AtomicBoolean运用
首先先看如下例子

 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