Saturday, July 25, 2009

CGLIB-For Dynamic Proxies

The Spring Framework implements method injection by dynamically generating a subclass overriding the method, using bytecode generation via the CGLIB library.

Please be aware that in order for this dynamic subclassing to work, you will need to have the CGLIB jar(s) on your classpath. Additionally, the class that the Spring container is going to subclass cannot be final, and the method that is being overridden cannot be final either.

By default, when the Spring container is creating a proxy for a bean that is marked up with the <aop:scoped-proxy/> element, a CGLIB-based class proxy will be created. This means that you need to have the CGLIB library on the classpath of your application.

Note: CGLIB proxies will only intercept public method calls! Do not call non-public methods on such a proxy; they will not be delegated to the scoped target object.

You can choose to have the Spring container create 'standard' JDK interface-based proxies for such scoped beans by specifying 'false' for the value of the 'proxy-target-class' attribute of the <aop:scoped-proxy/> element. Using JDK interface-based proxies does mean that you don't need any additional libraries on your application's classpath to effect such proxying, but it does mean that the class of the scoped bean must implement at least one interface, and all of the collaborators into which the scoped bean is injected must be referencing the bean via one of its interfaces.

Spring AOP can also use CGLIB proxies. This is necessary to proxy classes, rather than interfaces. CGLIB is used by default if a business object does not implement an interface. As it is good practice to program to interfaces rather than classes, business classes normally will implement one or more business interfaces. It is possible to force the use of CGLIB, in those (hopefully rare) cases where you need to advise a method that is not declared on an interface, or where you need to pass a proxied object to a method as a concrete type.

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