Interface - java.util.concurrent.ScheduledExecutorService
Created by : Mr Dk.
2021 / 02 / 16 🧧 15:48
Ningbo, Zhejiang, China
Definition
该接口扩展了 ExecutorService 接口，使任务可以被调度至 稍后执行，或 周期性执行。其中定义的 schedule() 函数能够指定在任意延时后执行任务，而 scheduleAtFixedRate() 函数指定以固定的周期执行任务。对于父类接口中定义的 execute() 等相当于以零延时 (立刻) 执行任务。
所有的调度函数参数接收的都是 相对时间，而不是绝对时间。
/**
* An {@link ExecutorService} that can schedule commands to run after a given
* delay, or to execute periodically.
*
* <p>The {@code schedule} methods create tasks with various delays
* and return a task object that can be used to cancel or check
* execution. The {@code scheduleAtFixedRate} and
* {@code scheduleWithFixedDelay} methods create and execute tasks
* that run periodically until cancelled.
*
* <p>Commands submitted using the {@link Executor#execute(Runnable)}
* and {@link ExecutorService} {@code submit} methods are scheduled
* with a requested delay of zero. Zero and negative delays (but not
* periods) are also allowed in {@code schedule} methods, and are
* treated as requests for immediate execution.
*
* <p>All {@code schedule} methods accept <em>relative</em> delays and
* periods as arguments, not absolute times or dates. It is a simple
* matter to transform an absolute time represented as a {@link
* java.util.Date} to the required form. For example, to schedule at
* a certain future {@code date}, you can use: {@code schedule(task,
* date.getTime() - System.currentTimeMillis(),
* TimeUnit.MILLISECONDS)}. Beware however that expiration of a
* relative delay need not coincide with the current {@code Date} at
* which the task is enabled due to network time synchronization
* protocols, clock drift, or other factors.
*
* <p>The {@link Executors} class provides convenient factory methods for
* the ScheduledExecutorService implementations provided in this package.
*
* <h3>Usage Example</h3>
*
* Here is a class with a method that sets up a ScheduledExecutorService
* to beep every ten seconds for an hour:
*
* <pre> {@code
* import static java.util.concurrent.TimeUnit.*;
* class BeeperControl {
* private final ScheduledExecutorService scheduler =
* Executors.newScheduledThreadPool(1);
*
* public void beepForAnHour() {
* final Runnable beeper = new Runnable() {
* public void run() { System.out.println("beep"); }
* };
* final ScheduledFuture<?> beeperHandle =
* scheduler.scheduleAtFixedRate(beeper, 10, 10, SECONDS);
* scheduler.schedule(new Runnable() {
* public void run() { beeperHandle.cancel(true); }
* }, 60 * 60, SECONDS);
* }
* }}</pre>
*
* @since 1.5
* @author Doug Lea
*/
public interface ScheduledExecutorService extends ExecutorService {
}
Schedule
在指定的延时后创建并执行任务。返回一个用于查询状态或停止执行的 Future 对象。
/**
* Creates and executes a one-shot action that becomes enabled
* after the given delay.
*
* @param command the task to execute
* @param delay the time from now to delay execution
* @param unit the time unit of the delay parameter
* @return a ScheduledFuture representing pending completion of
* the task and whose {@code get()} method will return
* {@code null} upon completion
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
* @throws NullPointerException if command is null
*/
public ScheduledFuture<?> schedule(Runnable command,
long delay, TimeUnit unit);
/**
* Creates and executes a ScheduledFuture that becomes enabled after the
* given delay.
*
* @param callable the function to execute
* @param delay the time from now to delay execution
* @param unit the time unit of the delay parameter
* @param <V> the type of the callable's result
* @return a ScheduledFuture that can be used to extract result or cancel
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
* @throws NullPointerException if callable is null
*/
public <V> ScheduledFuture<V> schedule(Callable<V> callable,
long delay, TimeUnit unit);
Schedule at Fixed Rate
任务将在一段 初始延时 之后，以固定的周期被启动执行。任务只能通过取消或关闭执行器的方式终结。如果任务的执行超过了执行周期，随后的任务将会暂缓开始，而不会并发执行。
注意，下一个任务的启动时间只和任务的执行周期有关，与任务执行期间耗费的时间无关。这里的 周期 指的是 前一个任务的开始时刻 到 下一个任务的开始时刻 的时间差。
/**
* Creates and executes a periodic action that becomes enabled first
* after the given initial delay, and subsequently with the given
* period; that is executions will commence after
* {@code initialDelay} then {@code initialDelay+period}, then
* {@code initialDelay + 2 * period}, and so on.
* If any execution of the task
* encounters an exception, subsequent executions are suppressed.
* Otherwise, the task will only terminate via cancellation or
* termination of the executor. If any execution of this task
* takes longer than its period, then subsequent executions
* may start late, but will not concurrently execute.
*
* @param command the task to execute
* @param initialDelay the time to delay first execution
* @param period the period between successive executions
* @param unit the time unit of the initialDelay and period parameters
* @return a ScheduledFuture representing pending completion of
* the task, and whose {@code get()} method will throw an
* exception upon cancellation
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
* @throws NullPointerException if command is null
* @throws IllegalArgumentException if period less than or equal to zero
*/
public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
long initialDelay,
long period,
TimeUnit unit);
Schedule with Fixed Delay
任务将在一段 初始延时 之后，以固定的 间隔 被执行。间隔的具体含义是 前一个任务的结束时刻 到 下一个任务的开始时刻 之间的时间差。
/**
* Creates and executes a periodic action that becomes enabled first
* after the given initial delay, and subsequently with the
* given delay between the termination of one execution and the
* commencement of the next. If any execution of the task
* encounters an exception, subsequent executions are suppressed.
* Otherwise, the task will only terminate via cancellation or
* termination of the executor.
*
* @param command the task to execute
* @param initialDelay the time to delay first execution
* @param delay the delay between the termination of one
* execution and the commencement of the next
* @param unit the time unit of the initialDelay and delay parameters
* @return a ScheduledFuture representing pending completion of
* the task, and whose {@code get()} method will throw an
* exception upon cancellation
* @throws RejectedExecutionException if the task cannot be
* scheduled for execution
* @throws NullPointerException if command is null
* @throws IllegalArgumentException if delay less than or equal to zero
*/
public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
long initialDelay,
long delay,
TimeUnit unit);