Spring AOP: JDK Dynamic Proxy vs. CGLib proxy

Spring AOP: JDK Dynamic Proxy vs. CGLib proxy

Spring's AOP  is proxy-based. Spring provides  two different options  to create the proxies. One is based on JDK dynamic proxies and works with interfaces, the other one utilizes CGLib and is based on classes. (That's why the property is called  proxyTargetClass  respectively  proxy-target-class .) For the moment I just want to provide a quick summary on the pros and cons of both options:

JDK dynamic proxies:

• The class has to implement interfaces. Otherwise you will get ClassCastExceptions saying that $Proxy0 can not be casted to the particular class.


• Eventually dynamic proxies force you to program to interfaces since you can not cast the proxy to the class - a feature I really like about them.

CGLib proxies:

• The proxies are created by sub-classing the actual class. This means wherever an instance of the class is used it is also possible to use the CGLib proxy.


• The class needs to provide a default constructor, i.e. without any arguments. Otherwise you'll get an IllegalArgumentException: "Superclass has no null constructors but no arguments were given." This makes constructor injection impossible.


• The proxying does not work with final methods since the proxy sub class can not override the class' implementation.


• The CGLib proxy is final, so proxying a proxy does not work. You will get an IllegalArgumentException saying "Cannot subclass final class $Proxy0". But this feature is usually not needed anyway. 


• Since two objects are created (the instance of the class and the proxy as instance of a sub class) the constructor is called twice. In general this should not matter. I consider changing the class' state based on constructor calls a code smell anyway.


• You have CGLib as additional dependency.
  
  ref:  http://static.springsource.org/spring/docs/3.1.1.RELEASE/spring-framework-reference/htmlsingle/spring-framework-reference.html#aop-introduction-proxies