It is late on a Friday afternoon, and your web application has stopped responding to requests. The server is still reachable, and the Apache Tomcat process is still running–there are no errors in the logs. You want to go home but you can’t until it is fixed. What do you do?
If your answer is “restart Tomcat and hope it stays up until Monday,” then this article is for you.
Rather than keeping your fingers crossed and hoping you don’t get an angry call from your boss over the weekend, this article will provide you with some simple steps you can take to diagnose the problem.
Step 1: What is Tomcat Doing? Thread Dumps Begin to Answer the Question
If the Tomcat process is running, then it must be doing something. The question is what is it doing when it should be responding to requests? The way to answer that question is with a thread dump–actually, a series of thread dumps. You need to take three thread dumps roughly 10 seconds apart and then compare them. I always compare them with a diff tool rather than by eye—a it is far too easy to miss subtle but important differences between the dumps.
How you generate a thread dump depends on your operating system and how you are running Tomcat. On Linux, FreeBSD, Solaris etc. use kill -3 to trigger a thread dump. On Windows use CTRL-BREAK if Tomcat is running in a console window. If Tomcat is running as a service, then the service wrapper should provide a way to trigger a thread dump. Commons Daemon (the service wrapper that ships with Tomcat) provides an option to trigger a thread dump via the system tray icon.