Java tutorial
/* * Copyright (c) 1999, 2018, Oracle and/or its affiliates. All rights reserved. * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. * * This code is free software; you can redistribute it and/or modify it * under the terms of the GNU General Public License version 2 only, as * published by the Free Software Foundation. Oracle designates this * particular file as subject to the "Classpath" exception as provided * by Oracle in the LICENSE file that accompanied this code. * * This code is distributed in the hope that it will be useful, but WITHOUT * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License * version 2 for more details (a copy is included in the LICENSE file that * accompanied this code). * * You should have received a copy of the GNU General Public License version * 2 along with this work; if not, write to the Free Software Foundation, * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. * * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA * or visit www.oracle.com if you need additional information or have any * questions. */ package java.lang; import jdk.internal.misc.VM; /** * Package-private utility class containing data structures and logic * governing the virtual-machine shutdown sequence. * * @author Mark Reinhold * @since 1.3 * * @see java.io.Console * @see ApplicationShutdownHooks * @see java.io.DeleteOnExitHook */ class Shutdown { // The system shutdown hooks are registered with a predefined slot. // The list of shutdown hooks is as follows: // (0) Console restore hook // (1) ApplicationShutdownHooks that invokes all registered application // shutdown hooks and waits until they finish // (2) DeleteOnExit hook private static final int MAX_SYSTEM_HOOKS = 10; private static final Runnable[] hooks = new Runnable[MAX_SYSTEM_HOOKS]; // the index of the currently running shutdown hook to the hooks array private static int currentRunningHook = -1; /* The preceding static fields are protected by this lock */ private static class Lock { }; private static Object lock = new Lock(); /* Lock object for the native halt method */ private static Object haltLock = new Lock(); /** * Add a new system shutdown hook. Checks the shutdown state and * the hook itself, but does not do any security checks. * * The registerShutdownInProgress parameter should be false except * registering the DeleteOnExitHook since the first file may * be added to the delete on exit list by the application shutdown * hooks. * * @params slot the slot in the shutdown hook array, whose element * will be invoked in order during shutdown * @params registerShutdownInProgress true to allow the hook * to be registered even if the shutdown is in progress. * @params hook the hook to be registered * * @throws IllegalStateException * if registerShutdownInProgress is false and shutdown is in progress; or * if registerShutdownInProgress is true and the shutdown process * already passes the given slot */ static void add(int slot, boolean registerShutdownInProgress, Runnable hook) { if (slot < 0 || slot >= MAX_SYSTEM_HOOKS) { throw new IllegalArgumentException("Invalid slot: " + slot); } synchronized (lock) { if (hooks[slot] != null) throw new InternalError("Shutdown hook at slot " + slot + " already registered"); if (!registerShutdownInProgress) { if (currentRunningHook >= 0) throw new IllegalStateException("Shutdown in progress"); } else { if (VM.isShutdown() || slot <= currentRunningHook) throw new IllegalStateException("Shutdown in progress"); } hooks[slot] = hook; } } /* Run all system shutdown hooks. * * The system shutdown hooks are run in the thread synchronized on * Shutdown.class. Other threads calling Runtime::exit, Runtime::halt * or JNI DestroyJavaVM will block indefinitely. * * ApplicationShutdownHooks is registered as one single hook that starts * all application shutdown hooks and waits until they finish. */ private static void runHooks() { synchronized (lock) { /* Guard against the possibility of a daemon thread invoking exit * after DestroyJavaVM initiates the shutdown sequence */ if (VM.isShutdown()) return; } for (int i = 0; i < MAX_SYSTEM_HOOKS; i++) { try { Runnable hook; synchronized (lock) { // acquire the lock to make sure the hook registered during // shutdown is visible here. currentRunningHook = i; hook = hooks[i]; } if (hook != null) hook.run(); } catch (Throwable t) { if (t instanceof ThreadDeath) { ThreadDeath td = (ThreadDeath) t; throw td; } } } // set shutdown state VM.shutdown(); } /* Notify the VM that it's time to halt. */ static native void beforeHalt(); /* The halt method is synchronized on the halt lock * to avoid corruption of the delete-on-shutdown file list. * It invokes the true native halt method. */ static void halt(int status) { synchronized (haltLock) { halt0(status); } } static native void halt0(int status); /* Invoked by Runtime.exit, which does all the security checks. * Also invoked by handlers for system-provided termination events, * which should pass a nonzero status code. */ static void exit(int status) { synchronized (lock) { if (status != 0 && VM.isShutdown()) { /* Halt immediately on nonzero status */ halt(status); } } synchronized (Shutdown.class) { /* Synchronize on the class object, causing any other thread * that attempts to initiate shutdown to stall indefinitely */ beforeHalt(); runHooks(); halt(status); } } /* Invoked by the JNI DestroyJavaVM procedure when the last non-daemon * thread has finished. Unlike the exit method, this method does not * actually halt the VM. */ static void shutdown() { synchronized (Shutdown.class) { runHooks(); } } }