Can Java Annotations help me with this?

Question

I'm wondering if there is a way to specify that a method gets called in advance of a class method. I know something like this should be posssible, since JUnit has before(), what I want to do is similar.

Here is a concrete example of what I'd like to do

class A {

 public void init(int a) {
  System.out.println(a);
 }

 @magic(arg=1)
 public void foo() { 
   //
 }

 public static void main() {
   A a = new A();
   a.foo();
 }
}

//Output: 1

Basically I want an annotation to tell either the compiler or the jvm call init() before foo()

Answer 1

If you have interface A you can wrap instances of this interface with Proxy and inside invoke method of its InvocationHandler you are free to check whether method is annotated and perform some actions depending on that:

class Initalizer implements InvocationHandler {
    private A delegate;
    Initializer(A delegate) {
        this.delegate = delegate;
    }

    public Object invoke(Object proxy, Method method, Object[] args) {
        if (method.isAnnotationPresent(magic.class)) {
            magic annotation = method.getAnnotation(magic.class);
            delegate.init(magic.arg);
        }
        method.invoke(delegate, args);
    }
} 
A realA = ...;
A obj = Proxy.newProxyInstance(A.class.getClassLoader(), new Class[] {A.class}, new Initializer(realA));

Or you can try using "before" advice of AspectJ. It will be something like the next:

@Aspect
public class Initializer {
    @Before("@annotation(your.package.magic) && target(obj) && @annotation(annotation)")
    private void initialize(A obj, magic annotation) {             
         a.init(annotation.arg);
    }
}

I'm not sure that snippets are working, they just illustrate idea.