Did you also get a message showing which Threads are involved here (the 'Locked By' etc) ?

If so: check the Stack Trace for the 'Locked by' (and 'Originally Locked by' - if different); the long running thread and triggered the Thread Dump will just show the thread-dump being requested; you want to see what was holding up the thread in the first place.

It's a good idea to understand the root cause of the thread dumps in UAT. It could be your chance to prevent some significant issue before things get into production. 

Open your log file with Notepad++ and search for --- Thread Dump Starts ---.

You will notice log entries related to threads at different states. Below are few examples.

Notice the WAITING on lock: parts.

"hz._hzInstance_2_c163b7d2a7487205b5c9848d4f29dc92.cached.thread-5" Id=1671251 in WAITING on lock=java.util.concurrent.SynchronousQueue$TransferStack@73518dbf
BlockedCount : 44, BlockedTime : -1, WaitedCount : 94161, WaitedTime : -1
    at sun.misc.Unsafe.park(Native Method)

"Thread-412 (HornetQ-client-global-threads-497038464)" Id=1316449 in TIMED_WAITING on lock=java.util.concurrent.SynchronousQueue$TransferStack@6b349e51
BlockedCount : 3, BlockedTime : -1, WaitedCount : 12652, WaitedTime : -1

Analyzing thread dumps could be a time consuming effort. Here is a great place to start.

Thanks,

Jay