AtomicBoolean介紹與使用

java.lang.Object      

java.util.concurrent.atomic.AtomicBoolean

繼承自Object。

 

• 介紹：

           

       在這個Boolean值的變化的時候不允許在之間插入，保持操作的原子性

• 方法和舉例
• compareAndSet(boolean expect, boolean update)

        這個方法主要兩個作用         1. 比較AtomicBoolean和expect的值，如果一致，執行方法內的語句。其實就是一個if語句         2. 把AtomicBoolean的值設成update         比較最要的是這兩件事是一氣呵成的，這連個動作之間不會被打斷，任何內部或者外部的語句都不可能在兩個動作之間運行。為多線程的控制提供了解決的方案。

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變量exists用來實現同一時間只有一個worker在工作. 但是假設exists的判斷和exists = true;之間有了 其他指令呢 Java代碼

 

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");
  }
 }

}

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");
   }
  }

 }

這時輸出是 bar2 enter bar2 working bar1 enter bar1 working bar1 leave bar2 leave 看到兩個線程同時工作了. 這時可以用AtomicBoolean Java代碼

 

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");
   }
  }

 }

 

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");
   }
  }

 }

因為它提供了原子性操作,其中exists.compareAndSet(false, true)這個操作把比較和賦值操作組成了一個原子操作, 中間不會提供可乘之機.輸出為 bar1 enter bar1 working bar2 give up