A warning about Runtime.getRuntime.exec() : You may be leaking File Descriptors

Normally Java’s online documentation is very informational, but this not one of those times. Recently, I had run into an issue where my application was issuing: java.io.IOException: error=24, Too many open files.

If you are making a lot of use of the Java’s Runtime.getRuntime.exec() or ProcessBuilder, then its very possible you are leaking File descriptors. Unix systems have a limit to how many files can be opened at any time by a given process. On Ubuntu 10.04, that limit is 1024 files. You can find out your system’s max limit per process by issuing: ulimit -n

Every time you execute a process using Java’s Runtime, STDIN, STDOUT, and STDERR are piped to Java as an Output or Input Stream. For each stream, there exists a pair of file descriptors. When the process has terminated, these streams are closed (in general) at the process side, closing on of the two file descriptors for that stream. If your Java program continues on, and you don’t use those streams in Java, then you are leaking the File descriptors on the Java side of the pipe (assuming your not specifically closing them or destroying the process). Normally, these will be closed when Garbage Collecting occurs, but you shouldn’t leave it up to the Garbage Collector to decide.

After you use ProcessBuilder or Runtime.getRuntime.exec(), you can do one of two things:

  1. Use a handler to the Process that was executed (say: Process p), and destroy it (Note: This also terminates the process if its running).  p.destroy();
  2. Close all the input streams if you haven’t already:
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Process p = Runtime.getRuntime().exec( <command></command> );
p.getInputStream().close();
p.getOutputStream().close();
p.getErrorStream().close();

Of course, if you used one of the above streams within say: a BufferedReader, you just need to close the buffered reader, and the underlying streams will be closed automatically, Once these streams are closed, their file descriptors are removed. If you want to take a look at a list of opens files: lsof

If you can get the PID of your Java process, you can filter out most of results from lsof by doing: lsof -p 1234

Getting a count is also easy:

lsof | wc -l

lsof -p 1234 | wc -l

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