切面编程(环绕通知与前后置通知区别)
本文系作者原创,转载请注明出处:http://www.cnblogs.com/further-further-further/p/7867034.html
解决问题
1、拥有前置通知和后置通知的功能,并能解决前置通知和后置通知在共享信息方面的不足(例如:统计切点方法执行时间);
2、在多线程并发条件下,能保证线程安全(因为在一个方法内定义的局部变量);
3、解决代码重复性,降低代码复杂程度;
内容说明
1、以下会给出前置通知、后置通知与环绕通知实例(观众观看表演),通过对比更能理解彼此之间的区别;
2、两者都通过@Component注解,扫描(Audience,Juggler)bean并注册到spring容器中时,需在XML配置文件中引入component-scan(前后置通知:<context:component-scan base-package="com.spring.example.aspectAspectJNoArgs"/> 环绕通知:<context:component-scan base-package="com.spring.example.aspectAround"/>)
3、切面是观众(Audience),切点是节目表演(Performance.perform())
前置通知:在节目表演之前,观众就坐(调用Audience的takeSeats方法),并关掉手机(调用Audience的turnOffCellPhones方法);
后置通知:在节目表演结束,观众鼓掌(调用Audience的applaud方法);
异常通知:节目表演出现异常,观众要求退票(调用Audience的demandRefund方法);
环绕通知:其他与上面相同,只是在节目表演开始与结束时打印时间,统计节目表演时长;
4、通过执行Juggler的perform方法,从而执行切面Audience中相应的方法,达到通知的效果;
应用实例:观众观看表演所做出的相应行为
先列出相关接口以及类代码
节目表演接口(切点方法)
1 package com.spring.example.aspectAround; 2 3 /** 4 * Created by weixw on 2017/11/16. 5 */ 6 public interface Performer { 7 8 void perform(); 9 }
切点类实现接口Juggler
1 package com.spring.example.aspectAround; 2 3 import org.springframework.stereotype.Component; 4 5 /** 6 * Created by weixw on 2017/11/16. 7 */ 8 @Component 9 public class Juggler implements Performer { 10 private int beanBags = 3; 11 public Juggler(){ 12 13 } 14 public Juggler(int beanBags){ 15 this.beanBags = beanBags ; 16 } 17 @Override 18 public void perform() { 19 System.out.println("JUGGLING "+ beanBags + " BEANBAGS"); 20 try { 21 Thread.sleep(1); 22 }catch (InterruptedException e){ 23 e.printStackTrace(); 24 } 25 } 26 27 28 }
上述代码都能共用,下面分别列举前后置通知与环绕通知区别代码
前后置通知(通过AspectJ注解实现,注意:<aop:aspectj-autoproxy/>不能少,它实现了切面相关方法绑定在切点上,切点方法执行就能触发相应通知)
XML配置文件:spring/aspect-aspectJnoArgs.xml(放在spring文件夹下)
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 <?xml version="1.0" encoding="UTF-8"?> 2 <beans xmlns="http://www.springframework.org/schema/beans" 3 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 4 xmlns:aop="http://www.springframework.org/schema/aop" 5 xmlns:context="http://www.springframework.org/schema/context" 6 xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> 7 8 <!--使用前置通知和后置通知唯一方式:在前置通知中记录开始时间,并在后置通知中报告表演耗费的时长,必须保存开始时间。因为Audience是单例,如果像这样保--> 9 <!--存状态,会存在线程安全问题;--> 10 <context:component-scan base-package="com.spring.example.aspectAspectJNoArgs"/> 11 <aop:aspectj-autoproxy/> 12 </beans>
前后置通知切面实现类
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 package com.spring.example.aspectAspectJNoArgs; 2 3 import org.aspectj.lang.annotation.*; 4 import org.springframework.stereotype.Component; 5 6 /** 7 * Created by weixw on 2017/11/16. 8 * 通过AspectJ注解实现切面编程 9 * 切点方法 id 默认是所依赖方法(public void performance(){})的小写方法名performance 10 */ 11 12 @Component 13 @Aspect 14 public class Audience { 15 @Pointcut("execution(* com.spring.example.aspectAspectJNoArgs.Performer.perform(..))") //定义切点 16 public void performance(){} 17 @Before("performance()")//表演之前 18 public void takeSeats(){ 19 System.out.println("The audience is taking their seats."); 20 } 21 @Before("performance()")//表演之前 22 public void turnOffCellPhones(){ 23 System.out.println("The audience is turning off their cellphones."); 24 } 25 @AfterReturning("performance()")//表演之后 26 public void applaud(){ 27 System.out.println("CLAP CLAP CLAP CLAP CLAP "); 28 } 29 @AfterThrowing("performance()") //表演失败之后 30 public void demandRefund(){ 31 System.out.println("Boo! We want our money back!"); 32 } 33 }
环绕通知
XML配置文件:spring/aspect-around.xml(放在spring文件夹下)
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 <?xml version="1.0" encoding="UTF-8"?> 2 <beans xmlns="http://www.springframework.org/schema/beans" 3 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 4 xmlns:aop="http://www.springframework.org/schema/aop" 5 xmlns:context="http://www.springframework.org/schema/context" 6 xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd"> 7 8 <!--前置通知和后置通知是在一个方法中实现,所以不需要保存变量值,自然是线程安全的;--> 9 10 <context:component-scan base-package="com.spring.example.aspectAround"/> 11 <!--通过component-scan自动扫描,@Component注解将Magician注册到spring容器--> 12 <aop:config> 13 <!--audience :切面 watchPerformance:切面方法 performance:切点--> 14 <aop:aspect ref="audience"> 15 <aop:pointcut id="performance" expression="execution(* com.spring.example.aspectAround.Performer.perform(..))"/> 16 <aop:around pointcut-ref="performance" method="watchPerformance" /> 17 </aop:aspect> 18 </aop:config> 19 </beans>
环绕通知切面实现类
![](https://images.cnblogs.com/OutliningIndicators/ContractedBlock.gif)
1 package com.spring.example.aspectAround; 2 3 import org.aspectj.lang.ProceedingJoinPoint; 4 import org.springframework.stereotype.Component; 5 6 /** 7 * Created by weixw on 2017/11/16. 8 */ 9 @Component 10 public class Audience { 11 public void takeSeats(){ 12 System.out.println("The audience is taking their seats."); 13 } 14 public void turnOffCellPhones(){ 15 System.out.println("The audience is turning off their cellphones."); 16 } 17 public void applaud(){ 18 System.out.println("CLAP CLAP CLAP CLAP CLAP"); 19 } 20 public void demandRefund(){ 21 System.out.println("Boo! We want our money back!"); 22 } 23 24 public void watchPerformance(ProceedingJoinPoint joinPoint){ 25 try{ 26 takeSeats(); //表演之前 27 turnOffCellPhones(); //表演之前 28 long start = System.currentTimeMillis(); 29 System.out.println("The performance start ......");//节目开始 30 joinPoint.proceed(); //执行被通知的方法 31 System.out.println("The performance end ......");//节目结束 32 long end = System.currentTimeMillis(); //表演之后 33 applaud();//表演之后 34 System.out.println("The performance took milliseconds:"+ (end - start) );//表演时长 35 }catch (Throwable t){ 36 demandRefund(); //表演失败之后 37 } 38 } 39 }
测试代码
环绕通知测试代码如下,前后置通知测试代码只需将配置文件名称改成spring/aspect-aspectJnoArgs.xml即可
1 package com.spring.example.aspectAround;/** 2 * Created by weixw on 2017/11/16. 3 */ 4 5 import javafx.application.Application; 6 import javafx.stage.Stage; 7 import org.springframework.context.ApplicationContext; 8 import org.springframework.context.support.ClassPathXmlApplicationContext; 9 10 public class Driver extends Application { 11 12 public static void main(String[] args) { 13 launch(args); 14 } 15 16 @Override 17 public void start(Stage primaryStage) { 18 try { 19 20 21 ApplicationContext ctx = new ClassPathXmlApplicationContext("spring/aspect-around.xml"); 22 Performer performer = (Performer) ctx.getBean("juggler"); 23 performer.perform(); 24 25 }catch (Exception e){ 26 e.printStackTrace(); 27 } 28 } 29 }
运行结果
环绕通知结果:
前后置通知结果:
总结
上述列出前后置通知和环绕通知样例。对于有变量缓存需求,线程安全的应用场景,前后置通知实现比较困难,而环绕通知实现就非常容易;
不要让懒惰占据你的大脑,不要让妥协拖垮你的人生。青春就是一张票,能不能赶上时代的快车,你的步伐掌握在你的脚下。