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

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'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!...

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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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