chapter04_面向切面的Spring_3_使用JavaConfig创建切面

• 添加 @Aspect注解，代表一个类是一个切面

  示例

    @Aspect
    public class Audience {
       ...
    }
  

• 通知的时刻有5种，详见"基本概念.md"

  示例

    @Aspect
    public class AroundAudience {
  
        @Around("execution(* com.springinaction.springidol.performance.Performance.perform())")
        public void watchPerformance(ProceedingJoinPoint joinpoint) {
  
            try {
  
                System.out.println("Now we use around aspectOfJavaConfig.");
  
                System.out.println("The audience is taking their seats.");
                System.out.println("The audience is turning off their cellphones");
  
                long start = System.currentTimeMillis();
  
                joinpoint.proceed();
  
                long end = System.currentTimeMillis();
  
                System.out.println("CLAP CLAP CLAP CLAP CLAP");
                System.out.println("The performance took " + (end - start)
                        + " milliseconds.");
  
            } catch (Throwable t) {
                System.out.println("Boo! We want our money back!");
            }
        }
    }
  

  具体的操作是在方法上方插入@Before(…), @AfterReturning(…), @AfterThrowing(…),@After(…),@Around(…)注解，注解中包括AspectJ切点表达式，切点表达式中必须要包含的是 execution

• 启用自动代理功能

  (1) 必须启用自动代理，否则 @Aspect注解不会被解析

  (2) 使用JavaConfig的话，可以添加 @EnableAspectJAutoProxy 注解启用自动代理

  示例

    @Configuration
    @EnableAspectJAutoProxy
    public class AroundAspectConfig {
  
        @Bean
        public AroundAudience aroundAudience() {
  
            return new AroundAudience();
        }
  
        @Bean
        public Performance performance() {
  
            return new MagicPerformance();
        }
    }
  

• 环绕通知 @Around

  (1) 它是另外四种通知的集合，可以实现另外四种通知的功能

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  (2) 它对应的函数接收 ProceedingJoinPoint类型的对象，这个对象是必须的。 joinpoint.proceed(); 代表执行被通知的方法，也可以获取返回值

  (3) 也可以不调用 joinpoint.proceed()方法实现阻塞效果；也可以在通知中多次进行joinpoint.proceed()调用

• 处理通知中的参数

  (1) 示例

  CompactDisc.java

    package com.springinaction.springidol.TrackCounter;
  
    public interface CompactDisc {
  
        void play();
  
        void playTrack(int index, String artist);
    }
  

  TrackCounter.java

    package com.springinaction.springidol.TrackCounter;
  
    @Aspect
    public class TrackCounter {
  
        private Map<Integer, Integer> trackCounts = new HashMap<Integer, Integer>();
        
        private Map<Integer, String> artistName = new HashMap<Integer, String>();
  
        @Before("execution(* com.springinaction.springidol.TrackCounter.CompactDisc.playTrack(int, String)) && args(trackNumber, artist)")
        public void countTrack(int trackNumber, String artist) {
  
            int currentCount
                = trackCounts.containsKey(trackNumber) ? trackCounts.get(trackNumber) : 0;
  
            trackCounts.put(trackNumber, currentCount + 1);
            artistName.put(trackNumber, artist);
        }
  
        public int getPlayCount(int trackNumber) {
  
            return (trackCounts.containsKey(trackNumber) ? trackCounts.get(trackNumber) : 0);
        }
  
        public String getArtistName(int trackNumber) {
  
            return artistName.get(trackNumber);
       }
    }
  

  (2) 说明

  args()用于指定要提供给通知方法的参数;中间用,隔开;参数名称要匹配

  (3) 通知方法必须是具体的实现类，不能是接口。接口的情况目前还没试验成功过