Verification
Check your system limits:

$ cat /proc/sys/fs/file-max
$ sysctl -a |grep fs.file-max


To check the number of file descriptors used by any given process use

$ ls -la /proc/<pid>/fd
or use the lsof (LiSt Open Files) command

$ lsof -p <pid of process>
or (my favourite)

$ lsof -p <pid> | wc -l
non of the above were sufficient under Ubuntu. Please run

$ ulimit -n
1024 is too few. Note that ulimit is built into bash, not a program of its own. If you use something else, type bash to change the shell for the ulimit command. Please apply both steps of the following solution


Solution
System wide limit change
Change limit by editing /etc/sysctl.conf and set fs.file-max to 400000
Then run

$ sysctl -p
to apply your changes to your system. May need reboot.


Per user/process limit change
For Linux systems running PAM (that is e.g. Ubuntu)  you will need to adjust /etc/security/limits.conf
tomcatservice hard nofile 5000



ulimit -n5000    has not worked for me in the past, but can always give it a try.