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:
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public class Timer {
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/**
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* The timer task queue. This data structure is shared with the timer
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* thread. The timer produces tasks, via its various schedule calls,
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* and the timer thread consumes, executing timer tasks as appropriate,
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* and removing them from the queue when they're obsolete.
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*/
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private TaskQueue queue = new TaskQueue();
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/**
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* The timer thread.
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*/
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private TimerThread thread = new TimerThread(queue);
TimerThread是Thread的子类,可以看出内部只有一个线程。下面看个例子:
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package com.zhy.concurrency.timer;
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import java.util.Timer;
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import java.util.TimerTask;
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public class TimerTest
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{
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private static long start;
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public static void main(String[] args) throws Exception
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{
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TimerTask task1 = new TimerTask()
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{
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public void run()
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{
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System.out.println( "task1 invoked ! "
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+ (System.currentTimeMillis() - start));
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try
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{
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Thread.sleep( 3000);
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} catch (InterruptedException e)
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{
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e.printStackTrace();
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}
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}
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};
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TimerTask task2 = new TimerTask()
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{
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public void run()
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{
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System.out.println( "task2 invoked ! "
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+ (System.currentTimeMillis() - start));
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}
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};
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Timer timer = new Timer();
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start = System.currentTimeMillis();
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timer.schedule(task1, 1000);
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timer.schedule(task2, 3000);
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}
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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:
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task1 invoked ! 1000
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task2 invoked ! 4000
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package com.zhy.concurrency.timer;
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import java.util.TimerTask;
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import java.util.concurrent.Executors;
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import java.util.concurrent.ScheduledExecutorService;
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import java.util.concurrent.TimeUnit;
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public class ScheduledThreadPoolExecutorTest
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{
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private static long start;
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public static void main(String[] args)
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{
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/**
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* 使用工厂方法初始化一个ScheduledThreadPool
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*/
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ScheduledExecutorService newScheduledThreadPool = Executors
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.newScheduledThreadPool( 2);
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TimerTask task1 = new TimerTask()
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{
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public void run()
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{
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try
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{
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System.out.println( "task1 invoked ! "
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+ (System.currentTimeMillis() - start));
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Thread.sleep( 3000);
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} catch (Exception e)
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{
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e.printStackTrace();
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}
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}
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};
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TimerTask task2 = new TimerTask()
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{
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public void run()
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{
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System.out.println( "task2 invoked ! "
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+ (System.currentTimeMillis() - start));
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}
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};
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start = System.currentTimeMillis();
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newScheduledThreadPool.schedule(task1, 1000, TimeUnit.MILLISECONDS);
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newScheduledThreadPool.schedule(task2, 3000, TimeUnit.MILLISECONDS);
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}
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}
输出结果:
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task1 invoked ! 1001
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task2 invoked ! 3001
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:
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package com.zhy.concurrency.timer;
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import java.util.Date;
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import java.util.Timer;
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import java.util.TimerTask;
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public class ScheduledThreadPoolDemo01
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{
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public static void main(String[] args) throws InterruptedException
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{
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final TimerTask task1 = new TimerTask()
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{
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public void run()
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{
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throw new RuntimeException();
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}
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};
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final TimerTask task2 = new TimerTask()
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{
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public void run()
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{
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System.out.println( "task2 invoked!");
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}
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};
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Timer timer = new Timer();
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timer.schedule(task1, 100);
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timer.scheduleAtFixedRate(task2, new Date(), 1000);
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}
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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:
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task2 invoked!
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Exception in thread "Timer-0" java.lang.RuntimeException
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at com.zhy.concurrency.timer.ScheduledThreadPoolDemo01$ 1.run(ScheduledThreadPoolDemo01.java: 24)
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at java.util.TimerThread.mainLoop(Timer.java: 512)
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at java.util.TimerThread.run(Timer.java: 462)
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:
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package com.zhy.concurrency.timer;
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import java.util.Date;
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import java.util.Timer;
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import java.util.TimerTask;
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import java.util.concurrent.Executors;
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import java.util.concurrent.ScheduledExecutorService;
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import java.util.concurrent.TimeUnit;
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public class ScheduledThreadPoolDemo01
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{
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public static void main(String[] args) throws InterruptedException
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{
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final TimerTask task1 = new TimerTask()
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{
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public void run()
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{
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throw new RuntimeException();
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}
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};
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final TimerTask task2 = new TimerTask()
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{
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public void run()
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{
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System.out.println( "task2 invoked!");
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}
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};
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ScheduledExecutorService pool = Executors.newScheduledThreadPool( 1);
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pool.schedule(task1, 100, TimeUnit.MILLISECONDS);
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pool.scheduleAtFixedRate(task2, 0 , 1000, TimeUnit.MILLISECONDS);
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}
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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:
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task2 invoked!
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task2 invoked!
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task2 invoked!
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task2 invoked!
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task2 invoked!<span style= "font-family: Arial, Helvetica, sans-serif;">...</span>
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。