ScheduledThreadPoolExecutor解析
ScheduledThreadPoolExecutor适用于延时执行,或者周期性执行的任务调度,ScheduledThreadPoolExecutor在实现上继承了ThreadPoolExecutor,所以你依然可以将ScheduledThreadPoolExecutor当成ThreadPoolExecutor来使用,但是ScheduledThreadPoolExecutor的功能要强大得多,因为ScheduledThreadPoolExecutor可以根据设定的参数来周期性调度运行,下面是四个和周期性相关的方法:
1.scheduled()
public class ScheduledThreadPoolExecutor extends ThreadPoolExecutor implements ScheduledExecutorService { public ScheduledFuture<?> schedule(Runnable command, long delay, TimeUnit unit) { if (command == null || unit == null) throw new NullPointerException(); RunnableScheduledFuture<?> t = decorateTask(command, new ScheduledFutureTask<Void>(command, null, triggerTime(delay, unit))); delayedExecute(t); return t; } public <V> ScheduledFuture<V> schedule(Callable<V> callable, long delay, TimeUnit unit) { if (callable == null || unit == null) throw new NullPointerException(); RunnableScheduledFuture<V> t = decorateTask(callable, new ScheduledFutureTask<V>(callable, triggerTime(delay, unit))); delayedExecute(t); return t; } }
2.scheduleAtFixedRate()
public ScheduledFuture<?> scheduleAtFixedRate(Runnable command, long initialDelay, long period, TimeUnit unit) if (command == null || unit == null) throw new NullPointerException(); if (period <= 0) throw new IllegalArgumentException(); ScheduledFutureTask<Void> sft = new ScheduledFutureTask<Void>(command, null, triggerTime(initialDelay, unit), unit.toNanos(period)); RunnableScheduledFuture<Void> t = decorateTask(command, sft); sft.outerTask = t; delayedExecute(t); return t; }
说明: 该方法会延时一段时间,然后根据设定的参数周期执行Runnable,在执行时将严格按照规划的时间路径来执行,比如周期为2,延时为0,那么执行的序列为0,2,4,6,8....,
3.scheduleWithFixedDelay()
public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command, long initialDelay, long delay, TimeUnit unit){ if (command == null || unit == null) throw new NullPointerException(); if (delay <= 0) throw new IllegalArgumentException(); ScheduledFutureTask<Void> sft = new ScheduledFutureTask<Void>(command, null, triggerTime(initialDelay, unit), unit.toNanos(-delay)); RunnableScheduledFuture<Void> t = decorateTask(command, sft); sft.outerTask = t; delayedExecute(t); return t; }
说明:该方法会延时一段时间,然后根据设定的参数周期执行Runnable,在执行时将基于上次执行时间来规划下次的执行,也就是在上次执行完成之后再次执行。比如上面的执行序列0,2,4,6,8...,如果第2秒没有被调度执行,而在第三秒的时候才被调度,那么下次执行的时间不是4,而是5,以此类推。
通过上面的代码我们可以发现,前两个方法是类似的,后两个方法也是类似的。前两个方法属于一次性调度,所以period都为0,区别在于参数不同,一个是Runnable,而一个是Callable,它们最后都变为了Callable了,以上四个方法最后都会调用一个方法: delayedExecute(t),下面看一下这个方法:
4.delayedExecute()
private void delayedExecute(RunnableScheduledFuture<?> task) { if (isShutdown()) reject(task); else { super.getQueue().add(task); if (isShutdown() && !canRunInCurrentRunState(task.isPeriodic()) && remove(task)) task.cancel(false); else ensurePrestart(); } }
说明:该方法先判断线程池是否被关闭了,如果被关闭了,则拒绝任务的提交,否则将任务加入到任务队列中去等待被调度执行。最后的ensurePrestart的意思是需要确保线程池已经被启动起来了。下面是这个方法: