Watchdog

可以使用 watchdog 控制某些耗时操作的超时时间，当超时的时候执行指定的操作，例如中断线程、重置 Redis key 的超时时间等。Apache 的 commons-exec 包中提供了 watchdog 的实现供我们使用 (源码为文章后面的 Watchdog.java)。

According to wikipedia - watchdog is an electronic timer that is used to detect and recover from computer malfunctions.

案例演示

下面的例子执行拼接字符串的耗时操作，定义了一个 watchdog，超时时间为 1S，超时后 watchdog 中断线程结束字符串拼接操作。

import org.apache.commons.exec.Watchdog;

import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.concurrent.TimeUnit;

public class Test {
    private static final SimpleDateFormat formatter = new SimpleDateFormat("hh:mm:ss");

    public static void main(String[] args) {
        Thread thread = new Thread(() -> {
            // 创建 watchdog
            Thread t = Thread.currentThread();
            Watchdog watchdog = new Watchdog(TimeUnit.SECONDS.toMillis(1));

            // 超时且未关闭 watchdog 时执行的操作
            watchdog.addTimeoutObserver(w -> {
                System.out.println("Watchdog trigger");
                t.interrupt();
            });

            // 启动 watchdog
            watchdog.start();

            // 执行耗时任务，超时的时候当前线程的 isInterrupted() 为 true，结束循环
            System.out.println("开始时间: " + formatter.format(new Date()));
            int count = 0;
            StringBuilder sb = new StringBuilder();
            while (!t.isInterrupted() && count++ < 10000) {
                sb.append(count);
            }
            System.out.printf("结束时间: %s，循环次数: %d", formatter.format(new Date()), count);

            // 关闭 watchdog
            watchdog.stop();
        });

        thread.start();
    }
}

输出:

开始时间: 01:51:40
Watchdog trigger
结束时间: 01:51:41，循环次数: 48060741

Watchdog.java

Watchdog 的实现很简单，创建一个 Daemon 线程，在 run 方法中等待超时，然后执行注册的 TimeoutObserver。

/* 
 * Licensed to the Apache Software Foundation (ASF) under one or more
 *  contributor license agreements.  See the NOTICE file distributed with
 *  this work for additional information regarding copyright ownership.
 *  The ASF 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 org.apache.commons.exec;

import java.util.Enumeration;
import java.util.Vector;

/**
 * Generalization of {@code ExecuteWatchdog}
 * 
 * @see org.apache.commons.exec.ExecuteWatchdog
 *
 * @version $Id: Watchdog.java 1636056 2014-11-01 21:12:52Z ggregory $
 */
public class Watchdog implements Runnable {

    private final Vector<TimeoutObserver> observers = new Vector<TimeoutObserver>(1);

    private final long timeout;

    private boolean stopped = false;

    public Watchdog(final long timeout) {
        if (timeout < 1) {
            throw new IllegalArgumentException("timeout must not be less than 1.");
        }
        this.timeout = timeout;
    }

    public void addTimeoutObserver(final TimeoutObserver to) {
        observers.addElement(to);
    }

    public void removeTimeoutObserver(final TimeoutObserver to) {
        observers.removeElement(to);
    }

    protected final void fireTimeoutOccured() {
        final Enumeration<TimeoutObserver> e = observers.elements();
        while (e.hasMoreElements()) {
            e.nextElement().timeoutOccured(this);
        }
    }

    public synchronized void start() {
        stopped = false;
        final Thread t = new Thread(this, "WATCHDOG");
        t.setDaemon(true);
        t.start();
    }

    public synchronized void stop() {
        stopped = true;
        notifyAll();
    }

    public void run() {
        final long startTime = System.currentTimeMillis();
        boolean isWaiting;
        synchronized (this) {
            long timeLeft = timeout - (System.currentTimeMillis() - startTime);
            isWaiting = timeLeft > 0;
            while (!stopped && isWaiting) {
                try {
                    wait(timeLeft);
                } catch (final InterruptedException e) {
                }
                timeLeft = timeout - (System.currentTimeMillis() - startTime);
                isWaiting = timeLeft > 0;
            }
        }

        // notify the listeners outside of the synchronized block (see EXEC-60)
        if (!isWaiting) {
            fireTimeoutOccured();
        }
    }
}