Java/Hibernate: Converting Hibernate proxy to real object and ORMLazyLoader

Hello,

To continue with Hibernate/ORM posts, I would submit a common problem faced with the use of lazy loading in ORM: the objects returned by the ORM layer contain lazy loading references (its type has a suffix as ‘_$$_javassist_11[….]’), and so the need of have an ORM session to access these objects.

Origin of lazy exceptions
Indeed, some object attributes are loaded as proxies due to lazy loading during a Hibernate session. For obvious performance reasons, it is not advisable to turn off the lazy loading: if an object with lazy loaded attributes is manipulated in persistence layer: no problem, the proxy will load the lazy loaded attribute once its necessary; during its ask. For example, if we have a “Chapter” class, that contains a “Book” property, when we do “chpater.getBook()”, the proxies will get the book from the database or Hibernate caches (1st and 2nd levels) when it’s requested – not when the chapter instance is initialized in the persistence layer.

However, there are still error scenarios:

• Problem 1: out of the persistence layer, in presentation layer (for example in a Spring MVC controller, Struts action), a access to a lazy loaded attribute
  view source

  print?

  1	chapter.getBook()

  will end up with the famous org.hibernate.LazyInitializationException because the Hibernate Session is closed and this lazy attribute don’t have the session attached and can’t load their lazy references;

• Problem 2: more, in some case, it could be necessary to manipulate directly the real object and not the proxy for example during object’s conversion, parsing object to xml or introspection/reflection and an access to a lazy loaded attribute
  view source

  print?

  1	chapter.getBook()

  will produce an error like:

  view source

  print?

  1	09:21:26 ERROR com.ho.controller.MyController handleException handleReadDetails error

  2	java.lang.ClassCastException: com.ho.data.Book_$$_javassist_11 cannot be cast to com.ho.data.Book

Bad solution
A simple ‘bad’ solution could be to have dedicated POJOs for each layer: a POJO ‘Book4Model’ for the persistence layer, a POJO ‘Book4Service’ for service layer and a POJO ‘Book4View’ for presentation layer. So, it will be necessary to maintain all conversions of these POJOs between layers.

Good solution
A other other solution is to ‘de-proxy’ the object due to a utility method like:

The offcial documentation http://docs.jboss.org/hibernate/orm/3.5/api/org/hibernate/Hibernate.html#initialize(java.lang.Object) explains that the method ‘initialize(Object proxy)’ ensures intialization of a proxy object or collection; it is not guaranteed that the elements INSIDE the collection will be initialized/materialized.

This utility method could be used directly inside the POJO of persistence layer:

ORMLazyLoader
However, this previous good solution resolves only the problem n°2 concerning the need of manipulate directly the real object and not the proxy. But the problem n° 1 about the access to a lazy loaded attribute between application’s layers is not target.

So, our complete ‘homemade’ solution is a utility named ORMLazyLoader based on previous ORMUtils class.
This unique Java class proposes an unique fonctionnality:

This lazyLoad fonctionnality will:

• be used in the persistence layer in order to load the lazy attributes of Hibernate or Collection types in a POJO Hibernate,
• receive as inputs a bean attached to Hibernate session and a list of levels to load,
• return the bean with attributes (Hibernate, Collection) detached from Hibernate session depending on specified levels,

Notes: The attributes (Hibernate, Collection) of levels not specified, will remain attached to the Hibernate session. So, if in the presentation layer, we try to access to an attribute attached to Hibernate Session, we will have the famous LazyInitializationException. More, a bean, whose the lazy attributes have been detached, can be used again in the persistence layer without load it again.

Uses of ORMLazyLoader
Simple example:

Other concrete examples:

• load all primitive attributes of:
  – “language” attribute of first level,
  – each instance “chapter” in “chapters” attribute of first level,
  – each instance “book” of second level in above “chapters/chapter” attribute of first level,
  – each instance “author” in “authors” of first level.
  view source

  print?

  1	(new ORMLazyLoader<Book>()).lazyLoad(bean, new String{"language", "chapters/book", "authors"});

• load all primitive attributes of the first level:
  view source

  print?

  1	(new ORMLazyLoader<Book>()).lazyLoad(bean, new String[] {}); :

• load all attributes (primitive, collection, hibernate types) of the first level:
  view source

  print?

  1	(new ORMLazyLoader<Book>()).lazyLoad(bean, new String[] {"*"}); :

• load all attributes (primitive, collection, hibernate types) of the first and second levels:
  view source

  print?

  1	(new ORMLazyLoader<Book>()).lazyLoad(bean, new String{"*&#47*"}); :

Here, the full code of the ‘homemade’ solution ORMLazyLoader:

This source code ORMLazyLoader is in the ZIP file attachement. This project needs the following librairies commons-beanutils-1.7.0.jar, commons-lang-2.4.jar, commons-logging-1.1.1.jar, hibernate-core-3.3.1.GA.jar, junit-4.1.jar and spring-2.5.5.jar.

Download: test_ORMTools.zip

That’s all!

Best regards,

Huseyin OZVEREN