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...