addShutdownHook

/**
     * Registers a new virtual-machine shutdown hook.
     *
     * <p> The Java virtual machine <i>shuts down</i> in response to two kinds
     * of events:
     *
     *   <ul>
     *
     *   <p> <li> The program <i>exits</i> normally, when the last non-daemon
     *   thread exits or when the <tt>{@link #exit exit}</tt> (equivalently,
     *   <tt>{@link System#exit(int) System.exit}</tt>) method is invoked, or
     *
     *   <p> <li> The virtual machine is <i>terminated</i> in response to a
     *   user interrupt, such as typing <tt>^C</tt>, or a system-wide event,
     *   such as user logoff or system shutdown.
     *
     *   </ul>
     *
     * <p> A <i>shutdown hook</i> is simply an initialized but unstarted
     * thread.  When the virtual machine begins its shutdown sequence it will
     * start all registered shutdown hooks in some unspecified order and let
     * them run concurrently.  When all the hooks have finished it will then
     * run all uninvoked finalizers if finalization-on-exit has been enabled.
     * Finally, the virtual machine will halt.  Note that daemon threads will
     * continue to run during the shutdown sequence, as will non-daemon threads
     * if shutdown was initiated by invoking the <tt>{@link #exit exit}</tt>
     * method.
     *
     * <p> Once the shutdown sequence has begun it can be stopped only by
     * invoking the <tt>{@link #halt halt}</tt> method, which forcibly
     * terminates the virtual machine.
     *
     * <p> Once the shutdown sequence has begun it is impossible to register a
     * new shutdown hook or de-register a previously-registered hook.
     * Attempting either of these operations will cause an
     * <tt>{@link IllegalStateException}</tt> to be thrown.
     *
     * <p> Shutdown hooks run at a delicate time in the life cycle of a virtual
     * machine and should therefore be coded defensively.  They should, in
     * particular, be written to be thread-safe and to avoid deadlocks insofar
     * as possible.  They should also not rely blindly upon services that may
     * have registered their own shutdown hooks and therefore may themselves in
     * the process of shutting down.
     *
     * <p> Shutdown hooks should also finish their work quickly.  When a
     * program invokes <tt>{@link #exit exit}</tt> the expectation is
     * that the virtual machine will promptly shut down and exit.  When the
     * virtual machine is terminated due to user logoff or system shutdown the
     * underlying operating system may only allow a fixed amount of time in
     * which to shut down and exit.  It is therefore inadvisable to attempt any
     * user interaction or to perform a long-running computation in a shutdown
     * hook.
     *
     * <p> Uncaught exceptions are handled in shutdown hooks just as in any
     * other thread, by invoking the <tt>{@link ThreadGroup#uncaughtException
     * uncaughtException}</tt> method of the thread's <tt>{@link
     * ThreadGroup}</tt> object.  The default implementation of this method
     * prints the exception's stack trace to <tt>{@link System#err}</tt> and
     * terminates the thread; it does not cause the virtual machine to exit or
     * halt.
     *
     * <p> In rare circumstances the virtual machine may <i>abort</i>, that is,
     * stop running without shutting down cleanly.  This occurs when the
     * virtual machine is terminated externally, for example with the
     * <tt>SIGKILL</tt> signal on Unix or the <tt>TerminateProcess</tt> call on
     * Microsoft Windows.  The virtual machine may also abort if a native method goes awry
     * by, for example, corrupting internal data structures or attempting to
     * access nonexistent memory.  If the virtual machine aborts then no
     * guarantee can be made about whether or not any shutdown hooks will be
     * run. <p>
     *
     * @param   hook
     *          An initialized but unstarted <tt>{@link Thread}</tt> object
     *
     * @throws  IllegalArgumentException
     *          If the specified hook has already been registered,
     *          or if it can be determined that the hook is already running or
     *          has already been run
     *
     * @throws  IllegalStateException
     *          If the virtual machine is already in the process
     *          of shutting down
     *
     * @throws  SecurityException
     *          If a security manager is present and it denies
     *          <tt>{@link RuntimePermission}("shutdownHooks")</tt>
     *
     * @see #removeShutdownHook
     * @see #halt(int)
     * @see #exit(int)
     * @since 1.3
     */
    public void addShutdownHook(Thread hook) {
	SecurityManager sm = System.getSecurityManager();
	if (sm != null) {
	    sm.checkPermission(new RuntimePermission("shutdownHooks"));
	}
	Shutdown.add(hook);
    }