JVM application shutdown hooks

[Laurent Bourgès]

Dear members,

I have a problem within an external library using one JVM Shutdown hook to
perform resource cleanup (socket, lock file deletion ...) that seems buggy
in java web start environment: sometimes the JVM reports an
IllegalStateException happenning during that shutdown hook.

This library uses java.util.logging to log warnings and exceptions but none
is logged (console or java trace files).

The 'lost' log messages is related to the LogManager's shutdown hook:
    // This private class is used as a shutdown hook.
    // It does a "reset" to close all open handlers.
    private class Cleaner extends Thread {

        private Cleaner() {
            /* Set context class loader to null in order to avoid
             * keeping a strong reference to an application classloader.
             */
            this.setContextClassLoader(null);
        }

        public void run() {
            // This is to ensure the LogManager.<clinit> is completed
            // before synchronized block. Otherwise deadlocks are possible.
            LogManager mgr = manager;

            // If the global handlers haven't been initialized yet, we
            // don't want to initialize them just so we can close them!
            synchronized (LogManager.this) {
                // Note that death is imminent.
                deathImminent = true;
                initializedGlobalHandlers = true;
            }

            // Do a reset to close all active handlers.
            reset();
        }
    }

Without any log handler, the log messages are lost during shutdown !
I think it is annoying as it avoids me getting log and exceptions ...

FYI, the ApplicationShutdownHooks class executes all 'application' hooks
concurrently:
    static void runHooks() {
        Collection<Thread> threads;
        synchronized(ApplicationShutdownHooks.class) {
            threads = hooks.keySet();
            hooks = null;
        }

        for (Thread hook : threads) {
            hook.start();
        }
        for (Thread hook : threads) {
            try {
                hook.join();
            } catch (InterruptedException x) { }
        }
    }
So it is not possible to define hook priorities ...

However, I looked at the JVM Shutdown class which supports hook priorities
and I think that the J.U.L Cleaner hook should run after all application
hooks.
Here are the current Shutdown priorities:
    // The system shutdown hooks are registered with a predefined slot.
    // The list of shutdown hooks is as follows:
    // (0) Console restore hook
    // (1) Application hooks
    // (2) DeleteOnExit hook

Moreover, as a RFE, it could be useful for applications to be able to
define hook priorities within an application:
I did that several times registering only 1 shutdown hook and handling
correct ordering on my own ... but could be supported by the JDK itself.

Finally, here are (below) the Open JDK8 usages of Runtime.addShutdownHook
[19 occurrences] which are more system hooks than application hooks:
com.sun.imageio.stream
StreamCloser.java
StreamCloser
addToQueue
<anonymous java.security.PrivilegedAction>
run
  101:  Runtime.getRuntime().addShutdownHook(streamCloser);
java.util.logging
LogManager.java
LogManager
LogManager
  255:  Runtime.getRuntime().addShutdownHook(new Cleaner());
sun.font
SunFontManager.java
SunFontManager
createFont2D
<anonymous java.security.PrivilegedAction>
run
2538:  Runtime.getRuntime().addShutdownHook(fileCloser);
sun.rmi.server
Activation.java
Activation
init
  240:  Runtime.getRuntime().addShutdownHook(shutdownHook);
sun.tools.jstat

sun.awt.shell
Win32ShellFolderManager2.java
Win32ShellFolderManager2
ComInvoker
ComInvoker
<anonymous java.security.PrivilegedAction<java.lang.Void>>
run
  493:  Runtime.getRuntime().addShutdownHook(
sun.awt.windows
WToolkit.java
WToolkit
registerShutdownHook
<anonymous java.security.PrivilegedAction<java.lang.Void>>
run
  285:  Runtime.getRuntime().addShutdownHook(shutdown);
sun.java2d.d3d
D3DScreenUpdateManager.java
D3DScreenUpdateManager
D3DScreenUpdateManager
<anonymous java.security.PrivilegedAction>
run
  112:  Runtime.getRuntime().addShutdownHook(shutdown);
java.util.prefs
FileSystemPreferences.java
FileSystemPreferences
<anonymous java.security.PrivilegedAction<java.lang.Void>>
run
  439:  Runtime.getRuntime().addShutdownHook(new Thread() {
sun.awt.X11
XToolkit.java
XToolkit
init
<anonymous java.security.PrivilegedAction<java.lang.Void>>
run
  350:  Runtime.getRuntime().addShutdownHook(shutdownThread);
sun.awt
X11GraphicsDevice.java
X11GraphicsDevice
setDisplayMode
<anonymous java.security.PrivilegedAction<java.lang.Void>>
run
  445:  Runtime.getRuntime().addShutdownHook(t);
java.util.prefs
MacOSXPreferencesFile.java
MacOSXPreferencesFile
timer
  356:  Runtime.getRuntime().addShutdownHook(flushThread);
sun.font
CFontManager.java
CFontManager
createFont2D
<anonymous java.security.PrivilegedAction<java.lang.Object>>
run
  232:  Runtime.getRuntime().addShutdownHook(fileCloser);
sun.lwawt
LWToolkit.java
LWToolkit
init
    83:  Runtime.getRuntime().addShutdownHook(

I hope that these hooks are already supporting concurrency execution (awt
?) but someone should check if there is no dependencies between them (like
systemd or rc scripts does).

PS: As shutdown hooks are given by the application as Thread class
instances, I suspect also a possible classloader leak ? In Java web start
environment, it may be the cause of my problems as I do not know if the
hook thread is still within the application class loader ...

Laurent Bourgès