第一章-请谈谈你对volatile的理解

是什么

volatile是Java虚拟机提供的轻量级的同步机制。

特性

• 保证可见性
• 不保证原子性
• 禁止指令重排

JMM是什么

1. 可见性

2. 原子性

   /**
    * @desc volatile可见性、非原子性验证
    * @Author xw
    * @Date 2019/8/19
    */
   public class VolatileDemo {
       public static void main(String[] args) {
           // 验证可见性
           seeOkVisibility();
           // 验证非原子性
           seeNonAtomic();
       }
   
       private static void seeNonAtomic() {
           MyData myData = new MyData();
           for (int i = 1; i <= 100; i++) {
               new Thread(() -> {
                   try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
                   for (int j = 1; j <= 100 ; j++) {
                       myData.addPlusPlus();
                   }
               }, "T" + i).start();
           }
           while (Thread.activeCount() > 2) {
   
           }
           System.out.println(Thread.currentThread().getName() + "\t mission is over value:" + myData.number);
       }
   
       // 设置可见性
       private static void seeOkVisibility() {
           MyData myData = new MyData();
           new Thread(() -> {
               System.out.println(Thread.currentThread().getName() + "\t come in");
               try { TimeUnit.SECONDS.sleep(3); } catch (InterruptedException e) { e.printStackTrace(); }
               myData.addTO60();
               System.out.println(Thread.currentThread().getName() + "\t updated number value: " + myData.number);
           }, "AAA").start();
           while (myData.number == 0) {
   
           }
           System.out.println(Thread.currentThread().getName() + "\t mission is over");
       }
   }
   
   class MyData {
       volatile int number;
       public void addTO60() {
           this.number = 60;
       }
       public void addPlusPlus() {
           this.number ++;
       }
   }

3. 有序性

   /**
    * @desc 指令重排
    * @Author xw
    * @Date 2019/8/8
    */
   public class ReSortSeqDemo {
       int a = 0;
       boolean flag = false;
       public void method01() {
           a = 1;
           flag = true;
       }
   
       // 多线程环境中线程交替执行，由于编译器优化重排的存在，两个线程中使用的变量能否保证一致性无法确定
       // JDK7以后做了优化
       public void method02() {
           if (flag){
               a = a + 5;
           }
       }
   
       public static void main(String[] args) {
           ReSortSeqDemo reSortSeqDemo = new ReSortSeqDemo();
           for(int i = 1; i <= 100; i++) {
               new Thread(() -> {
                   reSortSeqDemo.method01();
                   reSortSeqDemo.method02();
                   System.out.println(Thread.currentThread().getName() + " change number to " + reSortSeqDemo.a);
               },"T" + i).start();
           }
       }
   }

你在哪些地方用到过volatile？

单例模式DCL代码

/**
 * @desc DCL单例 + volatile（禁止指令重排）
 * @Author xw
 * @Date 2019/8/20
 */
public class SingletonDemo {
    private static volatile SingletonDemo instance = null;

    private SingletonDemo() {
        System.out.println(Thread.currentThread().getName() + "\t 我是构造方法");
    }

    public static SingletonDemo getInstance() {
        if (instance == null) {
            synchronized (SingletonDemo.class) {
                if (instance == null) {
                    instance = new SingletonDemo();
                }
            }
        }
        return instance;
    }

    public static void main(String[] args) {
        for (int i = 1; i < 100; i++) {
            new Thread(() -> {
                try { TimeUnit.SECONDS.sleep(1); } catch (InterruptedException e) { e.printStackTrace(); }
                SingletonDemo.getInstance();
            }, "T" + i).start();
        }
    }
}

单例模式volatile分析