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Java Thread Dump

A Java thread dump is a way of finding out what every thread in the JVM is doing at a particular point in time. This is especially useful if your Java application sometimes seems to hang when running under load, as an analysis of the dump will show where the threads are stuck.

You can generate a thread dump under Unix/Linux by running kill -QUIT <pid>, and under Windows by hitting Ctl + Break.

A great example of where this would be useful is the well-known Dining Philosophers deadlocking problem. Taking example code from Concurrency: State Models & Java Programs, we can cause a deadlock situation and then create a thread dump.

Dining Philosopers applet screenshot

In the example below (shown using tda), we can see that the 5 Philosopher threads each have a lock on a Fork object and are each waiting to obtain a lock on a second Fork object before they can eat. Unfortunately this never happens and all the philosophers starve.

Thread dump of the Dining Philosphers from Thread Dump Analyzer

Download the thread dump from here (7 KB).

Note that not all hangs are going to be due to deadlocks, and there are many tools (including Eclipse) that will help you analyse thread dumps.

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This entry was posted on Tuesday, April 10th, 2007 at 10:10 pm and is filed under General. You can follow any responses to this entry through the RSS 2.0 feed. You can skip to the end and leave a response. Pinging is currently not allowed.

3 Responses to “Java Thread Dump”

  1. Stuart Moncrieff Says:
    April 11th, 2007 at 8:45 am

    The Sun 1.5 JVM is able to detect some simple deadlocks.

    Using the example code from
    http://java.sun.com/docs/books/faq/src/thr/DeadlockExample.html the thread dump includes the following information:

    Found one Java-level deadlock:
    =============================
    “Thread-1″:
    waiting to lock monitor 0×00a891ec (object 0×06c216e0, a java.lang.String),
    which is held by “Thread-0″
    “Thread-0″:
    waiting to lock monitor 0×00a8920c (object 0×06c21708, a java.lang.String),
    which is held by “Thread-1″

  2. Stuart Moncrieff Says:
    April 11th, 2007 at 8:56 am

    Some additional points:

    · You may be able to generate a thread dump from the Admin Console of your application server. Under Unix, this is much simpler than doing a kill -3 from the shell, and it means that you do not have to be a member of the same group that the application server is a member of.
    · It is helpful to run multiple stack traces over a short period of time. If the same thread is still stuck at the same position of code, that is a good indication of a problem. This is a good thing to create a shell script for.
    · I quite liked this blog entry on stack traces and thread dumps: http://www.me.umn.edu/~shivane/blogs/cafefeed/2004/06/of-thread-dumps-and-stack-traces.html
    · Also, this one: http://java.sun.com/developer/technicalArticles/Programming/Stacktrace/

  3. Anthony Hallett Says:
    April 23rd, 2007 at 10:33 am

    Good to see you back posting

    :-)

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