Java Dynamic Proxy - How reference concrete class

Question

I have a question relating to dynamic proxies in java.

Suppose I have an interface called Foo with a method execute and class FooImpl implements Foo.

When I create a proxy for Foo and I have something like:

Foo f = (Foo) Proxy.newProxyInstance(Foo.class.getClassLoader(),
                                     new Class[] { Foo.class },
                                     handler);

Suppose my invocation handler looks like:

public class FooHandler implements InvocationHandler {
    public Object invoke(Object proxy, Method method, Object[] args) {
        ...
    }
}

If my invocation code looks something like

Foo proxyFoo = (Foo) Proxy.newInstance(Foo.getClass().getClassLoader(),
                                       new Class[] { Foo.class },
                                       new FooHandler());
proxyFoo.execute();

If the proxy can intercept the aforementioned call execute from the Foo interface, where does the FooImpl come in to play? Maybe I am looking at dynamic proxies in the wrong way. What I want is to be able to catch the execute call from a concrete implementation of Foo, such as FooImpl. Can this be done?

Many thanks

Johan Sjöberg · Accepted Answer · 2011-03-02 12:37:23Z

3

The way to intercept methods using dynamic proxies are by:

public class FooHandler implements InvocationHandler {
    private Object realObject;

    public FooHandler (Object real) {
        realObject=real;
    }


    public Object invoke(Object target, Method method, Object[] arguments) throws Throwable {
        if ("execute".equals(method.getName()) {
            // intercept method named "execute"
        }

        // invoke the original methods
        return method.invoke(realObject, arguments);
    }
}

Invoke the proxy by:

Foo foo = (Foo) Proxy.newProxyInstance(
            Foo.class.getClassLoader(),
            new Class[] {Foo.class}, 
            new FooHandler(new FooImpl()));

edited Mar 2, 2011 at 12:37

answered Mar 2, 2011 at 12:12

Johan Sjöberg

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5 Comments

Joeblackdev Over a year ago

Thanks guys. So an invocation of my code would look like: Foo f = new FooImpl(); FooHandler fh = new FooHandler(f); correct?

Costi Ciudatu Over a year ago

@Johan: I'm sure you didn't mean to write new Foo() there, as first of all Foo is an interface. I think that should be new FooHandler(new FooImpl()).

Joeblackdev Over a year ago

Guys, just one last thing - Why does the 'Foo.class.getClassLoader()' need to be passed in as an argument to the newProxyInstance method? What exactly does that do? Thanks

Johan Sjöberg Over a year ago

@Joeblackdev, In environments when using e.g., multiple jar-files they will each provide their own classloader and it might be desirable to select which to use.

Costi Ciudatu Over a year ago

@Joeblackdev: A class is only "equal to itself" in the same class loader. If Foo is loaded in some other CL it will be a different class. So you want to make sure that the returned Foo does not come from some other class loader, or otherwise you'll get a ClassCastException. The Proxy doesn't want to rely on the context classloader of the current thread as that's not guaranteed to be the one you expect.

Costi Ciudatu · Accepted Answer · 2011-03-02 12:14:01Z

2

If you want to delegate to some Foo implementation like FooImpl, just make your InvocationHandler a wrapper of an other Foo instance (passed to the constructor) and then send a FooImpl instance as the delegate. Then, inside the handler.invoke() method, call method.invoke(delegate, args).

answered Mar 2, 2011 at 12:14

Costi Ciudatu

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