Java code example to replace Timer with ScheduledExecutorService-javaTutorial-php.cn

Java code example to replace Timer with ScheduledExecutorService

Y2J

Release： 2017-04-24 11:57:44

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1、Timer管理延时任务的缺陷

a、以前在项目中也经常使用定时器，比如每隔一段时间清理项目中的一些垃圾文件，每个一段时间进行数据清洗；然而Timer是存在一些缺陷的，因为Timer在执行定时任务时只会创建一个线程，所以如果存在多个任务，且任务时间过长，超过了两个任务的间隔时间，会发生一些缺陷：下面看例子：

Timer的源码：

public class Timer {
    /**
     * The timer task queue.  This data structure is shared with the timer
     * thread.  The timer produces tasks, via its various schedule calls,
     * and the timer thread consumes, executing timer tasks as appropriate,
     * and removing them from the queue when they&#39;re obsolete.
     */
    private TaskQueue queue = new TaskQueue();

    /**
     * The timer thread.
     */
    private TimerThread thread = new TimerThread(queue);

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TimerThread是Thread的子类，可以看出内部只有一个线程。下面看个例子:

package com.zhy.concurrency.timer;

import java.util.Timer;
import java.util.TimerTask;

public class TimerTest
{
	private static long start;

	public static void main(String[] args) throws Exception
	{
		TimerTask task1 = new TimerTask()
		{
			@Override
			public void run()
			{

				System.out.println("task1 invoked ! "
						+ (System.currentTimeMillis() - start));
				try
				{
					Thread.sleep(3000);
				} catch (InterruptedException e)
				{
					e.printStackTrace();
				}

			}
		};
		TimerTask task2 = new TimerTask()
		{
			@Override
			public void run()
			{
				System.out.println("task2 invoked ! "
						+ (System.currentTimeMillis() - start));
			}
		};
		Timer timer = new Timer();
		start = System.currentTimeMillis();
		timer.schedule(task1, 1000);
		timer.schedule(task2, 3000);

	}
}

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定义了两个任务，预计是第一个任务1s后执行，第二个任务3s后执行，但是看运行结果：

task1 invoked ! 1000
task2 invoked ! 4000

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task2实际上是4后才执行，正因为Timer内部是一个线程，而任务1所需的时间超过了两个任务间的间隔导致。下面使用ScheduledThreadPool解决这个问题：

package com.zhy.concurrency.timer;
import java.util.TimerTask;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
public class ScheduledThreadPoolExecutorTest
{
	private static long start;
	public static void main(String[] args)
	{
		/**
		 * 使用工厂方法初始化一个ScheduledThreadPool
		 */
		ScheduledExecutorService newScheduledThreadPool = Executors
				.newScheduledThreadPool(2);
		TimerTask task1 = new TimerTask()
		{
			@Override
			public void run()
			{
				try
				{

					System.out.println("task1 invoked ! "
							+ (System.currentTimeMillis() - start));
					Thread.sleep(3000);
				} catch (Exception e)
				{
					e.printStackTrace();
				}

			}
		};

		TimerTask task2 = new TimerTask()
		{
			@Override
			public void run()
			{
				System.out.println("task2 invoked ! "
						+ (System.currentTimeMillis() - start));
			}
		};
		start = System.currentTimeMillis();
		newScheduledThreadPool.schedule(task1, 1000, TimeUnit.MILLISECONDS);
		newScheduledThreadPool.schedule(task2, 3000, TimeUnit.MILLISECONDS);
	}
}

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输出结果：

task1 invoked ! 1001
task2 invoked ! 3001

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符合我们的预期结果。因为ScheduledThreadPool内部是个线程池，所以可以支持多个任务并发执行。

2、Timer当任务抛出异常时的缺陷

如果TimerTask抛出RuntimeException，Timer会停止所有任务的运行：

package com.zhy.concurrency.timer;

import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;
public class ScheduledThreadPoolDemo01
{
	public static void main(String[] args) throws InterruptedException
	{
		final TimerTask task1 = new TimerTask()
		{
			@Override
			public void run()
			{
				throw new RuntimeException();
			}
		};
		final TimerTask task2 = new TimerTask()
		{
			@Override
			public void run()
			{
				System.out.println("task2 invoked!");
			}
		};
		Timer timer = new Timer();
		timer.schedule(task1, 100);
		timer.scheduleAtFixedRate(task2, new Date(), 1000);
	}
}

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上面有两个任务，任务1抛出一个运行时的异常，任务2周期性的执行某个操作，输出结果：

task2 invoked!
Exception in thread "Timer-0" java.lang.RuntimeException
	at com.zhy.concurrency.timer.ScheduledThreadPoolDemo01$1.run(ScheduledThreadPoolDemo01.java:24)
	at java.util.TimerThread.mainLoop(Timer.java:512)
	at java.util.TimerThread.run(Timer.java:462)

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由于任务1的一次，任务2也停止运行了。。。下面使用ScheduledExecutorService解决这个问题：

package com.zhy.concurrency.timer;
import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;
public class ScheduledThreadPoolDemo01
{
	public static void main(String[] args) throws InterruptedException
	{
		final TimerTask task1 = new TimerTask()
		{
			@Override
			public void run()
			{
				throw new RuntimeException();
			}
		};
		final TimerTask task2 = new TimerTask()
		{
			@Override
			public void run()
			{
				System.out.println("task2 invoked!");
			}
		};
		ScheduledExecutorService pool = Executors.newScheduledThreadPool(1);
		pool.schedule(task1, 100, TimeUnit.MILLISECONDS);
		pool.scheduleAtFixedRate(task2, 0 , 1000, TimeUnit.MILLISECONDS);
	}
}

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代码基本一致，但是ScheduledExecutorService可以保证，task1出现异常时，不影响task2的运行：

task2 invoked!
task2 invoked!
task2 invoked!
task2 invoked!
task2 invoked!<span style="font-family: Arial, Helvetica, sans-serif;">...</span>

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3、Timer执行周期任务时依赖系统时间

Timer执行周期任务时依赖系统时间，如果当前系统时间发生变化会出现一些执行上的变化，ScheduledExecutorService基于时间的延迟，不会由于系统时间的改变发生执行变化。

上述，基本说明了在以后的开发中尽可能使用ScheduledExecutorService(JDK1.5以后)替代Timer。

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