前奏：cglib的反向代理

public static void main(String[] args) {

		UserService target = new UserService();

		Enhancer enhancer = new Enhancer();
		enhancer.setSuperclass(UserService.class);
		enhancer.setCallbacks(new Callback[]{new MethodInterceptor() {
			@Override
			public Object intercept(Object o, Method method, Object[] objects, MethodProxy methodProxy) throws Throwable {
				//参数入参o就是代理对象，如果method.invoke传入的是代理对象o，那么就会出现死循环
				System.out.println("before");
				//相当于在执行被代理的方法
				//Object result = methodProxy.invoke(target, objects);
				//Object result = method.invoke(target, objects);
				Object result = methodProxy.invokeSuper(o, objects); //相当于在执行代理类的父类的方法，因为代理类一般是UserService extend UserService
				System.out.println("after");
				return result;
			}
		}});
		UserService userService = (UserService) enhancer.create();
		userService.test();

	}

package com.zhouyu.aop;

public class UserService {
	public void test() {
		System.out.println("aop");
	}
}

//注意这边必须使用接口
UserInterfaceImpl userInterface = (UserInterfaceImpl) Proxy.newProxyInstance(Test.class.getClassLoader(), new Class[]{UserInterfaceImpl.class}, new InvocationHandler() {
			@Override
			public Object invoke(Object o, Method method, Object[] objects) throws Throwable {
				System.out.println("before");
				method.invoke(target, objects);
				return null;
			}
		});
		userInterface.test();

升级版，使用ProxyFactory，这样不用关心底层是jdk还是cglib

public static void main(String[] args) {

		UserService target = new UserService();

		ProxyFactory proxyFactory = new ProxyFactory();
		proxyFactory.setTarget(target);
		proxyFactory.addAdvice(new MethodBeforeAdvice() {
			@Override
			public void before(Method method, Object[] args, Object target) throws Throwable {
				System.out.println("before");
			}
		});

		UserService proxy = (UserService) proxyFactory.getProxy();
		proxy.test();


	}

继续升级：

public class AfterReturningAdvice implements AfterReturningAdvice {

	@Override
	public void afterReturning(Object returnValue, Method method, Object[] args, Object target) throws Throwable {
		System.out.println("方法return后执行");
	}
}

public class AroundAdvice implements MethodInterceptor {

	@Nullable
	@Override
	public Object invoke(@NotNull MethodInvocation invocation) throws Throwable {
		System.out.println("方法执行Around前");
		Object proceed = invocation.proceed();
		System.out.println("方法执行Around后");
		return proceed;
	}
}

public class BeforeAdvice implements MethodBeforeAdvice {

	@Override
	public void before(Method method, Object[] args, Object target) throws Throwable {
		System.out.println("方法执行前执行");
	}
}

public class ThrowsAdvice implements ThrowsAdvice {

	public void afterThrowing(Method method, Object[] args, Object target, NullPointerException ex) {
		System.out.println("方法抛出异常后执行");
	}

}

public static void main(String[] args) {

		UserService target = new UserService();

		ProxyFactory proxyFactory = new ProxyFactory();
		proxyFactory.setTarget(target);
		proxyFactory.addAdvice(new ZhouyuBeforeAdvice());

		UserService proxy = (UserService) proxyFactory.getProxy();
		proxy.test();


	}

spring动态代理

@Aspect
@Component
public class ZhouyuAspect {

	@Before("execution(public void com.zhouyu.service.UserService.test())")
	public void zhouyuBefore(JoinPoint joinPoint) {
		System.out.println("zhouyuBefore");
	}

}

  其中当纯粹的使用@Aspect、@Before注解都是在ASPECTJ的jar包里面的，这些生效机制是：当编译的时候，发现了这些内容，然后对接口进行了切面增强。

  但是当使用spring的时候，是在spring启动的时候解析@Before注解内容，解析称为advice，aspectj的注解实现spring全部自己实现了。