java NIO MappedByteBuffer is not synchronizing the changes of the content

Question

I'm in a scene that multiple threads of one process A would write to several files concurrently, and then multiple threads of the process B will read from these files and resotre the content after process A is finished. The content might be huge and it is said that MappedByteBuffer could speed it up. However, I find that the changes I write does not go into the real file because when I check the content of the file using xxd command after the write process terminated, it shows all zero(I'm sure the real content bytes are not all zero because I print it out)

To address the concurrency problem, I maintained an AtomicInteger to make sure that only after all the threads has finished their jobs would I force() the changes into the disk. Here is the core code.

public void flush() {
    if(flushcount.incrementAndGet() >= threads_num.get()) //both are of type AtomicInteger
        finalflush();
}

private void finalflush() {
    flushlock.lock();
    try {
        //key is file name, value is the corresponding MappedByteBuffer
        //the limit of the file is set to 10M manually based on my needs
        for(Map.Entry<String, MappedByteBuffer> map:files.entrySet()){
            System.out.printf("%s : pos = %d, limit = %d\n", map.getKey(), map.getValue().position(), map.getValue().limit());
            map.getValue().force();
            map.getValue().load();
        }

        for(Map.Entry<String, FileChannel> map: channels.entrySet()) {
             map.getValue().close();
        }
    } catch (IOException e) {
        e.printStackTrace();
    } finally {
        flushlock.unlock();
    }
}

The output are like this

QUEUE_3 : pos = 43112, limit = 10485760
QUEUE_4 : pos = 42407, limit = 10485760
QUEUE_5 : pos = 43254, limit = 10485760
TOPIC_3 : pos = 395296, limit = 10485760
TOPIC_2 : pos = 401236, limit = 10485760
TOPIC_1 : pos = 398728, limit = 10485760
TOPIC_0 : pos = 392744, limit = 10485760

Seems pretty normal, but it just doesn't work... How can I sync these changes into the disk? Or, even if it doesn't have to be