public class Main {

    private static int THREAD_COUNT = 10;
    private static int ITEM_COUNT = 1000;

    private static ConcurrentHashMap<String, Long> getData(int count) {

return  LongStream.rangeClosed(1, count)
        .boxed().collect(Collectors.toConcurrentMap(i-> UUID.randomUUID().toString(), Function.identity(),(t1, t2)->t1,ConcurrentHashMap::new));
    }

    public static void main(String[] args) throws InterruptedException {
        ConcurrentHashMap<String, Long> stringLongConcurrentHashMap = getData(ITEM_COUNT-100);
        System.out.println("init siez:"+stringLongConcurrentHashMap.size());
        ForkJoinPool forkJoinPool = new ForkJoinPool(THREAD_COUNT);
        forkJoinPool.execute(()-> IntStream.rangeClosed(1,10).parallel().forEach(i->{
               

                int gap=ITEM_COUNT-stringLongConcurrentHashMap.size();
                System.out.println("gap siez:"+gap);

                stringLongConcurrentHashMap.putAll(getData(gap));

               
        }));
        forkJoinPool.shutdown();
        forkJoinPool.awaitTermination(1, TimeUnit.HOURS);
        System.out.println("finish siez:"+stringLongConcurrentHashMap.size());



    }
}

上面没添加锁输出以下结果

init siez:900
gap siez:100
gap siez:100
gap siez:100
gap siez:100
gap siez:100
gap siez:100
gap siez:100
gap siez:100
gap siez:100
gap siez:100
finish siez:1900

添加锁之后

public class Main {

    private static int THREAD_COUNT = 10;
    private static int ITEM_COUNT = 1000;

    private static ConcurrentHashMap<String, Long> getData(int count) {

return  LongStream.rangeClosed(1, count)
        .boxed().collect(Collectors.toConcurrentMap(i-> UUID.randomUUID().toString(), Function.identity(),(t1, t2)->t1,ConcurrentHashMap::new));
    }

    public static void main(String[] args) throws InterruptedException {
        ConcurrentHashMap<String, Long> stringLongConcurrentHashMap = getData(ITEM_COUNT-100);
        System.out.println("init siez:"+stringLongConcurrentHashMap.size());
        ForkJoinPool forkJoinPool = new ForkJoinPool(THREAD_COUNT);
        forkJoinPool.execute(()-> IntStream.rangeClosed(1,10).parallel().forEach(i->{
               synchronized (stringLongConcurrentHashMap){

                int gap=ITEM_COUNT-stringLongConcurrentHashMap.size();
                System.out.println("gap siez:"+gap);

                stringLongConcurrentHashMap.putAll(getData(gap));

               }
        }));
        forkJoinPool.shutdown();
        forkJoinPool.awaitTermination(1, TimeUnit.HOURS);
        System.out.println("finish siez:"+stringLongConcurrentHashMap.size());



    }
}

输出结果
init siez:900
gap siez:100
gap siez:0
gap siez:0
gap siez:0
gap siez:0
gap siez:0
gap siez:0
gap siez:0
gap siez:0
gap siez:0
finish siez:1000