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Dynamic Proxies in Java

Up until recently I never had to write any code that dealt with reflection. Before now I worked for CNET which is basically a series of differnent CMS’ with one request, one response. My first week at the new job required me to build a JMS system that would accept method invocations across the wire seamlessly for the producer. Java.lang.reflect was the answer.

Once you can wrap your head around the idea its pretty simple to write. Essentially what we are attempting to do is shield the caller from having to know anything about the other method that is going to invoked behind the scenes.

public Class MyConcreteProxyClass implements InvocationHandler {

   Object obj;
   public MyDynamicProxyClass(Object obj)
     { this.obj = obj; }

  public Object invoke(Object proxy, Method m, Object[] args)
         throws Throwable {
     try {
        // do stuff
     } catch (Exception e) {
        throw e;
     }
        // return something
     }
}

Now we have to create the proxy interface which the caller will implement. Any of the methods of this class that he implements and fire will invoke method on the MyConcreteProxyClass object.

public interface ProxyInterface{
    public Object myMethod();
}

Now its time to create wire everything up. There are several ways to do this which help hide this proxying like a static factory method but this simply demonstrates how this works.

ProxyInterface proxy = (ProxyInterface)
Proxy.newProxyInstance(obj.getClass().getClassLoader(),
         Class[] { ProxyInterface.class },
          new MyConcreteProxyClass(obj));


Categories: Computers, Software