A Custom reuseable LoadableDetachableModel for Spring/Hibernate Molindo Techblog - Molindo Techblog – formerly known as talk-on-tech.blogspot.com

A Custom reuseable LoadableDetachableModel for Spring/Hibernate

May 21st, 2008 by Michael Sparer | Published in Hibernate, Spring, Wicket | 2 Comments

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the annoyance

org.hibernate.LazyInitializationException - could not initialize proxy - the owning Session was closed org.hibernate.LazyInitializationException: could not initialize proxy - the owning Session was closed

the problem

Generally, detached objects in Hibernate are a bad thing when it comes to reusing objects between web-requests. Detached objects are objects that have been persistent but are not attached to a Hibernate session. They’re easily produced but can be a mess if it comes to Lazy-Loading or updating them. Just consider the following case

Foo foo = _myDAO.getFoo(); // attached object


add(new Link("fooLink", new Model(foo)) {
  protected void onClick() {
    Foo foo = (Foo) getModelObject(); // now it's detached as this is called in the next request
    Bar bar = foo.getBar(); // possible lazy-loading exception if bar gets loaded lazily

} }

the solution

First be sure to use the OpenSessionInViewFilter sothat a new hibernate session is opened when the request gets attached and closed when it gets detached.

Then the solution of course is a to use a LoadableDetachableModel that gets a fresh instance from the persistence-layer on each request. Could look like that:

IModel fooModel = new LoadableDetachableModel() {


  protected Object load() {
    return _myDAO.getFoo();
  }
}
add(new Link("fooLink", fooModel) {
  protected void onClick() {
    Foo foo = (Foo) getModelObject(); // attached as LDM has loaded a fresh instance
    Bar bar = foo.getBar(); // no lazy loading exception as still attached

} }

the improvement

LoadableDetachableModels are a nice thing but it’s quite a lot of work to inject your DAO (or service) and then create a new LDM for each of your objects in each and every one of your panels. so we came up with a more generic version of the LoadableDetachableModel. First, let your model-objects implement an Interface, say IPersistentObject with the single method

Serializable getId();

As your object are likely to have a primary key named id it’ll be no problem anyway. Then create the following class:

public class PersistentObjectModel extends LoadableDetachableModel {


  private static final long serialVersionUID = 1L;
  private final Class _clazz;
  private final Serializable _id;
  @SpringBean(name = "myDao")
  private IMyDao _myDao;
  @SuppressWarnings("unchecked")
  public PersistentObjectModel(final T object) {
    super(object);
           // object may be a hibernate proxy, so get the actual class
    if (object instanceof HibernateProxy) {
      _clazz = (Class) ((HibernateProxy) object).getHibernateLazyInitializer()
                                   .getImplementation().getClass();
    } else {
      _clazz = (Class) object.getClass();
    }
    _id = object.getId();
    // inject the bean
    InjectorHolder.getInjector().inject(this);
  }
  public PersistentObjectModel(final Class clazz, final Serializable id) {
    clazz = clazz;
    _id = id;
    // inject the bean
    InjectorHolder.getInjector().inject(this);
  }
  @Override
  protected T load() {
    // the DAO then simply passes the params to hibernate (or spring's hibernatetemplate
    return _myDao.getById(_clazz, _id);
  }
  @SuppressWarnings("unchecked")
  @Override
  public T getObject() {
    return (T) super.getObject();
  }

}

This makes working with hibernate objects really easy. There are two options:

Use the object e.g.
setModel(new PersistentObjectModel(foo));

Use only class and id.
This might happen e.g. if you pass your id using PageParameters (but better encrypt it )
setModel(new PersistentObjectModel(Foo.class, 1));

Hope that saved you some time writing boilerplate code have fun

Vitor says:

August 26th, 2011 at 5:40 pm (#)

Excelent!
I have a problem similar to that, but I’m not using a Hibernate conection, I’m using a tier named ServiceFacade, that is a Spring POJO with @Service, so, could you give some help do adapt that solution to my case?

Any help will be very appreciated!

Thanks a lot and Congratulations for this solution!

Vitor

August 26th, 2011 at 6:21 pm (#)

Forget!

I solved just calling InjectorHolder.getInjector().inject(this);
before the calling of my ServiceFacade tier.

You solution solved my problem indirectly!
Thanks a lot!

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