接口

public interface Fruit {
    public void show();
}

实现类

public class Apple implements Fruit{
    @Override
    public void show() {
        System.out.println("<<<<show method is invoked");
    }
}

代理类

public class DynamicAgent {

    //实现InvocationHandler接口，并且可以初始化被代理类的对象
    static class MyHandler implements InvocationHandler {
        private Object proxy;
        public MyHandler(Object proxy) {
            this.proxy = proxy;
        }
            
        //自定义invoke方法
        @Override
        public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
            System.out.println(">>>>before invoking");
            //真正调用方法的地方
            Object ret = method.invoke(this.proxy, args);
            System.out.println(">>>>after invoking");
            return ret;
        }
    }

    //返回一个被修改过的对象
    public static Object agent(Class interfaceClazz, Object proxy) {
        return Proxy.newProxyInstance(interfaceClazz.getClassLoader(), new Class[]{interfaceClazz},
                new MyHandler(proxy));
    }    
}

示例方法

public class ReflectTest {
    public static void main(String[] args) throws InvocationTargetException, IllegalAccessException {
        //注意一定要返回接口，不能返回实现类否则会报错
        Fruit fruit = (Fruit) DynamicAgent.agent(Fruit.class, new Apple());
        fruit.show();
    }
}