以下代码演示了 1000 个线程同时对 cnt 执行自增操作，操作结束之后它的值有可能小于 1000。

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class UnsafeExemple {
    public  class Exemple{
        private int cnt=0;
        public  void add(){
            cnt++;
        }
        public  int res(){
            return cnt;
        }
    }
    public static void main(String[]args)throws InterruptedException{
        int threadSize=1000;
        UnsafeExemple e=new UnsafeExemple();
        Exemple exemple=e.new Exemple();
        CountDownLatch countDownLatch=new CountDownLatch(threadSize);
        ExecutorService executorService= Executors.newCachedThreadPool();
        for(int i=0;i<threadSize;i++){
            executorService.execute(()->{
                exemple.add();
//                System.out.println(exemple.res());
            }
            );
            countDownLatch.countDown();
        }
        countDownLatch.await();
        executorService.shutdown();
        System.out.println(exemple.res());
    }
}

  解决线程不安全的方法为对add()方法上锁

import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class UnsafeExemple {
    public  class Exemple{
        private int cnt=0;
        public synchronized void add(){
            cnt++;
        }
        public  int res(){
            return cnt;
        }
    }
    public static void main(String[]args)throws InterruptedException{
        int threadSize=1000;
        UnsafeExemple e=new UnsafeExemple();
        Exemple exemple=e.new Exemple();
        CountDownLatch countDownLatch=new CountDownLatch(threadSize);
        ExecutorService executorService= Executors.newCachedThreadPool();
        for(int i=0;i<threadSize;i++){
            executorService.execute(()->{
                exemple.add();
//                System.out.println(exemple.res());
            }
            );
            countDownLatch.countDown();
        }
        countDownLatch.await();
        executorService.shutdown();
        System.out.println(exemple.res());
    }
}