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AtomicBoolean运用

Posted on 2014-08-15 10:15  南小爵  阅读(254)  评论(0)  编辑  收藏  举报
首先先看如下例子 
 1  private static class BarWorker implements Runnable {
 2 
 3   private static boolean exists = false;
 4 
 5   private String name;
 6 
 7   public BarWorker(String name) {
 8    this.name = name;
 9   }
10 
11   public void run() {
12    if (!exists) {
13     exists = true;
14     System.out.println(name + " enter");
15     System.out.println(name + " working");
16     System.out.println(name + " leave");
17     exists = false;
18    } else {
19     System.out.println(name + " give up");
20    }
21   }
22 
23  }
View Code

static变量exists用来实现同一时间只有一个worker在工作. 但是假设exists的判断和exists = true;之间有了 
其他指令呢 

 

 

 1  private static class BarWorker implements Runnable {
 2 
 3   private static boolean exists = false;
 4 
 5   private String name;
 6 
 7   public BarWorker(String name) {
 8    this.name = name;
 9   }
10 
11   public void run() {
12    if (!exists) {
13     try {
14      TimeUnit.SECONDS.sleep(1);
15     } catch (InterruptedException e1) {
16      // do nothing
17     }
18     exists = true;
19     System.out.println(name + " enter");
20     try {
21      System.out.println(name + " working");
22      TimeUnit.SECONDS.sleep(2);
23     } catch (InterruptedException e) {
24      // do nothing
25     }
26     System.out.println(name + " leave");
27     exists = false;
28    } else {
29     System.out.println(name + " give up");
30    }
31   }
32 
33  }
View Code

 

　　

这时输出是 
bar2 enter 
bar2 working 
bar1 enter 
bar1 working 
bar1 leave 
bar2 leave 
看到两个线程同时工作了. 
这时可以用AtomicBoolean 

 

 1 private static class BarWorker implements Runnable {
 2 
 3   private static AtomicBoolean exists = new AtomicBoolean(false);
 4 
 5   private String name;
 6 
 7   public BarWorker(String name) {
 8    this.name = name;
 9   }
10 
11   public void run() {
12    if (exists.compareAndSet(false, true)) {
13     System.out.println(name + " enter");
14     try {
15      System.out.println(name + " working");
16      TimeUnit.SECONDS.sleep(2);
17     } catch (InterruptedException e) {
18      // do nothing
19     }
20     System.out.println(name + " leave");
21     exists.set(false);
22    }else{
23     System.out.println(name + " give up");
24    }
25   }
26 
27  }
View Code
因为它提供了原子性操作,其中exists.compareAndSet(false, true)这个操作把比较和赋值操作组成了一个原子操作,
中间不会提供可乘之机.输出为 
bar1 enter 
bar1 working 
bar2 give up