Java ScheduledThreadPoolExecutor延迟或周期性执行任务

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:

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:

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。

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

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

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

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

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

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