How to Daemonize a Java Program?

If you can't rely on Java Service Wrapper cited elsewhere (for instance, if you are running on Ubuntu, which has no packaged version of SW) you probably want to do it the old fashioned way: have your program write its PID in /var/run/$progname.pid, and write a standard SysV init script (use for instance the one for ntpd as an example, it's simple) around it. Preferably, make it LSB-compliant, too.

Essentially, the start function tests if the program is already running (by testing if /var/run/$progname.pid exists, and the contents of that file is the PID of a running process), and if not run

logfile=/var/log/$progname.log
pidfile=/var/run/$progname.pid
nohup java -Dpidfile=$pidfile $jopts $mainClass </dev/null > $logfile 2>&1

The stop function checks on /var/run/$progname.pid, tests if that file is the PID of a running process, verifies that it is a Java VM (so as not to kill a process that simply reused the PID from a dead instance of my Java daemon) and then kills that process.

When called, my main() method will start by writing its PID in the file defined in System.getProperty("pidfile").

One major hurdle, though: in Java, there is no simple and standard way to get the PID of the process the JVM runs in.

Here is what I have come up with:

private static String getPid() {
    File proc_self = new File("/proc/self");
    if(proc_self.exists()) try {
        return proc_self.getCanonicalFile().getName();
    }
    catch(Exception e) {
        /// Continue on fall-back
    }
    File bash = new File("/bin/bash");
    if(bash.exists()) {
        ProcessBuilder pb = new ProcessBuilder("/bin/bash","-c","echo $PPID");
        try {
            Process p = pb.start();
            BufferedReader rd = new BufferedReader(new InputStreamReader(p.getInputStream()));
            return rd.readLine();
        }
        catch(IOException e) {
            return String.valueOf(Thread.currentThread().getId());
        }
    }
    // This is a cop-out to return something when we don't have BASH
    return String.valueOf(Thread.currentThread().getId());
}

Apache Commons Daemon will run your Java program as Linux daemon or WinNT Service.

I frequently find myself writing scripts or command lines which essentially look like this, if I want to:

1. Run a program that is immune to sighups
2. That is completely disconnected from the shell which spawns it, and
3. Produces a log file from stderr and stdout the contents of which are displayed as well, but
4. Allows me to stop viewing the log in progress and do other stuff without disrupting the running process

Enjoy.

nohup java com.me.MyProgram </dev/null 2>&1 | tee logfile.log &