http://tutorials.jenkov.com/java-util-concurrent/scheduledexecutorservice.html
http://www.infoq.com/cn/articles/java-blocking-queue
The
java.util.concurrent.ScheduledExecutorService is an ExecutorService which can schedule tasks to run after a delay, or to execute repeatedly with a fixed interval of time in between each execution. Tasks are executed asynchronously by a worker thread, and not by the thread handing the task to the ScheduledExecutorService.
The
java.util.concurrent.ScheduledExecutorService is an ExecutorService which can schedule tasks to run after a delay, or to execute repeatedly with a fixed interval of time in between each execution. Tasks are executed asynchronously by a worker thread, and not by the thread handing the task to the ScheduledExecutorService.schedule (Callable task, long delay, TimeUnit timeunit)
This method schedules the given
Callable for execution after the given delay.
The method returns a
ScheduledFuture which you can use to either cancel the task before it has started executing, or obtain the result once it is executed.
Here is an example:
ScheduledExecutorService scheduledExecutorService =
Executors.newScheduledThreadPool(5);
ScheduledFuture scheduledFuture =
scheduledExecutorService.schedule(new Callable() {
public Object call() throws Exception {
System.out.println("Executed!");
return "Called!";
}
},
5,
TimeUnit.SECONDS);
System.out.println("result = " + scheduledFuture.get());
scheduledExecutorService.shutdown();
This example outputs:
Executed! result = Called!
schedule (Runnable task, long delay, TimeUnit timeunit)
This method works like the method version taking a
Callable as parameter, except a Runnable cannot return a value, so the ScheduledFuture.get() method returns null when the task is finished.scheduleAtFixedRate (Runnable, long initialDelay, long period, TimeUnit timeunit)
This method schedules a task to be executed periodically. The task is executed the first time after the
initialDelay, and then recurringly every time the period expires.
If any execution of the given task throws an exception, the task is no longer executed. If no exceptions are thrown, the task will continue to be executed until the
ScheduledExecutorService is shut down.
If a task takes longer to execute than the period between its scheduled executions, the next execution will start after the current execution finishes. The scheduled task will not be executed by more than one thread at a time.
scheduleWithFixedDelay (Runnable, long initialDelay, long period, TimeUnit timeunit)
This method works very much like
scheduleAtFixedRate() except that the period is interpreted differently.
In the
scheduleAtFixedRate() method the period is interpreted as a delay between the start of the previous execution, until the start of the next execution.
In this method, however, the
period is interpreted as the delay between the end of the previous execution, until the start of the next. The delay is thus between finished executions, not between the beginning of executions.ScheduledExecutorService Shutdown
Just like an
ExecutorService, the ScheduledExecutorService needs to be shut down when you are finished using it. If not, it will keep the JVM running, even when all other threads have been shut down.
You shut down a
ScheduledExecutorService using the shutdown() or shutdownNow() methods which are inherited from the ExecutorService interface.http://www.infoq.com/cn/articles/java-blocking-queue
DelayQueue是一个支持延时获取元素的无界阻塞队列。队列使用PriorityQueue来实现。队列中的元素必须实现Delayed接口，在创建元素时可以指定多久才能从队列中获取当前元素。只有在延迟期满时才能从队列中提取元素。我们可以将DelayQueue运用在以下应用场景：
- 缓存系统的设计：可以用DelayQueue保存缓存元素的有效期，使用一个线程循环查询DelayQueue，一旦能从DelayQueue中获取元素时，表示缓存有效期到了。
- 定时任务调度。使用DelayQueue保存当天将会执行的任务和执行时间，一旦从DelayQueue中获取到任务就开始执行，从比如TimerQueue就是使用DelayQueue实现的。
队列中的Delayed必须实现compareTo来指定元素的顺序。比如让延时时间最长的放在队列的末尾。实现代码如下：
public int compareTo(Delayed other) {
if (other == this) // compare zero ONLY if same object
return 0;
if (other instanceof ScheduledFutureTask) {
ScheduledFutureTask x = (ScheduledFutureTask)other;
long diff = time - x.time;
if (diff < 0)
return -1;
else if (diff > 0)
return 1;
else if (sequenceNumber < x.sequenceNumber)
return -1;
else
return 1;
}
long d = (getDelay(TimeUnit.NANOSECONDS) -
other.getDelay(TimeUnit.NANOSECONDS));
return (d == 0) ? 0 : ((d < 0) ? -1 : 1);
}
如何实现Delayed接口
我们可以参考ScheduledThreadPoolExecutor里ScheduledFutureTask类。这个类实现了Delayed接口。首先：在对象创建的时候，使用time记录前对象什么时候可以使用，代码如下：
ScheduledFutureTask(Runnable r, V result, long ns, long period) {
super(r, result);
this.time = ns;
this.period = period;
this.sequenceNumber = sequencer.getAndIncrement();
}
然后使用getDelay可以查询当前元素还需要延时多久，代码如下：
public long getDelay(TimeUnit unit) {
return unit.convert(time - now(), TimeUnit.NANOSECONDS);
}
通过构造函数可以看出延迟时间参数ns的单位是纳秒，自己设计的时候最好使用纳秒，因为getDelay时可以指定任意单位，一旦以纳秒作为单位，而延时的时间又精确不到纳秒就麻烦了。使用时请注意当time小于当前时间时，getDelay会返回负数。
如何实现延时队列
延时队列的实现很简单，当消费者从队列里获取元素时，如果元素没有达到延时时间，就阻塞当前线程。
long delay = first.getDelay(TimeUnit.NANOSECONDS);
if (delay <= 0)
return q.poll();
else if (leader != null)
available.await();
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;
}
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;
}
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;
}
public interface ScheduledFuture<V> extends Delayed, Future<V> {
}
public interface Delayed extends Comparable<Delayed> {
* Returns the remaining delay associated with this object, in the
* given time unit.
long getDelay(TimeUnit unit);
}
