AtomicBoolean

 private static class BarWorker implements Runnable {

  private static boolean exists = false;

  private String name;

  public BarWorker(String name) {
   this.name = name;
  }

  public void run() {
   if (!exists) {
    exists = true;
    System.out.println(name + " enter");
    System.out.println(name + " working");
    System.out.println(name + " leave");
    exists = false;
   } else {
    System.out.println(name + " give up");
   }
  }

 }

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 

 private static class BarWorker implements Runnable {

  private static boolean exists = false;

  private String name;

  public BarWorker(String name) {
   this.name = name;
  }

  public void run() {
   if (!exists) {
    try {
     TimeUnit.SECONDS.sleep(1);
    } catch (InterruptedException e1) {
     // do nothing
    }
    exists = true;
    System.out.println(name + " enter");
    try {
     System.out.println(name + " working");
     TimeUnit.SECONDS.sleep(2);
    } catch (InterruptedException e) {
     // do nothing
    }
    System.out.println(name + " leave");
    exists = false;
   } else {
    System.out.println(name + " give up");
   }
  }

 }

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 

 private static class BarWorker implements Runnable {

   private static AtomicBoolean exists = new AtomicBoolean(false);

   private String name;

   public BarWorker(String name) {
    this.name = name;
   }

   public void run() {
    if (exists.compareAndSet(false, true)) {
     System.out.println(name + " enter");
     try {
      System.out.println(name + " working");
      TimeUnit.SECONDS.sleep(2);
     } catch (InterruptedException e) {
      // do nothing
     }
     System.out.println(name + " leave");
     exists.set(false);
    }else{
     System.out.println(name + " give up");
    }
   }

  }

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