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Ok, so I have a couple of Java programs that I'm running using a chron job on a linux server. These jobs run every ten minutes or so, take literally two minutes to run, and then exit. I need to add a way for the programs to detect, when they start up, if there is already an instance of themselves running, and if so to exit without going any further. I'm really not sure of the best way to handle this though and am hoping someone can offer some advice.

One approach I've considered is to run a command line argument from the java code that does some sort of PS command and looks through those to see if it's running. This seems pretty finicky and complex though for something so small. Plus, I'm not all that knowledgeable with linux and am not even sure the best way to do that. If anyone has some better thoughts, please let me know. Or if that is the best way, if you could provide the linux commands I'd need I'd appreciate it. Thanks.

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    Usually this is done by creating .lock file. Commented Jul 25, 2012 at 15:11
  • Not actually java related at all. Same issue applies to any language. Commented Jul 25, 2012 at 15:14
  • check out this link- rbgrn.net/content/43-java-single-application-instance Commented Jul 25, 2012 at 15:14
  • I would wrap the call of the java-program in a shell script and check per ps and grep like ps auxww | grep <Your mainclass> The problem with the lock file is, that it is possible it's not cleaned up in case of a crash of the java program. Commented Jul 25, 2012 at 15:17

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If you have a writable /tmp directory you can use a lockfile.

When your Java program starts up, check for a file with a name unique to your application (e.g. "my-lock-file.lock") in the /tmp directory. If none exists, create one, and remove it when you're done. If one exists, just exit.

You can check the existence of a file with the .exists() method of the java.io.File class.

If your code needs to be portable, you can use System.getProperty("java.io.tmpdir")); to get an appropriate temporary directory for the platform your code is running on.

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Rather use System.getProperty("java.io.tmpdir")); to get a operating system independent temp-directory.
Good point! The OP mentioned a Linux platform, but I'll go ahead and amend my answer.
@atomicinf Thanks for the info. Didn't know about lock files so thanks to your suggestion I checked it out a bit more and found a solution that I believe should work well and clean up after itself if the code exits abruptly.
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You could look at JMX and the Attach API to query for running JVMs.

Or, as Andrew logvinov mentioned, by using a lock file.

If you are using Java WebStart, there's already native support for this.

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Many programs solve this by creating a temporary file that points to their PID (often referred to as a "lock" file). The filename should encode all relevant information to distinguish this process from other processes that could legitimately run in parallel.

For example, if the process is bound to a user, it should contain the user name. If the process is bound to a machine, it should (also) contain the hostname (if you put it in machine-bound temp. directory, this is debatable. If you put it in a home directory, think of the case of multiple machines sharing a home via NFS).

The location of these files is typically /tmp. This is a great location, as /tmp is typically wiped during system boot, so no orphan files are left in case of a system crash. Another solution employed by some programs is to put the lock file in the user settings directory, if it is related to the settings. E.g. mozilla thunderbird has a file called /home/<username>/.thunderbird/<profilename>.default/lock.

The file should contain the PID of the process. The idea is simple: If the file contains the PID, it is easy to check whether this process is indeed still running. So if the process crashes, the file gets orphaned. The new process instance will check the PID in the file, see that it is not running any more, and ignore the file (overwrite).

Putting it all together, you could create a file like this:

/tmp/myawesomeservice-username-hostname-lock

With the content:

12345
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