volatile理解

volatile理解

volatile是java虚拟机提供的轻量级的同步机制。

一，voltaile的特性：保证可见性，不保证原子性，禁止指令重排

1.验证volatile的可见性代码：

class MyData{

     volatile int num = 0;

    void incrementNum(){
        this.num = num + 60;
    }
}

public class VolatileMain {

    public static void main(String[] args) {
        MyData myData = new MyData();
        new Thread(()->{
            try {
                TimeUnit.SECONDS.sleep(6);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            myData.incrementNum();
            System.out.println(Thread.currentThread().getName()+"--->"+myData.num);
        },"AAA").start();


        while (myData.num == 0){

        }
        System.out.println(Thread.currentThread().getName()+"t"+myData.num);
    }
}

2.验证volatile不保证原子性代码：

class MyData1{

    volatile int num = 0;

    void incrementNum(){
        this.num = num + 60;
    }
    void addPlus(){
        this.num++;
    }
}

public class VolatileAtomicValid {

    public static void main(String[] args) {
        MyData1 myData1 = new MyData1();
        for (int i = 0; i < 20; i++) {
            new Thread(()->{
                for (int j = 1; j <= 1000; j++) {
                    myData1.addPlus();
                }
            },String.valueOf(i)).start();
        }

        while (Thread.activeCount() > 2){
            Thread.yield();
        }

        System.out.println(Thread.currentThread().getName() + "t" + "finally number value:" + myData1.num);
    }
}
console 输出：main	finally number value:14885

3.volatile禁止指定重排：

class ReSortDemo{

   volatile int a = 0;
   volatile 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时无法确定的，结果无法预测
    public void method02(){
        if(flag){

            a = a + 5;
            System.out.println("******retValue" + a);
        }
    }
}

二，单例模式下在多线程环境下可能存在安全问题

    1. 单线程情况下

public class SingletonDemo {
    private static SingletonDemo singletonDemo = null;

    public SingletonDemo() {
        System.out.println(Thread.currentThread().getName() + "t 构造方法SingletonDemo");
    }
    public static SingletonDemo getInstance(){

        if(singletonDemo == null){
            singletonDemo = new SingletonDemo();
        }
        return singletonDemo;
    }

    public static void main(String[] args) {
        System.out.println(SingletonDemo.getInstance() == SingletonDemo.getInstance());
        System.out.println(SingletonDemo.getInstance() == SingletonDemo.getInstance());
        System.out.println(SingletonDemo.getInstance() == SingletonDemo.getInstance());
    }
}
console输出
main	 构造方法SingletonDemo
true
true
true

   2. 多线程情况下

public class SingletonDemo {
    private static SingletonDemo singletonDemo = null;

    public SingletonDemo() {
        System.out.println(Thread.currentThread().getName() + "t 构造方法SingletonDemo");
    }
    public static SingletonDemo getInstance(){

        if(singletonDemo == null){
            singletonDemo = new SingletonDemo();
        }
        return singletonDemo;
    }

    public static void main(String[] args) {
        for (int i = 0; i < 10; i++) {
            new Thread(()->{
                SingletonDemo.getInstance();
            },String.valueOf(i)).start();
        }
    }
}
console输出：
0	 构造方法SingletonDemo
7	 构造方法SingletonDemo
3	 构造方法SingletonDemo
5	 构造方法SingletonDemo
1	 构造方法SingletonDemo

  3.解决办法：

        （1）加synchronized

    public static synchronized SingletonDemo getInstance(){

        if(singletonDemo == null){
            singletonDemo = new SingletonDemo();
        }
        return singletonDemo;
    }
console输出：
0	 构造方法SingletonDemo
 缺点：在单例模式下加synchronized 能够保证数据的一致性，但并发性下降

       （2）DCL模式（double check lock）

    public static synchronized SingletonDemo getInstance(){

        if(singletonDemo == null){
            synchronized (SingletonDemo.class){
                if(singletonDemo == null){
                    singletonDemo = new SingletonDemo();
                }
            }
         
        }
        return singletonDemo;
    }
console输出：
0	 构造方法SingletonDemo
缺点：(双端检锁)机制不一定线程安全，原因时有指令重排的存在，加入volatile可以禁止指令重排

     （3） 缺点：(双端检锁)机制不一定线程安全，原因时有指令重排的存在，加入volatile可以禁止

                         指令重排。

  private static volatile SingletonDemo singletonDemo = null;