0.3
• Hibernate-core : 3.5.1-Final
• Hibernate-annotations : 3.5.1-Final
• hibernate-common-annotations : 3.2.0-Final
• hibernate-entitymanager : 3.5.1-Final
• Mysql database
• Junit 4

User has Many Bank Accounts; Bank Account can have many users

User.java

@ManyToMany(fetch = FetchType.EAGER, mappedBy="user") 
private List<BankAccount> bankAccounts = new ArrayList<BankAccount>();

BankAccount.java

@ManyToMany(fetch = FetchType.EAGER)
@JoinTable(name = "user_bankaccount", 
           joinColumns = @JoinColumn(name="bank_account_id"), 
           inverseJoinColumns = @JoinColumn(name = "user_id")
)
private List<User> user = new ArrayList<User>();

DB Tables

Users
user_id PK

Bankaccount
bank_account_id PK

user_bankaccount
bank_account_id PK ( references bankaccount.bank_account_id )
user_id PK ( references user.user_id )

issues

1. when I try to get all the users data (getAllUsers) using a JUnit test case, I get unable to fetch multiple bags simultaneously error.
2. If I use Set and HashSet instead of List and ArrayList respectively, I get stackoverflow error.

Please help me and let me know if code is wrong or its a known hibernate issue with specific version of libs that I am using.

Answer 1

You cannot map the many to many relationship on both of the lists, hibernate will then try to fetch a collection for every nested element i.e. every user in the users list has a list of bank accounts which have a list of users... . think of it of a never ending recursion.

Answer 2

(Let's leave the fetch attribute aside for now). Your mapping is perfectly valid and is the right way to map a bidirectional many-to-many relationship. From the JPA 2.0 specification:

2.10.4 Bidirectional ManyToMany Relationships

...

Example:

@Entity
public class Project {
    private Collection<Employee> employees;

    @ManyToMany
    public Collection<Employee> getEmployees() {
        return employees;
    }

    public void setEmployees(Collection<Employee> employees) {
        this.employees = employees;
    }
    ...
}

@Entity
public class Employee {
    private Collection<Project> projects;

    @ManyToMany(mappedBy="employees")
    public Collection<Project> getProjects() {
        return projects;
    }
    public void setProjects(Collection<Project> projects) {
        this.projects = projects;
    }
    ...
}

In this example:

• Entity Project references a collection of Entity Employee.
• Entity Employee references a collection of Entity Project.
• Entity Project is the owner of the relationship.

...

That being said, I'm unsure of the behavior when using EAGER fetching on both sides (will it lead to an infinite cycle?), the JPA specification is pretty blurry about this and I can't find any clear mention that it is forbidden. But I bet that it's part of the problem.

But in the particular case of Hibernate, I'd expect Hibernate to be able to handle cycles as mentioned in this comment from Emmanuel Bernard:

LAZY or EAGER should be orthogonal to an infinite loop issue in the codebase. Hibernate knows how to handle cyclic graphs

Funnily enough, I've answered a very similar question recently (very close problem). Maybe an hint that something is wrong in Hibernate (my understanding of the above comment is that using EAGER fetching on both sides should work).

I'll thus conclude my answer in the same way: if you can provide a test case allowing to reproduce the problem, open a Jira issue.