How can I debug a hanging java thread?

Question

I have the following problem:
I deploy a web application in Tomcat (Linux) and after shutdown of Tomcat, if I do ps -ef I still can see the java process running.
I believe this happens due to some hanging thread but I don't know how can I track this thread down.
How can I debug this issue?

Answer 1

You can generate 4-5 thread dumps as described below and then analyze them using tools like Samurai.

What you want to check is when a stuck thread or long running transaction happens, all the thread dumps will show a certain thread id is at the same line in your java stack trace. In simpler terms, the transaction is spanning across multiple thread dumps and hence needs more investigation.

Now when you run these through Samurai, it will highlight these in Red colour so you can quickly click on it and get to the lines showing issues.

See an example of this here. Look at the Samurai output image in that link. The Green cells are fine. Red and Grey cells need looking at.

Generating a Thread Dump:

(Linux)

If the JVM is running in a console then simply press Ctrl-\. If the JVM is running in the background then send it the QUIT signal:

kill -QUIT process_id

There process_id is the process number of the running Java process. The thread dump will be sent to wherever standard out is redirected too. You can generally get the process numbers of of all running Java processes with the command:

ps axf | grep java

Answer 2

You say your java process still exists, right? Processes exist as long as they have attached threads, right? If so, I would go for the following approach: - run the process with the MBean server attached and managed internally by the JVM

Then connect to the process after you send the QUIT signal and get the thread dump (there should be a JMX for that. See which threads look suspicious to you.

I think you can also use JVisualVM to take thread dumps...