How is AOP being implemented to Change Java interface content?

I am current using Seasar2 Framework on a project that I am in. The framework is quite popular here in Japan but I am having problem in finding English documentations. Even on their official English translation site, they just discuss that the framework use Dependency Injection and AOP.

I was intrigued with the way they use it in one of their component S2Dao. Basically you only need to create interface DAO class and the framework automatically, changes the c开发者_如何学Code on runtime and creates intermediate class that get called in the middle. Hence DB transactions codes are automatically added to the class. I was wondering, is there any step by step explanation on how this is done? Can java change code on runtime and change the method on runtime?

Are good reference on how this is done? I just want to know how the framework is doing this.

Yes, it is possible to do dynamic implementations of an interface at runtime, and to manipulate the compiled bytecode also.

Java provides a built-in mechanism to implement interfaces at run-time, called dynamic proxy classes.

There are also good libraries like cglib or javassist, that allow you not only to implement interfaces, but also to extend classes and to manipulate bytecode at run-time (to change the behavior of a method, for example). Frameworks like Spring and Hibernate use libraries like these to make their magic, so your framework may be using some of these also.

NOTE: If you are curious, these libraries can "tweak" the bytecode because instead of using the default ClassLoader of the JVM, they load your classes using their own ClassLoader, so they have total control of every single byte of the loaded class, and they can do whatever they want with them :).