This article shows how to use AOP with nothing but java - No code generation or bytecode manipulation.

Sample Scenario:
You want to insert logging statements for every call to every method of a certain class. This sounds like the default example used to promote AOP, but there is an easier solution if you know about java.lang.reflect.Proxy and java.lang.reflect.InvocationHandler.

Let’s take a random interface like

public interface MyInterface {
  public void methodA(int i, String s, String t) throws IOException ;
}

Use a factory to get the Implementation or the proxy and anything you like to switch between using a proxy and not using it.

public class MyInterfaceFactory {
  public static MyInterface getMyInterface()  {
    Object impl = new MyInterfaceImpl();
    String proxySwitch = System.getProperty("myinterface.logging");

    if (proxySwitch == null) {
      return (MyInterface) impl;
    }
    InvocationHandler proxy = new LoggingProxy(impl);
    MyInterface bf2 = (MyInterface) Proxy.newProxyInstance(MyInterface.class.getClassLoader(),
                  new Class[] { MyInterface.class }, proxy);
    return bf2;
  }
}

An implement InvocationHandler to create a LoggingProxy. To keep this example simple, I used System.out.println, use any flavor of logging you like in the real world.

public class LoggingProxy implements InvocationHandler {
  private Object target;
  public LoggingProxy(Object t) {
    target = t;
  }

  public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
    System.out.println(”Method ” + method.getName());
    for (int i = 0, l = args.length; i < l; i++) {
      System.out.println(”Object[" + i + "] ” + args[i].getClass().getName() + ” ” + args[i]);
    }

      return method.invoke(target, args);
  }
}

This implementation looks like it will work, but it has the slight problem, that it might change the behaviour of your application

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