java.exe does not end - only one OS-thread remains

Question

A scheduler starts a java application, which ends without any problem most of the time.

Sometimes however, the java.exe process doesn't end. As far we can see, the java application itself has ended, but the java.exe process not. There is not solution but to kill it.

The Process Explorer shows only one thread (in sense of the OS, not of java) for this process. It hat the state "Wait:WrKeyedEvent" and following stack

ntoskrnl.exe!KeWaitForMultipleObjects+0x0a
ntoskrnl.exe!ExfReleasePushLock+0x8ec
ntoskrnl.exe!_misaligned_access+0x331
ntdll.dll!ZwReleaseKeyedEvent+0xa
ntdll.dll!RtlFindMostSignificatBit+0xa0
ntdll.dll!RtlProcessFlsData+0xdc
ntdll.dll!LdrShutdownProcess+0xa9
ntdll.dll!RtlExitUserProcess+0x90
msvcrt.dll!wcstoui64+0x2fa
jvm.dll!JVM_Clone
jvm.dll!JVM_Clone
jvm.dll!JVM_Clone
jvm.dll!JVM_Clone
jvm.dll!JVM_Clone
jvm.dll!JVM_FindSignal
msvcrt.dll!srand
msvcrt.dll!ftime64_s
kernel32.dll!BaseThreadInitThunk
ntdll.dll!RtlUserThreadStart

Has somebody an idea, why java.exe does not end?

Edit about the response "one no-deamon thread is still running"

The problem is not a remaining thread in the Java application. An Java application has several running (Java-)threads, even if only one of it is a no-deamon one.

This is also true for a ShutdownHook. See this example:

/** Main class */
public class TestShutdown
{
    public static void main (final String[] args) throws FileNotFoundException
    {
        Runtime.getRuntime().addShutdownHook(new HookThread());
    }

}

/** The hook Thread */
final class HookThread extends Thread
{

    public HookThread()
    {
        super(HookThread.class.getName();
    }

    @Override
    public void run()
    {
        try
        {
            sleep(5000l);
        } catch (final InterruptedException ex) { throw new Error(); }

        final StringBuilder sb = new StringBuilder();
        Map<Thread, StackTraceElement[]> traces = Thread.getAllStackTraces();
        sb.append("Number of threads: " + traces.size() + "\n");
        for (final Thread th: traces.keySet())
        {
            sb.append("-----------------\n");
            sb.append(th.toString()+"  "+ (th.isDaemon() ? " (deamon) " : " (not deamon) ") + "\n");
            final StackTraceElement[] trace = traces.get(th);
            for (int j=0; j < trace.length; j++)
                sb.append("  at " + trace[j] + "\n");
        }
        System.out.println(sb.toString());
    }

}

and the result:

Number of threads: 6
-----------------
Thread[Reference Handler,10,system]   (deamon)
  at java.lang.Object.wait(Native Method)
  at java.lang.Object.wait(Unknown Source)
  at java.lang.ref.Reference$ReferenceHandler.run(Unknown Source)
-----------------
Thread[Finalizer,8,system]   (deamon)
  at java.lang.Object.wait(Native Method)
  at java.lang.ref.ReferenceQueue.remove(Unknown Source)
  at java.lang.ref.ReferenceQueue.remove(Unknown Source)
  at java.lang.ref.Finalizer$FinalizerThread.run(Unknown Source)
-----------------
Thread[HookThread,5,main]   (not deamon)
  at java.lang.Thread.dumpThreads(Native Method)
  at java.lang.Thread.getAllStackTraces(Unknown Source)
  at HookThread.run(TestShutdown.java:59)
-----------------
Thread[Attach Listener,5,system]   (deamon)
-----------------
Thread[DestroyJavaVM,5,main]   (not deamon)
  at java.lang.Object.wait(Native Method)
  at java.lang.Thread.join(Unknown Source)
  at java.lang.Thread.join(Unknown Source)
  at java.lang.ApplicationShutdownHooks.runHooks(Unknown Source)
  at java.lang.ApplicationShutdownHooks$1.run(Unknown Source)
  at java.lang.Shutdown.runHooks(Unknown Source)
  at java.lang.Shutdown.sequence(Unknown Source)
  at java.lang.Shutdown.shutdown(Unknown Source)
-----------------
Thread[Signal Dispatcher,9,system]   (deamon)

(By the way: in this case we have two not deamons: the hook thread and the DestroyJavaVM, which has started the hook and waits until it ends.)

But moreover: my hanging java.exe has only one OS-thread! And it's clear that the program java.exe must hava more threads as the application which is running in its virtual machine... (whith the above programm, 6 Java-threads are running in the VM but the java.exe process itself hat 17 OS-threads.)

Answer 1

This can happen if a shutdown hook or a finalizer does not complete. Do you have shutdown hooks or do you have Runtime.runFinalizersOnExit enabled?

Anyway, if you are the developer, you can start an extra thread that monitors what other threads are doing. See Get a List of all Threads currently running in Java

Answer 2

Your java application does not end because some thread in it loops forever or has a pending lock on a resource - thats proof of a bug, you need to contact the java application developer and show him your current problem.

For starters : its generally a good idea for applications which are used in a productive environment to have some form of watchdog - there are several, well-tested libraries for that kind of purpose -- like Apache commons Watchdog. Classes like these simply suspend/wait until a timeout occurs - at which point they will call your provided function, it is advisable to call System.exit(1) in that function, effectively forcing a shutdown. This works because the watchdog is a daemon thread which will get killed once everything else is already "dead" - so the watchdog will never fire if your application ends within a pre-set timeout.