源码解读ExcutorService之ScheduledExecutorService

ScheduledExecutorService继承自ExcutorService，也就是它是一种线程池，它不仅能实现任务的跟踪与管理，还能定时、定频执行任务。

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• An ExecutorService that can schedule commands to run after a given delay, or to execute periodically.

它是一种可以任务调度的线程池，你可以使其延时、或者定频执行。

• The schedule methods create tasks with various delays and return a task object that can be used to cancel or check execution. The scheduleAtFixedRate and scheduleWithFixedDelay methods create and execute tasks that run periodically until cancelled.

你可以指定任意的延迟来使用schedule方法以创建一个任务，它会返回一个task对象用以取消或者跟踪执行情况。scheduleAtFixedRate、scheduleWithFixedDelay 方法可以创建一个定频任务，直到你取消它。

• Commands submitted using the Executor.execute(Runnable) and ExecutorService submit methods are scheduled with a requested delay of zero. Zero and negative delays (but not periods) are also allowed in schedule methods, and are treated as requests for immediate execution.

你可以使用execute方法或者submit方法来提交一个任务，任务会被零延迟地立刻执行。你可以在schedule方法，也就是创建任务时指定零延迟或者负延迟，它们会被当作立即执行的任务。

• All schedule methods accept relative delays and periods as arguments, not absolute times or dates. It is a simple matter to transform an absolute time represented as a java.util.Date to the required form. For example, to schedule at a certain future date, you can use: schedule(task, date.getTime() - System.currentTimeMillis(), TimeUnit.MILLISECONDS). Beware however that expiration of a relative delay need not coincide with the current Date at which the task is enabled due to network time synchronization protocols, clock drift, or other factors.

所有的schedule方法都可以接收 延迟 和 定频 作为参数，参数不是一个绝对的时间或者日期。将一个绝对的时间转换为需要的形式是很容易实现的。比方说，要实现未来某个时刻的任务调度，我们可以调用

    schedule(task, date.getTime() - System.currentTimeMillis(), TimeUnit.MILLISECONDS)

但要注意，这个延迟到期时，可能因为各种原因，例如网络时间同步协议、时间不同步等等、导致真正的延迟时间和你设定的不一致。（比如说现在是9点，我设置了10点.getTime() - System.currentTimeMillis()的delay，这时候服务器上的时间是9点50分，任务延迟10分钟就会执行，而不是预想的一小时。）

• The Executors class provides convenient factory methods for the ScheduledExecutorService implementations provided in this package.

在这个包中，Executors这个类提供了很多方便的Factory方法实现了ScheduledExecutorService，比如说：