For many years LazyInitializationExceptions have become most frustrating point of Hibernate. This exception occurs when you try to access an un-initialised lazy association of a detached entity. In general, entities become detached in three different ways;
- session close
- session clear
- session evict
You have to be sure that any lazy attributes you will access be initialised before those three cases happen. Otherwise you will come up with the infamous LazyInitializationException.
So what are the ways to get rid of that exception? There are several ways available in Hibernate and employed among the community;
- You can invoke Hibernate.initialize(proxy) on each lazy attribute before their owning entity becomes detached.
- You have to access their specific properties such as ids before detachment such as x.getLazySet().size() or x.getLazyManyToOneAssoc().getId().
- For web applications you can also employ OpenSessionInViewFilter which keeps session open until the end of view render process so that entities won’t become detached before those lazy attributes accessed.
- You can also reattach detached entities before accessing their lazy attributes using session merge or lock operations.
However, with Hibernate 4.1.6 a new feature is introduced to handle those lazy problems. When you enable hibernate.enable_lazy_load_no_trans property in hibernate.properties or in hibernate.cfg.xml, you will have no LazyInitializationException any more. It works for any sort of lazy associations not only many sided (1:M, N:M) but also one sided (1:1,:M:1) as well. So what does this property do? It simply signals Hibernate that it should open a new session if session which is set inside current un-initialised proxy is closed.
You should be aware that if you have any other open session which is also used to manage current transaction, this newly opened session will be different and it may not participate into the current transaction unless it is JTA. Because of this TX side effect you should be careful against possible side effects in your system.
Another important point is that this feature only works for proxies whose sessions are closed or their owning entity is evicted from the session. If the entity become detached with session clear then you will still have the lazy problem. According to code comments in Hibernate’s AbstractPersistentCollection class, Hibernate team aims to handle session clear in the next major release.