netty5/common/src/main/java/io/netty/util/ThreadDeathWatcher.java

/*
 * Copyright 2014 The Netty Project
 *
 * The Netty Project licenses this file to you under the Apache License,
 * version 2.0 (the "License"); you may not use this file except in compliance
 * with the License. You may obtain a copy of the License at:
 *
 *   http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
 * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
 * License for the specific language governing permissions and limitations
 * under the License.
 */

package io.netty.util;

import io.netty.util.concurrent.DefaultThreadFactory;
import io.netty.util.internal.MpscLinkedQueue;
import io.netty.util.internal.PlatformDependent;
import io.netty.util.internal.logging.InternalLogger;
import io.netty.util.internal.logging.InternalLoggerFactory;

import java.util.ArrayList;
import java.util.List;
import java.util.Queue;
import java.util.concurrent.ThreadFactory;
import java.util.concurrent.atomic.AtomicBoolean;

/**
 * Checks if a thread is alive periodically and runs a task when a thread dies.
 * <p>
 * This thread starts a daemon thread to check the state of the threads being watched and to invoke their
 * associated {@link Runnable}s.  When there is no thread to watch (i.e. all threads are dead), the daemon thread
 * will terminate itself, and a new daemon thread will be started again when a new watch is added.
 * </p>
 */
public final class ThreadDeathWatcher {

    private static final InternalLogger logger = InternalLoggerFactory.getInstance(ThreadDeathWatcher.class);
    private static final ThreadFactory threadFactory =
            new DefaultThreadFactory(ThreadDeathWatcher.class, true, Thread.MIN_PRIORITY);

    private static final Queue<Entry> pendingEntries = PlatformDependent.newMpscQueue();
    private static final Watcher watcher = new Watcher();
    private static final AtomicBoolean started = new AtomicBoolean();

    /**
     * Schedules the specified {@code task} to run when the specified {@code thread} dies.
     *
     * @param thread the {@link Thread} to watch
     * @param task the {@link Runnable} to run when the {@code thread} dies
     *
     * @throws IllegalArgumentException if the specified {@code thread} is not alive
     */
    public static void watch(Thread thread, Runnable task) {
        if (thread == null) {
            throw new NullPointerException("thread");
        }
        if (task == null) {
            throw new NullPointerException("task");
        }
        if (!thread.isAlive()) {
            throw new IllegalArgumentException("thread must be alive.");
        }

        pendingEntries.add(new Entry(thread, task));

        if (started.compareAndSet(false, true)) {
            Thread watcherThread = threadFactory.newThread(watcher);
            watcherThread.start();
        }
    }

    private ThreadDeathWatcher() { }

    private static final class Watcher implements Runnable {

        private final List<Entry> watchees = new ArrayList<Entry>();

        @Override
        public void run() {
            for (;;) {
                fetchWatchees();
                notifyWatchees();

                try {
                    Thread.sleep(1000);
                } catch (InterruptedException ignore) {
                    // Ignore the interrupt; do not terminate until all tasks are run.
                }

                if (watchees.isEmpty() && pendingEntries.isEmpty()) {

                    // Mark the current worker thread as stopped.
                    // The following CAS must always success and must be uncontended,
                    // because only one watcher thread should be running at the same time.
                    boolean stopped = started.compareAndSet(true, false);
                    assert stopped;

                    // Check if there are pending entries added by watch() while we do CAS above.
                    if (pendingEntries.isEmpty()) {
                        // A) watch() was not invoked and thus there's nothing to handle
                        //    -> safe to terminate because there's nothing left to do
                        // B) a new watcher thread started and handled them all
                        //    -> safe to terminate the new watcher thread will take care the rest
                        break;
                    }

                    // There are pending entries again, added by watch()
                    if (!started.compareAndSet(false, true)) {
                        // watch() started a new watcher thread and set 'started' to true.
                        // -> terminate this thread so that the new watcher reads from pendingEntries exclusively.
                        break;
                    }

                    // watch() added an entry, but this worker was faster to set 'started' to true.
                    // i.e. a new watcher thread was not started
                    // -> keep this thread alive to handle the newly added entries.
                }
            }
        }

        private void fetchWatchees() {
            for (;;) {
                Entry e = pendingEntries.poll();
                if (e == null) {
                    break;
                }

                watchees.add(e);
            }
        }

        private void notifyWatchees() {
            List<Entry> watchees = this.watchees;
            for (int i = 0; i < watchees.size();) {
                Entry e = watchees.get(i);
                if (!e.thread.isAlive()) {
                    watchees.remove(i);
                    try {
                        e.task.run();
                    } catch (Throwable t) {
                        logger.warn("Thread death watcher task raised an exception:", t);
                    }
                } else {
                    i ++;
                }
            }
        }
    }

    private static final class Entry extends MpscLinkedQueue.Node<Entry> {
        final Thread thread;
        final Runnable task;

        Entry(Thread thread, Runnable task) {
            this.thread = thread;
            this.task = task;
        }

        @Override
        public Entry value() {
            return this;
        }
    }
}