public class ConcurrentHashMapExample2 {

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

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

private static Map<String,Integer> map = new ConcurrentHashMap<>();

public static void main(String[] args) throws Exception{
    ExecutorService executorService = Executors.newCachedThreadPool();
    final Semaphore semaphore = new Semaphore(threadTotal);
    final CountDownLatch countDownLatch = new CountDownLatch(clientTotal);
    final String key = "key";
    for (int i=0;i<clientTotal;i++){

        executorService.execute(() ->{
            try {
                semaphore.acquire();
                add(key);
                semaphore.release();
            } catch (Exception e) {
                log.error("exception",e);
            }
            countDownLatch.countDown();
        });
    }
    countDownLatch.await();//保证减少到0
    executorService.shutdown();
    log.info("size:{}", map.get(key));
}

private static void add(String key){
    Integer value = map.get(key);
    if (value == null) {
        map.put(key, 1);
    } else {
        map.put(key, value + 1);
    }
}

}
我能知道线程不安全的原因是判空的地方有问题，但是应该怎么改才能变成线程安全的，除了在方法上加synchronized？