高并发-volatile

synchronized是阻塞式同步，在线程竞争激烈的情况下会升级为重量级锁。而volatile可以说是java虚拟机提供的最轻量级的同步机制。

根据Java内存模型，各个线程会将共享变量从主内存中拷贝到工作内存，然后执行引擎会基于工作内存中的数据进行操作处理。线程在工作内存进行操作后何时写到主内存是不确定的。volatile修饰的变量给java虚拟机特殊的约定，线程对volatile变量的修改会立刻被其他线程所感知，不会出现数据脏读的现象，从而保证数据的“可见性”。被volatile修饰的变量确保每个线程能够获取该变量的最新值。

 

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package com.bjsxt.base.sync007;   public class RunThread extends Thread{       private volatile boolean isRunning = true;     private void setRunning(boolean isRunning){         this.isRunning = isRunning;     }           public void run(){         System.out.println("进入run方法.."+Thread.currentThread().getName());         int i = 0;         while(isRunning == true){             //.. //          System.out.println("线程是否开启："+isRunning);         }         System.out.println("线程停止.."+Thread.currentThread().getName());     }           public static void main(String[] args) throws InterruptedException {         RunThread rt = new RunThread();         rt.start();         Thread.sleep(10);         rt.setRunning(false);         System.out.println("isRunning的值已经被设置了false.."+Thread.currentThread().getName());     } }

 打印

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1	进入run方法..Thread-0<br>isRunning的值已经被设置了false..main<br>线程停止..Thread-0

 例1

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package com.bjsxt.base.conn008;   import java.util.ArrayList; import java.util.List;   public class ListAdd1 {             private volatile static List list = new ArrayList();              public void add(){         list.add("bjsxt");     }     public int size(){         return list.size();     }           public static void main(String[] args) {                   final ListAdd1 list1 = new ListAdd1();                   Thread t1 = new Thread(new Runnable() {             @Override             public void run() {                 try {                     for(int i = 0; i <10; i++){                         list1.add();                         System.out.println("当前线程：" + Thread.currentThread().getName() + "添加了一个元素..");                         Thread.sleep(500);                     }                   } catch (InterruptedException e) {                     e.printStackTrace();                 }             }         }, "t1");                   Thread t2 = new Thread(new Runnable() {             @Override             public void run() {                 while(true){                     if(list1.size() == 5){                         System.out.println("当前线程收到通知：" + Thread.currentThread().getName() + " list size = 5 线程停止..");                         throw new RuntimeException();                     }                 }             }         }, "t2");                         t1.start();         t2.start();     }             }

 

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当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. Exception in thread "t2" java.lang.RuntimeException     at com.bjsxt.base.conn008.ListAdd1$2.run(ListAdd1.java:43)     at java.lang.Thread.run(Unknown Source) 当前线程收到通知：t2 list size = 5 线程停止.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素..

 例2 使用wait 和 notify  ， 配合synchronized关键字

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package com.bjsxt.base.conn008;   import java.util.ArrayList; import java.util.List; import java.util.Queue; import java.util.concurrent.CountDownLatch; import java.util.concurrent.LinkedBlockingDeque; import java.util.concurrent.LinkedBlockingQueue; /**  * wait notfiy 方法，wait释放锁，notfiy不释放锁  *  */ public class ListAdd2 {     private volatile static List list = new ArrayList();              public void add(){         list.add("bjsxt");     }     public int size(){         return list.size();     }           public static void main(String[] args) {                   final ListAdd2 list2 = new ListAdd2();                   // 1 实例化出来一个 lock         // 当使用wait 和 notify 的时候 ， 一定要配合着synchronized关键字去使用         final Object lock = new Object();                   final CountDownLatch countDownLatch = new CountDownLatch(1);                   Thread t1 = new Thread(new Runnable() {             @Override             public void run() {                 try {                     synchronized (lock) {                         for(int i = 0; i <10; i++){                             list2.add();                             System.out.println("当前线程：" + Thread.currentThread().getName() + "添加了一个元素..");                             Thread.sleep(500);                             if(list2.size() == 5){                                 System.out.println("已经发出通知.."); //                              countDownLatch.countDown();                                 lock.notify();                             }                         }                                           }                 } catch (InterruptedException e) {                     e.printStackTrace();                 }               }         }, "t1");                   Thread t2 = new Thread(new Runnable() {             @Override             public void run() {                 synchronized (lock) {                     if(list2.size() != 5){                         try {                             System.out.println("t2进入...");                             lock.wait(); //                          countDownLatch.await();                         } catch (InterruptedException e) {                             e.printStackTrace();                         }                     }                     System.out.println("当前线程：" + Thread.currentThread().getName() + "收到通知线程停止..");                     throw new RuntimeException();                 }             }         }, "t2");                     t2.start();         t1.start();               }       }

 

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t2进入... 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 已经发出通知.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t1添加了一个元素.. 当前线程：t2收到通知线程停止.. Exception in thread "t2" java.lang.RuntimeException     at com.bjsxt.base.conn008.ListAdd2$2.run(ListAdd2.java:71)     at java.lang.Thread.run(Unknown Source)

 例3 countDownLatch