java动态代理的原理

在许多mvc框架中，经常用到注解来实现面向切面（aop）的编程。面向切面编程，可以对业务逻辑中各部分进行分离，提高程序的重用性，降低各逻辑业务部分的耦合度。

jdk中利用反射原理使用Proxy类对对象进行代理。

jdk动态代理的应用前提是，目标类基于统一的接口。

接口类：

public interface Service {
    public void doService();
}

继承Service的实现类：

 public class ServiceImpl1 implements Service {
     @Override
     public void doService() {
         System.out.println("impl 1 do the service....");

     }
 }
 public class ServiceImpl2 implements Service {
     @Override
     public void doService() {
         System.out.println("impl 2 do the service...");
     }
 }

代理工厂类的getService方法，传入参数是被代理的service的class。创建InvocationHandler类对象，对被代理对象的方法进行处理。

 public <T extends Service> T getService(Class<T> clazz) {
         String simpleName = clazz.getSimpleName();
         String implName = getImplName(simpleName);
         try {
             final T service = (T) Class.forName(implName).newInstance();
             T froxyService = (T) Proxy.newProxyInstance(service.getClass().getClassLoader(),
                     service.getClass().getInterfaces(), new InvocationHandler() {

                 @Override
                 public Object invoke(Object proxy, Method method, Object[] args)
                         throws Throwable {
                     System.out.println("do something before proxy method...");
                     Object obj = method.invoke(service, args);
                     System.out.println("do something after proxy method...");
                     return obj;
                 }
             });
             return froxyService;
         } catch (Exception e) {
             e.printStackTrace();
             throw new RuntimeException();
         }
     }

测试类和结果：

 public class ProxyTest {
     public static void main(String[] args) {
         Factory factory = new Factory();
         Service impl1 = new ServiceImpl1();
         Service proxyImpl1 = factory.getService(impl1.getClass());
         proxyImpl1.doService();
         Service impl2 = new ServiceImpl2();
         Service proxyImpl2 = factory.getService(impl2.getClass());
         proxyImpl2.doService();
     }
 }

 结果是：
 do something before proxy method...
 impl 1 do the service....
 do something after proxy method...
 do something before proxy method...
 impl 2 do the service...
 do something after proxy method...