How to take Java Thread Dump?

A java thread dump is a way of finding out what each thread in the JVM is doing at a particular point of time. This is especially useful when your java application seems to hang when running under load. Thread dump will help you to find out where the threads are stuck.

How to take thread dump on UNIX:

First, find the process id by looking in the process table. You can generally get the process numbers of all running Java processes with the command:

ps axf | grep java

or
ps -ef | grep java | grep -v grep

Run the following command to take the thread dump.

Kill –QUIT process_id

The thread dump will be sent to where ever the standard output is redirected to. (In tomcat, normally the thread dump will be sent to TOMCAT_HOME/logs/Catalina.out)
QUIT signal does not actually kill the java process. The thread dump will be sent to the standard output and the process will continue.

How to take thread dump on Windows:

press CTRL+Break

The thread dump is printed in the command window, and you must mark and cut / paste to a separate file in order to continue working on it.

What happens when you take thread dump?

i). The Java process is paused — all threads simply stop dead in their tracks
ii)). The Main java process asks each thread in turn to give a complete account of what they’re doing
iii). The thread dump is sent to standard error, or somewhere else, depending on your Java vendor
iv)-. The Java process is unpaused — all threads simply continue where they left off.
v)-The Java process usually keeps on running, and the whole process only takes a few seconds. Any activity, even input/output is suspended. After the thread dump has completed, everything returns back to normal, just as if nothing had happened.


A sample thread dump is given below.