This post was originally published as part of the Java Advent series.

There are cases where you would like to start a Java process identical to the current one (or at least using the the same JVM with tweaked parameters). Some concrete cases where this would be useful:

• Auto-tuning the maximum memory parameters (ie. you have an algorithm to determine the optimal value – for example: 80% of the system memory – and your JVM wasn’t started with that particular value)
• Creating a cluster of processes for high(er)-availability (true HA implies multiple physical nodes) or because processes have different roles (like the components in MongoDB).
• Daemonizing the current process (that is, the background process should run even after the launching process has terminated) – this is a very frequent modus-operandi for programs on *nix systems where you have the foreground “control” process and the background “daemon” process (not to be confused with the “daemon” threads).

Doing this is relatively simple – and can be done in pure Java – after you find the correct API calls:

List arguments = new ArrayList<>();
// the java executable
arguments
  .add(String.format("%s%sbin%sjava",
    System.getProperty("java.home"), File.separator,
    File.separator));
// pre-execuable arguments (like -D, -agent, etc)
arguments.addAll(ManagementFactory.getRuntimeMXBean()
  .getInputArguments());

String classPath = System.getProperty("java.class.path"), javaExecutable = System
  .getProperty("sun.java.command");
if (classPath.equals(javaExecutable)) {
 // was started with -jar
 arguments.add("-jar");
 arguments.add(javaExecutable);
} else {
 arguments.add("-classpath");
 arguments.add(classPath);
 arguments.add(javaExecutable);
}

// we might add additional arguments here which will be received by the
// launched program
// in its args[] paramater
arguments.add("runme");

// launch it!
new ProcessBuilder().command(arguments).start();

Some explanations about to the code:

• It is largely inspired from this project
• We suppose that the java executable is named java and is located in bin/java relative to java.home. We use File.separator for the code to be portable.
• getInputArguments is used to get specific arguments passed to the JVM (like -Xmx). It does not include the classpath.
• Which is taken from java.class.path
• Finally, there is one heuristic step: we try to detect if we were launched using the -jar myjar.jar syntax or the MyMainClass syntax and replicate it.

This is it! After that we use ProcessBuilder (which we should always favour over Runtime.exec because it auto-escapes the parts of the command line for us).

A final thought: if you intend to use this method to “daemonize” a process (that is: to ensure that it stays running after its parent process has terminated) you should do two things:

• Redirect the standard input and output. By default they are redirected into temporary buffers and the JVM will seemingly randomly terminate when those buffers (pipes) fill up.
• Under Windows use javaw instead of java. This ensures that the process won’t be tied to the console it was started from (however it will still be tied to the user login session and will terminate when the user logs out – for a more heavy-duty solution look into the Java Service Wrapper).

This is it for today, hope you enjoyed it, fond it useful. If you run the code and it doesn’t work as advertised, let me know so that I can update it (I’m especially interested if it works with non Sun/Oracle JVMs). Come back tomorrow for an other article!

Meta: this post is part of the Java Advent Calendar and is licensed under the Creative Commons 3.0 Attribution license.