Why add synchronization keywords in the getOrInitExecutors method after using concurrent collection classes?

 private static Map<String, ExecutorService> executors = new ConcurrentHashMap<>();



 
    public static ExecutorService getOrInitExecutors(String poolName, int poolSize){
        ExecutorService executor = executors.get(poolName);
        if(null == executor){
            synchronized (TaskProcessUtil.class){
                executor = executors.get(poolName);
                if(null == executor){
                    executor = init(poolName, poolSize);
                    executors.put(poolName, executor);
                }
            }
        }
        return executor;
    }

I want to know the reason for writing this way