Question

What is the difference between Unidirectional and Bidirectional associations?

Since the table generated in the db are all the same,so the only difference I found is that each side of the bidiretional assocations will have a refer to the other,and the unidirectional not.

This is a Unidirectional association

public class User {
    private int     id;
    private String  name;
    @ManyToOne
    @JoinColumn(
            name = "groupId")
    private Group   group;
}

public class Group {
    private int     id;
    private String  name;
}

The Bidirectional association

public class User {
    private int     id;
    private String  name;
    @ManyToOne
    @JoinColumn(
            name = "groupId")
    private Group   group;
}
public class Group {
    private int         id;
    private String      name;
    @OneToMany(mappedBy="group")
    private List<User>  users;
}

The difference is whether the group holds a reference of the user.

So I wonder if this is the only difference? which is recommended?

Answer 1

The main difference is that bidirectional relationship provides navigational access in both directions, so that you can access the other side without explicit queries. Also it allows you to apply cascading options to both directions.

Note that navigational access is not always good, especially for "one-to-very-many" and "many-to-very-many" relationships. Imagine a Group that contains thousands of Users:

• How would you access them? With so many Users, you usually need to apply some filtering and/or pagination, so that you need to execute a query anyway (unless you use collection filtering, which looks like a hack for me). Some developers may tend to apply filtering in memory in such cases, which is obviously not good for performance. Note that having such a relationship can encourage this kind of developers to use it without considering performance implications.

• How would you add new Users to the Group? Fortunately, Hibernate looks at the owning side of relationship when persisting it, so you can only set User.group. However, if you want to keep objects in memory consistent, you also need to add User to Group.users. But it would make Hibernate to fetch all elements of Group.users from the database!

So, I can’t agree with the recommendation from the Best Practices. You need to design bidirectional relationships carefully, considering use cases (do you need navigational access in both directions?) and possible performance implications.

See also:

• Deterring “ToMany” Relationships in JPA models
• Hibernate mapped collections performance problems