What can cause Java to keep running after System.exit()?

Question

I have a Java program which is being started via ProcessBuilder from another Java program. System.exit(0) is called from the child program, but for some of our users (on Windows) the java.exe process associated with the child doesn't terminate. The child program has no shutdown hooks, nor does it have a SecurityManager which might stop System.exit() from terminating the VM. I can't reproduce the problem myself on Linux or Windows Vista. So far, the only reports of the problem come from two Windows XP users and one Vista user, using two different JREs (1.6.0_15 and 1.6.0_18), but they're able to reproduce the problem every time.

Can anyone suggest reasons why the JVM would fail to terminate after System.exit(), and then only on some machines?

Edit 1: I got the user to install the JDK so we could get a thread dump from the offending VM. What the user told me is that the VM process disappears from VisualVM as soon as he clicks on the 'Quit' item in my menu---but, according to Windows Task Manager, the process hasn't terminated, and no matter how long the user waits (minutes, hours), it never terminates.

Edit 2: I have confirmed now that Process.waitFor() in the parent program never returns for at least one of the users having the problem. So, to summarize: The child VM seems to be dead (VisualVM doesn't even see it) but the parent still sees the process as live and so does Windows.

Answer 1

This can happen if your code (or a library you use) has a shutdown hook or a finalizer that doesn't finish cleanly.

A more vigorous (so should only be used in extreme cases!) way to force shutdown is by running:

Runtime.getRuntime().halt(0); 

Answer 2

The parent process has one thread dedicated to consuming each of the child's STDOUT and STDERR (which passes that output through to a log file). So far as I can see, those are working properly, since we're seeing all the output we expect to see in the log

i had a similar problem with my program not disappearing from task mgr when i was consuming the stdout/stderr. in my case, if I closed the stream that was listening before calling system.exit() then the javaw.exe hung around. strange, it wasn't writing to the stream...

the solution in my case was to simply flush the stream rather than close it before existing. of course, you could always flush and then redirect back to stdout and stderr before exit.