Dynamic Proxies in Java

Java Proxy class in reflection package lets you dynamically create a proxy class(and its instances) that implements interfaces of your choice. Proxy instance contains a InvocationHandler supplied by you. Any method calls to the proxy instance call the handle method of passed invocation handler where you can determine the behavior that you want on the call.
So, for example, you can very easily wrap around implementation of an interface and do some magic before/after the method invocation(and much more of course). Using this you can get lightweight "aop" like functionality for method interception.

A much more informative article on the same topic is http://www.ibm.com/developerworks/java/library/j-jtp08305/index.html