Java IOException “Too many open files”

I’m doing some file I/O with multiple files (writing to 19 files, it so happens). After writing to them a few hundred times I get the Java IOException: Too many open files. But I actually have only a few files opened at once. What is the problem here? I can verify that the writes were successful.

Answer

On Linux and other UNIX / UNIX-like platforms, the OS places a limit on the number of open file descriptors that a process may have at any given time. In the old days, this limit used to be hardwired¹, and relatively small. These days it is much larger (hundreds / thousands), and subject to a “soft” per-process configurable resource limit. (Look up the ulimit shell builtin …)

Your Java application must be exceeding the per-process file descriptor limit.

You say that you have 19 files open, and that after a few hundred times you get an IOException saying “too many files open”. Now this particular exception can ONLY happen when a new file descriptor is requested; i.e. when you are opening a file (or a pipe or a socket). You can verify this by printing the stacktrace for the IOException.

Unless your application is being run with a small resource limit (which seems unlikely), it follows that it must be repeatedly opening files / sockets / pipes, and failing to close them. Find out why that is happening and you should be able to figure out what to do about it.

FYI, the following pattern is a safe way to write to files that is guaranteed not to leak file descriptors.

Writer w = new FileWriter(...);
try {
    // write stuff to the file
} finally {
    try {
        w.close();
    } catch (IOException ex) {
        // Log error writing file and bail out.
    }
}

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^{1 – Hardwired, as in compiled into the kernel. Changing the number of available fd slots required a recompilation … and could result in less memory being available for other things. In the days when Unix commonly ran on 16-bit machines, these things really mattered.}

UPDATE

The Java 7 way is more concise:

try (Writer w = new FileWriter(...)) {
    // write stuff to the file
} // the `w` resource is automatically closed 

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UPDATE 2

Apparently you can also encounter a “too many files open” while attempting to run an external program. The basic cause is as described above. However, the reason that you encounter this in exec(...) is that the JVM is attempting to create “pipe” file descriptors that will be connected to the external application’s standard input / output / error.