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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:
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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:
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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。
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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
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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); |
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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 |
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for ( int i
= 0 ;
i < 3 ;
i++) { Thread.sleep( 1000 ); WorkerThread
worker = new WorkerThread( "do
heavy processing" ); scheduledThreadPool.scheduleWithFixedDelay(worker, 0 , 1 , TimeUnit.SECONDS); } |
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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 |