Java ConcurrentSkipListMap卡在某些线程中

Question

I have this container

private ConcurrentMap<Integer,SortedMap<Long,Object>> users;

Which gets initialized in constructor users = new ConcurrentHashMap<>();

and for each user it gets new ConcurrentSkipListMap<>()

I have this method which is called over and over again in threadpool

private void process(Object obj){
    // SOME CODE

    SortedMap<Long,Object> q = users.get( obj.userId);

    logger.debug("SECTION 1");

    q.headMap( someNumber ).clear();

    logger.debug("SECTION 2");

    q.put( obj.someId, obj );

    logger.debug("SECTION 3");
}

this method is called using executorService = Executors.newScheduledThreadPool(8);

executorService.execute(new Runnable() {
            @Override
            public void run() {
                process(obj);
            }
        });

sometimes it works fine but sometimes (I suppose on high load) for some user process methods (all calls) get stuck and never outputs SECTION 2

and if I swap q.headMap( someNumber ).clear(); and q.put( obj.someId, obj ); it still doesn't output SECTION 2

I also tried LinkedBlockingQueue instead of ConcurrentSkipListMap but I have same issue

it looks like a deadlock but I'm not using any synchronized statement on that structure.

please share your idea if you have one

Answer 1

There could multiple causes; the most obvious one is unsynchronized code. Take thread-dumps to easily identify the cause.

Some issues:

In ConcurrentSkipListMap JavaDocs it says at headMap() :

The returned map will throw an IllegalArgumentException on an attempt to insert a key outside its range.

So if obj.someId.compareTo(someNumber) < 0 you can get an IllegalArgumentException at put() which is not treated anywhere.

Also this can lead to race conditions:

q.headMap( someNumber ).clear();
q.put( obj.someId, obj )

if two threads have (thread1) someNumber = (thread2) obj.someId you get a race condition and who know what happens underneath. Moreover, as JavaDocs say clear() is not an atomical operation.

And you did not mention the other place where you use users referring object; how are users populated.

Answer 2

It seams that problem is not that code gets stuck but it throws exception which is handled silently by theadpool. I tried to push custom ThreadFactory with logging enabling but it still doesn't works. solution is to provide proper try catching for RuntimeException s

So lesson I've learned is that if part of your code doesn't get executed inside threadpool it might be because of the RuntimeException and you should provide more try catching to find the cause