Java多线程技术-Volatile关键字解析

2018-06-18 03:49:51来源：未知阅读 ()

分析volatile关键字可以从这三个方面分析，什么是程序的原子性，什么是程序的可见性，什么是程序的有序性

什么是程序的原子性

以下语句那些是原子操作？

public class ThreadCounter implements Runnable {
      private int count = 0;
      @Override
      public void run() {
            ++count;
//          count++;
      }
      public static void main(String[] args) throws InterruptedException {
            ThreadCounter thread = new ThreadCounter();
            for(int i = 0; i< 10000; i++){
                  new Thread(thread).start();
            }
            Thread.sleep(1000);//确保线程执行完
            System.out.println(thread.count);
      }
}

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演示结果：语句一和二是非原子操作，语句三和四是原子操作

执行指令：javap -s -c ThreadCounter

run方法的指令码（count++）：

count++这行代码分成了4个指令来执行，在多线程的情况下会不一致。

解决方法：

public class ThreadCounter implements Runnable {
      private int count = 0;
      @Override
      public void run() {
            synchronized (this) {
                  ++count;
            //    count++;
            }
      }
      public static void main(String[] args) throws InterruptedException {
            ThreadCounter thread = new ThreadCounter();
            for(int i = 0; i< 10000; i++){
                  new Thread(thread).start();
            }
            Thread.sleep(1000);//确保线程执行完
            System.out.println(thread.count);
      }
}

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什么是程序的可见性？

public class VolatileExample {
      boolean v =false;
      
      private void write(){
            v =true;
      }
      
      private void read(){
            while(!v){
            }
            System.out.println("程序结束！");
      }
      
      public static void main(String[] args) throws InterruptedException {
            final VolatileExample example = new VolatileExample();
            Thread thread1 = new Thread(()->{example.read();});
            thread1.start();
            Thread.sleep(1000);
            Thread thread2 = new Thread(()->{example.write();});
            thread2.start();
      }
}

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演示结果：

程序没有结束，read方法中的v没有因write方法的修改而退出循环！

解决方法：为变量v添加volatile关键字

public class VolatileExample {
      volatile boolean v =false;
      
      private void write(){
            v =true;
      }
      
      private void read(){
            while(!v){
            }
            System.out.println("程序结束！");
      }
      
      public static void main(String[] args) throws InterruptedException {
            final VolatileExample example = new VolatileExample();
            Thread thread1 = new Thread(()->{example.read();});
            thread1.start();
            Thread.sleep(1000);
            Thread thread2 = new Thread(()->{example.write();});
            thread2.start();
      }
}

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什么是程序的有序性？

Volatile应用场景

1. 状态标记量

public class ThreadTest {
      private volatile boolean isContinue = false;
      
      private class HandleThread extends Thread {
            @Override
            public void run() {
                  while (isContinue) {
                        // do something
                  }
            };
      }
}