chapter04_面向切面的Spring_3_使用JavaCo

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

(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