How do you diagnose a deadlock in a running JVM?

jcmd <pid> Thread.print or jstack <pid> gives you the dump, and HotSpot does the analysis for you: it walks the monitor ownership graph and prints a "Found one Java-level deadlock" section naming both threads, the locks each holds and the lock each wants.

Two caveats worth raising, because they are what the follow-up question is about. Deadlock detection covers monitors and, since Java 6, java.util.concurrent locks that implement AbstractOwnableSynchronizer, but not a Semaphore misuse or a latch nobody counts down, which is a hang without a cycle. And a thread waiting on a ReentrantLock shows as WAITING at LockSupport.park, not BLOCKED, and its frame only says - parking to wait for <0x...> (a java.util.concurrent.locks.ReentrantLock$NonfairSync). To find the owner, take the dump with jstack -l (or jcmd <pid> Thread.print -l) and look for that address under another thread's "Locked ownable synchronizers".

Take three dumps a few seconds apart. One dump shows where threads are; three show whether they are moving.