开发中,往往遇到另起线程执行其他代码的情况,用java定时任务接口ScheduledExecutorService来实现。
ScheduledExecutorService是基于线程池设计的定时任务类,每个调度任务都会分配到线程池中的一个线程去执行,也就是说,任务是并发执行,互不影响。
注意,只有当调度任务来的时候,ScheduledExecutorService才会真正启动一个线程,其余时间ScheduledExecutorService都是处于轮询任务的状态。
1.scheduleAtFixedRate方法
例子:
import java.text.SimpleDateFormat; import java.util.Date; import java.util.concurrent.Executors; import java.util.concurrent.ScheduledExecutorService; import java.util.concurrent.TimeUnit; public class ScheduleAtFixedRateDemo { public static void main(String[] args) { ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1); SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");//设置日期格式 executorService.scheduleAtFixedRate(new Runnable(){ @Override public void run() { System.out.println("++++++++++++++++++++thread:" + df.format(new Date())); } }, 2, 3, TimeUnit.SECONDS); System.out.println("++++++++++++++++++++main:" + df.format(new Date())); } }
运行结果:
++++++++++++++++++++main:2017-10-20 15:20:52
++++++++++++++++++++thread:2017-10-20 15:20:54
++++++++++++++++++++thread:2017-10-20 15:20:57
++++++++++++++++++++thread:2017-10-20 15:21:00
++++++++++++++++++++thread:2017-10-20 15:21:03
++++++++++++++++++++thread:2017-10-20 15:21:06
可以看出来,在2s后,子线程开始执行,并且每过3s轮询执行一次。
2.scheduleWithFixedDelay方法
例子:
import java.text.SimpleDateFormat; import java.util.Date; import java.util.concurrent.Executors; import java.util.concurrent.ScheduledExecutorService; import java.util.concurrent.TimeUnit; /** * ScheduleWithFixedDelay的用法 * @author Administrator * */ public class ScheduleWithFixedDelayDemo { public static void main(String[] args) { ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1); SimpleDateFormat df = new SimpleDateFormat("yyyy-MM-dd HH:mm:ss");//设置日期格式 executorService.scheduleWithFixedDelay(new Runnable(){ @Override public void run() { System.out.println("++++++++++++++++++++thread:" + df.format(new Date())); } }, 2, 3, TimeUnit.SECONDS); System.out.println("++++++++++++++++++++main:" + df.format(new Date())); } }
运行结果:
++++++++++++++++++++main:2017-10-20 15:24:32
++++++++++++++++++++thread:2017-10-20 15:24:34
++++++++++++++++++++thread:2017-10-20 15:24:37
++++++++++++++++++++thread:2017-10-20 15:24:40
++++++++++++++++++++thread:2017-10-20 15:24:43
3.两个区别
ScheduleAtFixedRate每次执行时间为上一次任务开始起向后推一个时间间隔,即每次执行时间为initialDelay,initialDelay+period,initialDelay+2*period。。。。。
ScheduleWithFixedDelay每次执行时间为上一次任务结束起向后推一个时间间隔,即每次执行时间为:initialDelay,initialDelay+executeTime+delay,initialDelay+2*executeTime+2*delay。。。。。
由此可见,ScheduleAtFixedRate是基于固定时间间隔进行任务调度,ScheduleWithFixedDelay取决于每次任务执行的时间长短,是基于不固定时间间隔进行任务调度。