Understanding Dynamic Proxy: Spring AOP Basics

Abhishek SomaniApril 23rd, 2013Last Updated: April 23rd, 2013

2 176 2 minutes read

Why AOP :

To Understand AOP(Aspect Oriented Programming), we need to understand Cross Cutting Concerns in software development . In every project , there is some amount of code which gets repeated in multiple classes , across several modules.For example logging which needs to be done for almost all classes and for all module.

This kind of code reduces code’s re usability , maintainability and scalability.For example , if you want to take the class and reuse it somewhere else , where logging is not needed , you have to change this class . Similarly, if validation or logging is changed , we have to change almost each and every class wherever this is used .

A class should focus on it’s core functionality , things like logging ,validation , transaction etc are not part of class functionality . Aspect Oriented Programming is the paradigm to help us removing these cross cutting concerns. It provides us the tools and methodology to separate our cross cutting code from classes .

Proxy Pattern:

We can create a proxy of the object , which will take care of the cross cutting concern code.There are two kind of proxy patterns :

Static Proxy :
Where we create a proxy object for every class. This is not feasible and practical
Dynamic Proxy :
In this , proxies are created dynamically through reflection . This functionality is added from JDK 1.3 . dynamic Proxy form the basic building block of Spring AOP

Here Class Example1 implements interface BasicFunc .

package com.somaniab.blog.ex;

public class Example1 implements Basicfunc{

 @Override
 public void method1() {
  System.out.println("executing method 1");  
 }

}

Here is the Interface Basicfunc

package com.somaniab.blog.ex;

public interface Basicfunc{

 public void method1();       

}

Now if we want to calculate execution time of method1 , we have to write that code in method itself , or we can create a proxy object. For creating proxy object , we create a Invocationhandler like this :

package com.somaniab.blog.ex;

import java.lang.reflect.InvocationHandler;
import java.lang.reflect.Method;

public class MyInvocationHandler implements InvocationHandler{

 private Object target ;

        public MyInvocationHandler(Object target)
 {
  this.target = target;
 }

 public Object getTarget() {
  return target;
 }

 public void setTarget(Object target) {
  this.target = target;
 }

 @Override
 public Object invoke(Object proxy, Method method, Object[] params)
   throws Throwable {
  long a = System.currentTimeMillis();
  Object result =method.invoke(target ,params);
  System.out.println("total time taken  "+
                         (System.currentTimeMillis()-a));
  return result;
 }

}

In this invocation handler , we call the actual method as well as calculate the time taken.Now , in main class we create the proxy object by using Proxy class.

package com.somaniab.blog.ex;

import java.lang.reflect.Proxy;

public class MainClass {
      public static void main(String[] args) {

       Example1 ex = new Example1();

       Basicfunc proxied =(Basicfunc)Proxy
                           .newProxyInstance(MainClass.class.getClassLoader(),
         ex.getClass().getInterfaces() ,new MyInvocationHandler(ex));
       proxied.method1();
 }
}

For creating Proxy we pass classloader[mostly the same classloader as origin class],interfaces,and the invocationHandler (pass the original target object in the invocation handler ). The original class must implement a interface ,only those method declared in interface get proxied and then we cast the proxy to the interface type .

If you get Exception like this : java.lang.ClassCastException: $Proxy0 cannot be cast to com.somaniab.blog.ex.Example1 , it means your target class does not implement the interface.

In CGLib Proxy , there is no necessity of declaring interface. So , this is how we made sure that our Example class write code for only method1 and we kept execution time calculation code out of it. It is a very basic example , but it is the basic of Spring AOP. Other frameworks implementing aop other than Spring AOP are AspectJ and JBOSS AOP

Reference: Understanding Dynamic Proxy: Spring AOP Basics from our JCG partner Abhishek Somani at the Java, J2EE, Server blog.