AtomicBoolean 原子布尔值

AtomicBoolean介绍与使用

java.util.concurrent.atomic.AtomicBoolean

继承自java.lang.Object

• 介绍：在这个Boolean值的变化的时候不容许在之间插入，保持操做的原子性
• 方法和举例：compareAndSet(boolean expect, boolean update)这个方法主要两个做用

1. 比较AtomicBoolean和expect的值，若是一致，执行方法内的语句。其实就是一个if语句
2. 把AtomicBoolean的值设成update 比较最要的是这两件事是一鼓作气的，这两个动做之间不会被打断，任何内部或者外部的语句都不可能在两个动做之间运行。为多线程的控制提供了解决的方案。

// 使用：

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在工做， 可是假设exists的判断和exists = true;之间有了其余指令呢？

Java代码

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做了. 这时能够用AtomicBoolean

Java代码

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");  
   }  
  }  
  
 }

由于它提供了原子性操做,其中exists.compareAndSet(false, true)这个操做把比较和赋值操做组成了一个原子操做, 中间不会提供可乘之机.输出为 bar1 enter bar1 working bar2 give up