Java并发编程 - 线程安全性之可见性

 

volatile 原理分析

package com.mmall.concurrency.example.count;

import com.mmall.concurrency.annoations.NotThreadSafe;
import lombok.extern.slf4j.Slf4j
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

@Slf4j
@NotThreadSafe
public class CountExample4 {

    // 请求总数
    public static int clientTotal = 5000;

    // 同时并发执行的线程数
    public static int threadTotal = 200;

    public static volatile int count = 0;

    public static void main(String[] args) throws Exception {
        ExecutorService executorService = Executors.newCachedThreadPool();
        final Semaphore semaphore = new Semaphore(threadTotal);
        final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
        for (int i = 0; i < clientTotal ; i++) {
            executorService.execute(() -> {
                try {
                    semaphore.acquire();
                    add();
                    semaphore.release();
                } catch (Exception e) {
                    log.error("exception", e);
                }
                countDownLatch.countDown();
            });
        }
        countDownLatch.await();
        executorService.shutdown();
        log.info("count:{}", count);
    }

    private static void add() {
        count++;
        // 1、读取count：这一步2个线程同时获取都是最新状态
        // 2、+1：这时2个线程也都 +1 操作
        // 3、写进count：然后这时2个也同时写入就丢失了 1 次
    }
}

// 输出
4993

• 所以 volatile 不具备原子性。

volatile 应用

• 两个线程同时启动，目的是为了线程2的任务必须要等线程1完成了才可以执行，感觉一下~