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：

 1 public class ServiceImpl1 implements Service {
 2     @Override
 3     public void doService() {
 4         System.out.println("impl 1 do the service....");
 5 
 6     }
 7 }
 8 public class ServiceImpl2 implements Service {
 9     @Override
10     public void doService() {
11         System.out.println("impl 2 do the service...");
12     }
13 }

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。

 1 public <T extends Service> T getService(Class<T> clazz) {
 2         String simpleName = clazz.getSimpleName();
 3         String implName = getImplName(simpleName);
 4         try {
 5             final T service = (T) Class.forName(implName).newInstance();
 6             T froxyService = (T) Proxy.newProxyInstance(service.getClass().getClassLoader(), 
 7                     service.getClass().getInterfaces(), new InvocationHandler() {
 8                 
 9                 @Override
10                 public Object invoke(Object proxy, Method method, Object[] args)
11                         throws Throwable {
12                     System.out.println("do something before proxy method...");
13                     Object obj = method.invoke(service, args);
14                     System.out.println("do something after proxy method...");
15                     return obj;
16                 }
17             });
18             return froxyService;
19         } catch (Exception e) {
20             e.printStackTrace();
21             throw new RuntimeException();
22         }
23     }

测试类和结果：

 1 public class ProxyTest {
 2     public static void main(String[] args) {
 3         Factory factory = new Factory();
 4         Service impl1 = new ServiceImpl1();
 5         Service proxyImpl1 = factory.getService(impl1.getClass());
 6         proxyImpl1.doService();
 7         Service impl2 = new ServiceImpl2();
 8         Service proxyImpl2 = factory.getService(impl2.getClass());
 9         proxyImpl2.doService();
10     }
11 }
12 
13 结果是：
14 do something before proxy method...
15 impl 1 do the service....
16 do something after proxy method...
17 do something before proxy method...
18 impl 2 do the service...
19 do something after proxy method...