Javaでのマルチスレッドの安全性のケース分析

ケーススタディ

package com.duyang.thread.basic.basethread;

/**
 * @author ：jiaolian
 * @date ：Created in 2020-12-16 14:02
 * @description：线程不安全分析
 * @modified By：
 * 公众号:叫练
 */
public class ThreadUnsafe {

    public static void main(String[] args) {
        Thread task = new Task();
        Thread threadA = new Thread(task,"A");
        Thread threadB = new Thread(task,"B");
        Thread threadC = new Thread(task,"C");
        Thread threadD = new Thread(task,"D");
        Thread threadE = new Thread(task,"E");
        threadA.start();
        threadB.start();
        threadC.start();
        threadD.start();
        threadE.start();
    }

    private static class Task extends Thread {

        int count = 5;
        @Override
        public void run() {
            /**
             * jvm分3步骤;
             * 1.获取count(从主内存获取值)
             * 2.count减1(在各自寄存器完成)
             * 3.保存count(刷新到主内存)
             *
             * 说下可能执行的过程...
             * A线程获取cpu的count值为5，A线程先减去1，保存count值为4刷新到主内存,此时还没有执行System.out.println count
             * 切换到B线程，此时B线程的count值为4，因为B线程是从主内存取的，B线程count值减去1为3，此时刷新到主内存，主内存值变为3
             * 切换到A线程，执行System.out.println count=3
             * 切换到B线程，执行System.out.println count=3
             * 情况就是这样的
             *
             */
            count--;
            System.out.println(Thread.currentThread().getName() + " "+count);
        }
    }
}