The other day, one of my websites was not available anymore. Looking at the log files, I found the following exception:

Dec 7, 2011 1:22:39 AM org.apache.jk.common.ChannelSocket acceptConnections
WARNING: Exception executing accept
java.net.SocketException: Too many open files
	at java.net.PlainSocketImpl.socketAccept(Native Method)
	at java.net.PlainSocketImpl.accept(PlainSocketImpl.java:384)
	at java.net.ServerSocket.implAccept(ServerSocket.java:450)
	at java.net.ServerSocket.accept(ServerSocket.java:421)
	at org.apache.jk.common.ChannelSocket.accept(ChannelSocket.java:307)
	at org.apache.jk.common.ChannelSocket.acceptConnections(ChannelSocket.java:661)
	at org.apache.jk.common.ChannelSocket$SocketAcceptor.runIt(ChannelSocket.java:872)
	at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:690)
	at java.lang.Thread.run(Thread.java:595)

This was the first time I was getting this exception. What is even stranger is that I didn’t change anything on this application for quite a while!
Anyway, I first did what I usually do with Tomcat: restart it! This fixed the issue but only for a few hours before it crashed again.

After some investigation, it seems that Tomcat was reaching the limit of open file descriptors allowed in this machine (1024 in my case).
To get the maximum number of open file descriptors, simply type the following command:

ulimit -n

It is possible to increase this value by editing the file /etc/security/limits.conf and adding the new limit for the user running Tomcat. However, this is not recommended as 1024 should be sufficient.

The second thing I did was to check the list of open files used by the Tomcat process:

lsof -p 

What I found by running this command was a bit odd. It seems that Tomcat was having a multitude of opened connections to one of the web services used by the application. So it looks like the connections between my website and the web service were never closed!  😐
Because I didn’t change the code on my side, I asked the third party who owns the web service to check their code. I don’t know what was the root cause of the problem but they fixed it on their side and it is now working fine.

In conclusion, if you get the same exception, try to find where the problem is coming from before increasing the maximum number of open file descriptors. 😉