12Spring_AOP编程(AspectJ)_前置通知

接下里的博客会一篇一篇的讲解每一个通知。其实AOP_AspectJ的编程与传统的AOP的编程的最大的区别就是写一个Aspect 支持多个Advice和多个PointCut 。而且我们写AOP_Aspctj不需要继承任何的接口，不像传统的AOP还要继承接口。

前置通知是在运行我们被代理的方法之前先执行的。

前置通知具有如下的特点：

1.在目标方法执行前 进行增强代码。

2.AspectJ 提供Advice无需实现任何借口， 可以将很多通知代码 写入一个类 （切面类）

3.前置通知定义方法： 无返回值，可以传入参数 JoinPoint 连接点

先拿出案例，再具体讲解：

案例结构图如下：

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" 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CustomerService.java是目标类（被代理的类）。

MyAspect.java是切面类（里面可以写很多个通知）

TestAop2.java是junit的测试类。

applicationContext.xml是Spring的配置文件

第一步：我们先写目标类（要被代理的类）

package cn.itcast.spring.d_aspectj;

//目标业务类
public class CustomerService {
    public void update() {
        System.out.println("this is update");
    }

    public void delete() {
        System.out.println("this is delete");
    }

    public void search() {
        System.out.println("this is search");
    }

}

第二步：我们写切面类

package cn.itcast.spring.d_aspectj;

import org.aspectj.lang.JoinPoint;

//切面类(内部可以写很多的Advice的方法)
public class MyAspect {
    //前置通知1，2

    public void before1()
    {
        System.out.print("前置通知1");

    }
    //这个方法传入了参数JoinPoint，通过这个参数我们其实可以得到很多的信息，
    //比如现在执行的是那个切点
    public void before2( JoinPoint joinPoint)
    {

        System.out.print("前置通知2");
    }

}

第三步:写配置文件

<!-- AspectJ AOP -->
<!-- 配置目标 -->
<bean id="CustomerService" class="cn.itcast.spring.d_aspectj.CustomerService"></bean>
 <!-- 配置切面类 -->
<bean id="MyAspect" class="cn.itcast.spring.d_aspectj.MyAspect"></bean>
 <aop:config>
 <!-- ref引用切面类 -->
 <aop:aspect ref="MyAspect">

     <aop:pointcut expression="execution(* cn.itcast.spring.d_aspectj.CustomerService.*(..))" id="mypointcut2"/>
     <aop:before method="before1" pointcut-ref="mypointcut2"  />
      <aop:before method="before2" pointcut-ref="mypointcut2"  />
 </aop:aspect>
 <!-- 解释一下上面的执行流程
 <bean id="CustomerService" class="cn.itcast.spring.d_aspectj.CustomerService"></bean>
 <bean id="MyAspect" class="cn.itcast.spring.d_aspectj.MyAspect"></bean>都是是创建目标的对象。没什么好说的

  <aop:pointcut expression="execution(* cn.itcast.spring.d_aspectj.CustomerService.*(..))" id="mypointcut2"/>
     <aop:before method="before1" pointcut-ref="mypointcut2"  />
      <aop:before method="before2" pointcut-ref="mypointcut2"  />
      是说我们先找到pointcut-ref="mypointcut2"然后根据mypointcut2找到cn.itcast.spring.d_aspectj.CustomerService.*(..)，然后
       当每次执行cn.itcast.spring.d_aspectj.CustomerService.*(..)这里的方法时
      都是执行"MyAspect"(cn.itcast.spring.d_aspectj.MyAspect)里面的before1这个方法。
      同理我们找到pointcut-ref="mypointcut2"然后根据mypointcut2找到cn.itcast.spring.d_aspectj.CustomerService.*(..)，然后
       当每次执行cn.itcast.spring.d_aspectj.CustomerService.*(..)这里的方法时
      都是执行"MyAspect"(cn.itcast.spring.d_aspectj.MyAspect)里面的before2这个方法。

 -->
</aop:config>

第四步：写JUnit测试代码：

//整合spring和junit

//指定spring配置文件位置
@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration(locations="classpath:applicationContext.xml")
public class TestAop2 {
    @Autowired
    private CustomerService customerService;
    @Test
    public void testbefore()
    {System.out.print(customerService.getClass().getName());
        customerService.delete();

    }

}

最后的输出的结果是：

前置通知1

前置通知2

this is delete

由上面可知MyAspect"这个切面类里面的before1（Advice）和before2（Advice）都可以去执行不同的pointCut(切点)。这就是所谓的一个Aspect 支持多个Advice和多个PointCut。

前置通知细节：

1、 默认不能阻止目标方法执行，如果抛出异常，目标方法无法执行

2、 可以传入JoinPoint 连接点参数 ， 通过该参数可以获得当前拦截对象和方法信息

最后再提醒一点：所有的所有都是基于动态代理模式，这里没有用到接口，所以这里的代理是Cglib代理。

验证一下：

在TestAop2 的代码上中写

System.out.println(customerSetvice.getClass().getName());

输出是$$EnhancerByCGLIB$$