Java 比较StringBuilder和Stringbuffer 线程安全性

港控/mmm° 2021-11-09 13:58 284阅读 0赞

直接上代码：
    1.StringBuffer

package com.thread;
    
    import javax.annotation.concurrent.ThreadSafe;
    import java.util.concurrent.CountDownLatch;
    import java.util.concurrent.ExecutorService;
    import java.util.concurrent.Executors;
    import java.util.concurrent.Semaphore;
    import java.util.concurrent.atomic.AtomicInteger;
    
    @ThreadSafe
    public class SafeCountExample {
    
        //请求总数
        private static int clientTotal = 5000;
        //线程数量
        private static int threadTotal = 200;
        private static StringBuffer stringBuffer =  new StringBuffer();
        //private static StringBuilder stringBuilder =  new StringBuilder();
    
        public static void main(String[] args) throws  Exception{
    
            ExecutorService exec = Executors.newCachedThreadPool();
            final Semaphore semaphore = new Semaphore(threadTotal);
            //每次固定数量的线程获取许可
            final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
            for (int i = 0; i < clientTotal ; i++) {
                exec.execute(()->{
                    try {
                        semaphore.acquire();
                        add();
                        semaphore.release();
                    }catch (Exception e){
                        e.printStackTrace();
                    }
                    countDownLatch.countDown();
                });
            }
            countDownLatch.await();
            exec.shutdown();
            System.out.println(stringBuffer.length());
           // System.out.println(stringBuilder .length());
    
        }
        private static void add(){
            stringBuffer.append("1");
           // stringBuilder .append("1");
        }
    }

1.1 多次运行，发现是线程安全的，结果都一样如下，：

![20190801153252395.png][]

2.StringBuilder.我们放开红色被注释的代码，多次运行，发现线程是不安全的，结果都不一样：

![20190801155554173.png][]

![2019080115593368.png][]

[20190801153252395.png]: /images/20211109/f8c40920c6c74503b44aaba0d3885cf3.png
[20190801155554173.png]: /images/20211109/584e59c32d774a538d3291537746b0f7.png
[2019080115593368.png]: /images/20211109/bd2f5b4dc2c54a0fbf22eb9c47d714ed.png