Dynamic Proxy: how to handle nested method calls

Question

I'm trying to learn Dynamic Proxies in Java.

I know how they work but I can't find a solution to my problem: given an interface and its implementation with methods a(), b() and c() nested one into the other (let's say a() calls b() which calls c()), I would like to proxy my object to log EACH call to the methods.

So I code my InvocationHandler such as the invoke() method prints a log-line before the execution.

But when I call proxy.a(), only the call of method a() is logged and not the whole chain of methods.

What am I missing? Is the target of the proxy have to be a proxy itself?

forty-two · Accepted Answer · 2012-02-23 11:29:27Z

Well, the object itself doesn't know that it is being proxied, so when a() calls b(), it will be a normal "intra object" call.

If the target of the proxy is the proxy itself, you will have a loop.

One way to solve this, if it's really needed, would be to introduce a delegate to the target object and set it up with the proxy or with itself as delegate. Strange, but might work. Look out for loops, though.

do you mean using a proxied obj instead of the obj inside the invocation handler? i tried instead to let the invocation handler extends my object's class and apply the invoke method on the same invochandler instance ( method.invoke(this, args), i thought it would pretend to be the object but having each method call filtered by the proxy ), but once again it fails.

Tomas Narros · Accepted Answer · 2012-02-23 11:32:12Z

It is because, while from your test code you are calling proxy.a(), your final a() method is not calling proxy.b(), but straight to the self instance b() .

As a workaround, you can overload every method passing it a delegate instance. Suposing a class name of MyClass and a interface name of MyInterface:

void a() { //to keep the non-proxy working, the default method have to pass the //self intance a(this); } void a(MyInterface target) { target.b(target); } void b() { b(this); } void b(MyInterface target) { target.c(target); } void c() { c(this); } void c(MyInterface target) { //do whatever }

Then, from your test code you'll be able to call proxy.a(proxy), and get the expected result.

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