Finding the cause for waiting/sleeping threads

Question

I noticed that my java application (runs on tomcat6) spawns a lot of threads which do not terminate.

So I created a thread dump and noticed that there are tons of threads waiting, like this:

"pool-1-thread-22" prio=5 tid=101b4b000 nid=0x127122000 waiting on condition [127121000]    java.lang.Thread.State: WAITING (parking)     at sun.misc.Unsafe.park(Native Method)     - parking to wait for  <6c340cee0> (a java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject)     at java.util.concurrent.locks.LockSupport.park(LockSupport.java:156)     at java.util.concurrent.locks.AbstractQueuedSynchronizer$ConditionObject.await(AbstractQueuedSynchronizer.java:1987)     at java.util.concurrent.LinkedBlockingQueue.take(LinkedBlockingQueue.java:399)     at java.util.concurrent.ThreadPoolExecutor.getTask(ThreadPoolExecutor.java:947)     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:907)     at java.lang.Thread.run(Thread.java:680)    Locked ownable synchronizers:     - None 

Now the question is: WHAT are these threads waiting for? I have suspect class which seems to spawn these threads but I don't know what exactly is making these threads stuck.

Is there anything I can do to find the cause for this except for tearing the class apart line by line and keep monitoring thread behavior?

Answer 1

On tomcat, they're usually request worker threads waiting for someone to connect. Nothing to worry about. They're ready to handle those 100 users connecting at once to your server.

Answer 2

Those threads are part of a ThreadPool. More specificaly java.util.concurrent.ThreadPoolExecutor. The thread is waiting on a Runnable/Callable to be submitted to the pool. For example

ExecutorService e = Executors.newFixedThreadPool(10); 

Will create 10 threads that will sit an wait until

e.submit(new Runnable(){   public void run(){ ...} }); 

Then one thread will be notified and invoke that runnable. What they are being used for I cannot tell. You'll have to find out what started the thread pool. Maybe its handling client requests to the application server.