Sunday, December 19, 2004

What are Java annotations?

This must be described elsewhere, but a quick Googling didn't give it to me. A little experimentation reveals to me that JDK 5.0 annotations are dynamic proxies under the hood.

To find this out, I made an annotation named @annotate and exammined annotation.class; it revealed itself to be an interface. I then decorated a method with @annotate, got the Method with reflection, pulled the appropriate annotation class off with getAnnotation(annotate.class).getClass() and examined that: dynamic proxy, $Proxy3(java.lang.reflect.InvocationHandler).

I wonder how I can use this knowlege for some real Java-fu.

UPDATE: An even stronger answer: Proxy.isProxyClass(Class) returns true on the method annotation class, and the proxy invocation handler is a sun.reflect.annotation.AnnotationInvocationHandler. Good thing Sun provides source for the non-public bits.

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