many-to-many persistence

I am new to Java EE, and I set up a learning case for myself where I can register users and persist the data to the database, I use Glassfish, EclipseLink JPA 2.0 and JSF framework. The database contains three tables: user(id[pk],name,gender,...), hobby(id[pk],hobby_name), and user_hobby which describes the many-to-many relationship and contains only user_id and hobby_id.

in the User.java entity class, I declare the relationship as such:

...
@ManyToMany(fetch=FetchType.EAGER)
@JoinTable(
    name="user_hobby",
    joinColumns=@JoinColumn(name="user_id"),
    inverseJoinColumns=@JoinColumn(name="hobby_id")
)
private List<Hobby> hobbies;

public List<Hobby> getHobbies() {
    return hobbies;
}

public void setHobbies(List<Hobby> hobbies) {
    this.hobbies = hobbies;
}
...

and in the Hobby.java class, I declare bidirection as such:

...
@ManyToMany(mappedBy="hobbies",fetch=FetchType.EAGER)
private List<User> users;

public List<User> getUsers() {
    return users;
}

public void setUsers(List<User> users) {
    this.users = users;
}
...

In the JSF view page, I output all the hobbies from the hobby table which contain pre-inserted values like such:

<h:selectManyListbox id ="hobb" label="Hobbies" value="#{userController.user.hobbies}" >
      <f:selectItems value="#{itemController.hobbyItems}"/>
</h:selectManyListbox>

and corresponding itemcontroller has this method:

public List<SelectItem> getHobbyItems(){
    List<SelectItem> items=new ArrayList<SelectItem>();
    //fetch all hobbies from database
    List<Hobby> hobbies=hobbyFacade.findAll();
    for(Hobby h:hobbies)items.add(new SelectItem(h,h.getHobby()));
    return items;
} 

Here you can see that I inject a list of hobby objects into the view and let user.hobbies get hold on it.

When I create users, if I don't select hobbies, then the creation succeeds and data is persisted to the database, but if I select any hobby, the server throws a null pointer exception, which I couldn't figure out why, I have spend almost a day to debug this, but could not make any progress. Could anybody maybe help me with this issue, I would much appreciate it. If you need any more information about settings, please let me know.

Update exception(first three lines):

WARNING: java.lang.NullPointerException
at org.eclipse.persistence.internal.sessions.UnitOfWorkImpl.checkForUnregisteredExistingObject(UnitOfWorkImpl.java:745)
at org.eclipse.persistence.internal.sessions.UnitOfWorkImpl.discoverAndPersistUnregisteredNewObjects(Unit开发者_如何学PythonOfWorkImpl.java:4124)

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after a long struggling with this issues, I finally solved the problem. the problem was about conversion between string in view and object in backing bean. I used mapping solution which is one of the methods proposed in this post: http://balusc.blogspot.com/2007/09/objects-in-hselectonemenu.html. thanks.

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Please include your code to create users that is failing and include the full exception stack trace.

Are you adding a null value to a list?

Ensure that you are using the managed versions of the objects and merging your objects correctly.