Thursday, May 31, 2012

Byte code instrumentation and the ORM magic

All ORM tools use some kind of byte-code instumentation to do the persistance magic behind the scenes. But as an architect, it is important to understand what Hibernate or any JPA tool does to the entity classes?

Hibernate 'enhances' entity classes at runtime using a byte-code library called Javaassist. For e.g. it adds a '_dirty' flag to each field. It also adds a '_loaded' flag for each field to support lazy loading. A good blog explaining these concepts is here. So Hibernate reads the XML configuration or obtains annotations at runtime using reflection to apply byte-code instumentation.

There are various ways of doing byte-code instumentation using libraries such as CGLib, ASM, Javaassist, etc.
This byte-code enhancement can be done at compile-time or run-time. For Hibernate, besides a few special cases which require compile time 'enhancement' to byte-code; all common scenarios can be satisfied with runtime instrumentation.

The following link gives a good overview of all the enhancement options available in JPA.

In the .NET world, NHibernate uses the Linfu or Castle Dynamic Proxies byte-code enhancement providers.