Java提供的Time类可以周期性地或者延期执行任务，但是有时我们需要并行执行同样的任务，这个时候如果创建多个Time对象会给系统带来负担，解决办法是将定时任务放到线程池中执行。
Java的ScheduledThreadPoolExecutor类实现了ScheduledExecutorService接口中定义的以不同方法执行任务的方法。
之前，我写过一篇关于Java ThreadPoolExecutor的文章中使用了Executors创建线程池。Executors类也提供了工厂方法创建ScheduledThreadPoolExecutor，并且可以设置线程池中的线程。
假设有下面简单的Runnable类
WorkerThread.java:
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 | package com.journaldev.threads; import java.util.Date; public class WorkerThread implements Runnable{ private String command; public WorkerThread(String s){ this .command=s; } @Override public void run() { System.out.println(Thread.currentThread().getName()+ " Start. Time = " + new Date()); processCommand(); System.out.println(Thread.currentThread().getName()+ " End. Time = " + new Date()); } private void processCommand() { try { Thread.sleep( 5000 ); } catch (InterruptedException e) { e.printStackTrace(); } } @Override public String toString(){ return this .command; } } |
下面的例子中worker线程将被延期10s执行上面的Rnnable类大约花费5s执行任务
ScheduledThreadPool.java:
| 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 | package com.journaldev.threads; import java.util.Date; import java.util.concurrent.Executors; import java.util.concurrent.ScheduledExecutorService; import java.util.concurrent.TimeUnit; public class ScheduledThreadPool { public static void main(String[] args) throws InterruptedException { ScheduledExecutorService scheduledThreadPool = Executors.newScheduledThreadPool( 5 ); //schedule to run after sometime System.out.println( "Current Time = " + new Date()); for ( int i= 0 ; i< 3 ; i++){ Thread.sleep( 1000 ); WorkerThread worker = new WorkerThread( "do heavy processing" ); scheduledThreadPool.schedule(worker, 10 , TimeUnit.SECONDS); } //add some delay to let some threads spawn by scheduler Thread.sleep( 30000 ); scheduledThreadPool.shutdown(); while (!scheduledThreadPool.isTerminated()){ //wait for all tasks to finish } System.out.println( "Finished all threads" ); } } |
运行上面的程序，可以得到下面的输出，由此可以确认任务在10s后才执行。
| 1 2 3 4 5 6 7 8 | Current Time = Tue Oct 29 15:10:03 IST 2013 pool-1-thread-1 Start. Time = Tue Oct 29 15:10:14 IST 2013 pool-1-thread-2 Start. Time = Tue Oct 29 15:10:15 IST 2013 pool-1-thread-3 Start. Time = Tue Oct 29 15:10:16 IST 2013 pool-1-thread-1 End. Time = Tue Oct 29 15:10:19 IST 2013 pool-1-thread-2 End. Time = Tue Oct 29 15:10:20 IST 2013 pool-1-thread-3 End. Time = Tue Oct 29 15:10:21 IST 2013 Finished all threads |
注意到所有的schedule方法都返回了ScheduledFuture实例，可以用于获取线程状态信息和延迟时间。ScheduledFuture接口继承Future接口，更多信息见Java Callable Future Example.
在ScheduledExecutorService中至少有2个方法可用于周期性执行任务。
scheduleAtFixedRate(Runnable command,long initialDelay,long period,TimeUnit unit)
我们可以使用该方法延迟执行任务，设置任务的执行周期。时间周期从线程池中首先开始执行的线程算起，所以假设period为1s，线程执行了5s，那么下一个线程在第一个线程运行完后会很快被执行。
比如下面的代码
| 1 2 3 4 5 6 | for ( int i = 0 ; i < 3 ; i++) { Thread.sleep( 1000 ); WorkerThread worker = new WorkerThread( "do heavy processing" ); // schedule task to execute at fixed rate scheduledThreadPool.scheduleAtFixedRate(worker, 0 , 10 , TimeUnit.SECONDS); |
输出
| 1 2 3 4 5 6 7 8 9 | Current Time = Tue Oct 29 16:10:00 IST 2013 pool-1-thread-1 Start. Time = Tue Oct 29 16:10:01 IST 2013 pool-1-thread-2 Start. Time = Tue Oct 29 16:10:02 IST 2013 pool-1-thread-3 Start. Time = Tue Oct 29 16:10:03 IST 2013 pool-1-thread-1 End. Time = Tue Oct 29 16:10:06 IST 2013 pool-1-thread-2 End. Time = Tue Oct 29 16:10:07 IST 2013 pool-1-thread-3 End. Time = Tue Oct 29 16:10:08 IST 2013 pool-1-thread-1 Start. Time = Tue Oct 29 16:10:11 IST 2013 pool-1-thread-4 Start. Time = Tue Oct 29 16:10:12 IST 2013 |
scheduleWithFixedDelay(Runnable command,long initialDelay,long delay,TimeUnit unit) 该方法可被用于延迟周期性执行任务，delaytime是线程停止执行到下一次开始执行之间的延迟时间，假设有下面的代码
| 1 2 3 4 5 6 | for ( int i = 0 ; i < 3 ; i++) { Thread.sleep( 1000 ); WorkerThread worker = new WorkerThread( "do heavy processing" ); scheduledThreadPool.scheduleWithFixedDelay(worker, 0 , 1 , TimeUnit.SECONDS); } |
输出结果
| 1 2 3 4 5 6 7 8 9 | Current Time = Tue Oct 29 16:14:13 IST 2013 pool-1-thread-1 Start. Time = Tue Oct 29 16:14:14 IST 2013 pool-1-thread-2 Start. Time = Tue Oct 29 16:14:15 IST 2013 pool-1-thread-3 Start. Time = Tue Oct 29 16:14:16 IST 2013 pool-1-thread-1 End. Time = Tue Oct 29 16:14:19 IST 2013 pool-1-thread-2 End. Time = Tue Oct 29 16:14:20 IST 2013 pool-1-thread-1 Start. Time = Tue Oct 29 16:14:20 IST 2013 pool-1-thread-3 End. Time = Tue Oct 29 16:14:21 IST 2013 pool-1-thread-4 Start. Time = Tue Oct 29 16:14:21 IST 2013 |
