Java多线程volatile原理及用法解析

package com.designmodal.design.juc01;

import java.util.concurrent.TimeUnit;

/**
 * @author D-L
 * @Classname T001_volatile
 * @Version 1.0
 * @Description volatile 保证线程的可见性
 * @Date 2020/7/19 17:30
 */
public class T001_volatile {
  //定义一个变量running
  volatile boolean running = true;

  public void m(){
    while(running){
      //TODO 不做任何的处理
      System.out.println("while is running When can I stop -------------");
    }
    System.out.println("method is end ---------------");
  }

  public static void main(String[] args) {
    T001_volatile t001_volatile = new T001_volatile();
    new Thread(t001_volatile::m , "Thread t1").start();

    //停一秒
    try {
      TimeUnit.SECONDS.sleep(1);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    //修改running的值
    t001_volatile.running = false;
  }
}

package com.designmodal.design.juc01;

import java.util.ArrayList;
import java.util.List;

/**
 * @author D-L
 * @Classname VolatileAndSynchronized
 * @Version 1.0
 * @Description synchronized can not be replaced by volatile
 *        volatile 不能代替synchronized
 *        只能保证可见性 不能保证原子性
 *        count++ 不是原子性操作
 * @Date 2020/xx/xx 23:25
 */
public class VolatileAndSynchronized {
  volatile int count = 0;
  public synchronized void m(){
    for (int i = 0; i < 1000; i++) {
      //非原子性操作 汇编指令至少有三条
      count++;
    }
  }

  public static void main(String[] args) {
    VolatileAndSynchronized v = new VolatileAndSynchronized();
    List<Thread> threads = new ArrayList<>();
    for (int i = 0; i < 10; i++) {
      threads.add(new Thread(v::m , "Thread"+ i));
    }
    threads.forEach(o ->o.start());
    threads.forEach(o ->{
      try {
        o.join();
      } catch (InterruptedException e) {
        e.printStackTrace();
      }
    });
    System.out.println(v.count);
  }
}

package com.designmodal.design.juc01;

import java.util.concurrent.TimeUnit;

/**
 * @author D-L
 * @Classname T002_volatile
 * @Version 1.0
 * @Description volatile 指令重排序
 * @Date 2020/7/20 00:48
 */
public class T002_volatile {
  //创建私有的 T002_volatile 有人会问这里的volatile要不要使用，这里的答案是肯定的
  private static /**volatile*/ volatile T002_volatile INSTANCE;

  public T002_volatile() {}

  public T002_volatile getInstance(){
    //模拟业务代码 这里为了synchronized更加细粒度，所以使用了双重检查
   if(INSTANCE == null){
     synchronized (this){
       //双重检查
       if(INSTANCE == null){
         //避免线程之间的干扰 在这里睡一秒
         try {
           TimeUnit.SECONDS.sleep(1);
         } catch (InterruptedException e) {
           e.printStackTrace();
         }
         //创建实例对象
         INSTANCE = new T002_volatile();
       }
     }
   }
   return INSTANCE;
  }

  /**
   * 创建100个线程 调用getInstance() 打印hashcode值
   * @param args
   */
  public static void main(String[] args) {
    T002_volatile t001_volatile = new T002_volatile();
    for (int i = 0; i < 100; i++) {
      new Thread(() ->{
        T002_volatile instance = t001_volatile.getInstance();
        System.out.println(instance.hashCode());
      }).start();
    }

  }
}