How to fix java.net.SocketException: Too many files open java.io.IOException in Tomcat, Weblogic Server

Not many Java programmers know that socket connections are treated like files and they use file descriptors, which is a limited resource. The different operating system has different limits on the number of file handles they can manage. One of the common reasons for java.net.SocketException: Too many files open in Tomcat, Weblogic, or any Java application server is, too many clients connecting and disconnecting frequently a very short span of time. Since Socket connection internally uses TCP protocol, which says that a socket can remain in TIME_WAIT state for some time, even after they are closed. One of the reasons to keep the closed socket in the TIME_WAIT state is to ensure that delayed packets reached the corresponding socket.


The different operating system has different default time to keep sockets in TIME_WAIT state, in Linux it's 60 seconds, while in Windows is 4 minutes. Remember longer the timeout, the longer your closed socket will keep file handle, which increases the chances of java.net.SocketException: Too many files open exception. 

This also means, if you are running Tomcat, Weblogic, Websphere, or any other web server in windows machine, you are more prone to this error than Linux based systems e.g. Solaris or Ubuntu. 

By the way, this error is the same as java.io.IOException: Too many files open exception, which is thrown by code from IO package if you try to open a new FileInputStream or any stream pointing to file resource.

How to solve java.net.SocketException: Too many files open

Now, we know that this error is coming because clients are connecting and disconnecting frequently. If that seems unusual to your application, you can find the culprit client and prohibit them from reconnecting from making a connection, but if that is something, your application may expect and you want to handle it on your side, you have two options :

1) Increase the number of open file handles or file descriptors per process.

2) Reduce timeout for TIME_WAIT state in your operating system

In UNIX based operating system e.g. Ubuntu or Solaris, you can use the command ulimit -a to find out how many open file handles per process is allowed.

$ ulimit -a

core file size        (blocks, -c) unlimited

data seg size         (kbytes, -d) unlimited

file size             (blocks, -f) unlimited

open files                    (-n) 256

pipe size          (512 bytes, -p) 10

stack size            (kbytes, -s) 8192

cpu time             (seconds, -t) unlimited

max user processes            (-u) 2048

virtual memory        (kbytes, -v) unlimited

You can see that, open files (-n) 256, which means only 256 open file handles per process is allowed. If your Java program, remember Tomcat, weblogic or any other application server are Java programs and they run on JVM, exceeds this limit, it will throw java.net.SocketException: Too many files open error.

You can change this limit by using ulimit -n to a larger number e.g. 4096, but do it with the advice of the UNIX system administrator and if you have a separate UNIX support team, than better escalate to them.

Another important thing to verify is that your process is not leaking file descriptors or handles, well that's a tedious thing to find out, but you can use lsof command to check how many open file handles is owned by a particular process in UNIX or Linux. You can run lsof command by providing PID of your process, which you can get it from ps command.

Similarly, you can change TIME_WAIT timeout but do with consultation of UNIX support, as a really low time means, you might miss delayed packets. In UNIX based systems, you ca n see current configuration in /proc/sys/net/ipv4/tcp_fin_timeout file. In Windows based system, you can see this information in the windows registry. You can change the TCP TIME_WAIT timeout in Windows by following below steps :

1) Open Windows Registry Editor, by typing regedit in run command window

2) Find the key HKEY_LOCAL_MACHINE\System\CurrentControlSet\Services\tcpip\Parameters

3) Add a new key value pair TcpTimedWaitDelay asa decimal and set the desired timeout in seconds (60-240)

4) Restart your windows machine.

Remember, you might not have permission to edit windows registry, and if you are not comfortable, better not to do it. Instead ask Windows Network support team, if you have any,  to do that for you. Bottom line to fix java.net.SocketException: Too many files open, is that either increasing number of open file handles or reducing TCP TIME_WAIT timeout. java.net.SocketException: Too many files open issue is also common among FIX Engines, where client use TCP/IP protocol to connect with brokers FIX servers. 

Since FIX engines needs correct value of incoming and outgoing sequence number to establish FIX session, and if client tries to connect with a smaller sequence number than expected at brokers end, it disconnects the session immediately. 

If the client is well behind, and keep retrying by increasing sequence number by 1, it can cause java.net.SocketException: Too many files open at brokers end. To avoid this, let's FIX engine keep track of it's sequence number, when it restart. In short, "java.net.SocketException: Too many files open" can be seen any Java Server application e.g. Tomcat, Weblogic, WebSphere etc, with client connecting and disconnecting frequently.