Tomcat too many open files

tomcat运行一段时间之后出现文件句柄数过多的情况,
java.net.SocketException: Too many open files

网上解决方法

A couple of days ago we ran into the infamous “too many open files” when our Tomcat web server was under load.

There are several blogs around the internet that tries to deal with this issue but none of them seemed to do the trick for us. Usually what you do is to set the ulimit to a greater value (it’s something like 1024 by default). But in order to make it permanent after reboot the first thing suggested is to update the /proc/sys/fs/file-max file and increase the value then edit the /etc/security/limits.conf and add the following line * - nofile 2048 (see here for more details). But none of this worked for us. We saw that when doing

cat /proc/<tomcat pid>/limits

the limit was still set to the initial value of 1024:

Limit                     Soft Limit           Hard Limit           Units
Max cpu time              unlimited            unlimited            seconds
Max file size             unlimited            unlimited            bytes
Max data size             unlimited            unlimited            bytes
Max stack size            8388608              unlimited            bytes
Max core file size        0                    unlimited            bytes
Max resident set          unlimited            unlimited            bytes
Max processes             63810                63810                processes
<b>Max open files                1024                     1024                    files</b>
Max locked memory         65536                65536                bytes
Max address space         unlimited            unlimited            bytes
Max file locks            unlimited            unlimited            locks
Max pending signals       63810                63810                signals
Max msgqueue size         819200               819200               bytes
Max nice priority         0                    0
Max realtime priority     0                    0
Max realtime timeout      unlimited            unlimited            us

It was not until we found this thread that the reason and solution became clear. Our Tomcat instance was started as a service during boot and there’s a bug discovered and filed (with patch) in 2005 that doesn’t seem to have been resolved yet. The bug reveals itself by ignoring the max number of open files limit when starting daemons in Ubuntu/Debain. So the work-around suggested by “BOK” was to edit /etc/init.d/tomcat and add:

ulimit -Hn 16384
ulimit -Sn 16384

Finally the max number of open files for Tomcat was increased!

以上摘自：https://blog.jayway.com/2012/02/11/how-to-really-fix-the-too-many-open-files-problem-for-tomcat-in-ubuntu/

分析

1. 打开的文件过多，一般来说是由于应用程序对资源使用不当造成，比如没有及时关闭Socket或数据库连接等。但也可能应用确实需要打开比较多的文件句柄，而系统本身的设置限制了这一数量。

2. Tomcat将主动打开尽可能多的文件句柄，并将硬限制设置为如此高的数量将导致有问题的虚拟机可以消耗大量内存。

3. 查看系统允许打开的最大文件数
   cat /proc/sys/fs/file-max
   
   tomcat文件句柄数
   cat /proc/<tomcat pid>/limits

4. 不仅要关注系统最大文件句柄数，还要关注tomcat启动能使用的最大句柄数。

5. 在I/O操作之后需要在finally里调用close()。