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