jax-b xml inverse reference with many to many relationship

Question

I have a few many to many relationships in my data model. I have been trying to use JAX=B to get XML representations of the data models, however I have read that for one to many relationships an:

@XmlInverseReference

is needed for the inverse side of the mapping. I believe this is for using different fetch types (ie. LAZY and EAGER). I am unsure of exactly how this annotation works. Does it use back pointers to ensure that data is not fetched when it is specified on certain fields? I also do not know if I need to annotate my many to many relationship with the above annotation or not.

Here is the user class that has a many to many relationship with itself, ie. a user can be friends with many other users. Should I annotate the getter with an @XmlInverseReference?

@Entity
@Table(name = "users")
public class User implements Serializable {

...

// bi-directional many-to-many association to User
    @ManyToMany(cascade = { CascadeType.PERSIST, CascadeType.MERGE,
            CascadeType.REFRESH })
    @JoinTable(name = "friends", joinColumns = { @JoinColumn(name = "uid") }, inverseJoinColumns = { @JoinColumn(name = "frienduId") })
    private List<User> friends;

/**
 * @return
 * 
 *         gets the list of users this user is friends with
 */
public List<User> getFriends() {
    return this.friends;
}

/**
 * @param friendsList
 * 
 *            sets the users friends list
 */
public void setFriends(List<User> friendsList) {
    this.friends = friendsList;
}

Any help or guidance is much appreciated.

Answer 1

Note: I'm the EclipseLink JAXB (MOXy) lead and a member of the JAXB (JSR-222) expert group.

@XmlInverseReference is a EclipseLink JAXB (MOXy) extension that enables you to map bidirectional relationships:

• http://blog.bdoughan.com/2010/07/jpa-entities-to-xml-bidirectional.html
• http://blog.bdoughan.com/2013/03/moxys-xmlinversereference-is-now-truly.html

@XmlInverseReference serves two roles:

• During marshalling it prevents an infinite loop from occurring. If a bidirectional relationship exists between Foo and Bar, it will marshal Foo then Bar and then it will stop before trying to marshal Foo again.
• During unmarshalling it will populate the back pointer.