DEMO

public interface HelloWorld {
    void helloWorld();
}

public class HelloWorldImpl implements HelloWorld {

    @Override
    public void helloWorld() {
        System.out.println("Hello World!");
    }
}

public class InvocationHandlerImpl implements InvocationHandler {

    //需要代理的对象
    private Object object;

    public InvocationHandlerImpl(Object object) {
        this.object = object;
    }

    @Override
    public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
        long time = System.nanoTime();
        System.out.println("Invocation of Method " + method.getName() + " start!");
        method.invoke(object, args);
        System.out.println("Invocation of Method " + method.getName() + " end! takes " + (System.nanoTime() - time)  + " nanoseconds.");
        return null;
    }
}


public class DynamicProxyTest {

    public static void main(String[] args) {
        //代理对象
        HelloWorld helloworld = new HelloWorldImpl();

        InvocationHandler handler = new InvocationHandlerImpl(helloworld);

        ClassLoader loader = helloworld.getClass().getClassLoader();
        Class[] interfaces = helloworld.getClass().getInterfaces();

        //创建代理类
        HelloWorld proxyclz = (HelloWorld) Proxy.newProxyInstance(loader, interfaces, handler);

        proxyclz.helloWorld();


    }

}

Invocation of Method helloWorld start!
Hello World!
Invocation of Method helloWorld end! takes 289726 nanoseconds.

Process finished with exit code 0

实际上,proxyclz是继承真实对象接口的代理类,他的内部方法会通过反射的方式去调用handler的invoke方法.
动态代理原理图:

动态代理

其中代理类的创建是:

HelloWorld proxyclz = (HelloWorld) Proxy.newProxyInstance(loader, interfaces, handler);

最后会创建一个继承接口的代理类,实现了接口中的所有方法,所有方法均会反向调用handler的invoke的方法并传入当前调用method的method对象和入参如,上面例子中的proxy会有如下方法（伪代码）

public final void helloWorld()  
  {  
     Method method = Class.forName("HelloWorld").getMethod("helloWorld", new Class[0]);  
     handler.invoke(this, method, null);  
  }