如何在执行一定次数后停止计划重复执行的 Runnable

Question

情况

我有一个可运行的。我有一个类，它使用带有 scheduleWithFixedDelay。

目标

我想更改此类以安排 Runnable 无限期地进行固定延迟执行，或直到它运行一定次数，具体取决于某个参数传递给构造函数。

如果可能的话，我想使用相同的 Runnable，因为它在概念上是应该“运行”的相同事物。

可能的方法

方法#1

有两个 Runnable，一个在多次执行后取消计划（它会记录计数），另一个则不会：

public class MyClass{
    private ScheduledExecutorService scheduler = Executors.newSingleThreadScheduledExecutor();

    public enum Mode{
        INDEFINITE, FIXED_NO_OF_TIMES
    }

    public MyClass(Mode mode){
        if(mode == Mode.INDEFINITE){
            scheduler.scheduleWithFixedDelay(new DoSomethingTask(), 0, 100, TimeUnit.MILLISECONDS);
        }else if(mode == Mode.FIXED_NO_OF_TIMES){
            scheduler.scheduleWithFixedDelay(new DoSomethingNTimesTask(), 0, 100, TimeUnit.MILLISECONDS);
        }
    }

    private class DoSomethingTask implements Runnable{
        @Override
        public void run(){
            doSomething();
        }
    }

    private class DoSomethingNTimesTask implements Runnable{
        private int count = 0;

        @Override
        public void run(){
            doSomething();
            count++;
            if(count > 42){
                // Cancel the scheduling.
                // Can you do this inside the run method, presumably using
                // the Future returned by the schedule method? Is it a good idea?
            }
        }
    }

    private void doSomething(){
        // do something
    }
}

我宁愿只使用一个 Runnable 来执行 doSomething 方法。将调度与 Runnable 联系起来感觉是错误的。您对此有何看法？

方法#2

使用单个 Runnable 来执行我们想要定期运行的代码。有一个单独的预定可运行对象，用于检查第一个可运行对象已运行多少次，并在达到一定数量时取消。这可能不准确，因为它是异步的。感觉有点麻烦。您对此有何看法？

方法 #3

扩展 ScheduledExecutorService 并添加方法“scheduleWithFixedDelayNTimes”。也许这样的类已经存在了？目前，我正在使用 Executors.newSingleThreadScheduledExecutor(); 来获取 ScheduledExecutorService 实例。我可能必须实现类似的功能来实例化扩展的 ScheduledExecutorService。这可能很棘手。您对此有何看法？

无调度程序方法 [编辑]

我无法使用调度程序。我可以改为：

for(int i = 0; i < numTimesToRun; i++){
    doSomething();
    Thread.sleep(delay);
}

并在某个线程中运行它。你对此有何看法？您仍然可以使用 runnable 并直接调用 run 方法。

---

欢迎任何建议。我正在寻找一场辩论，以找到实现我的目标的“最佳实践”方式。

Answer 1

您可以在 Future 上使用 cancel() 方法。来自 scheduleAtFixedRate

Otherwise, the task will only terminate via cancellation or termination of the executor

这里是一些示例代码将 Runnable 包装在另一个 Runnable 中，跟踪原始运行的次数，并在运行 N 次后取消。

public void runNTimes(Runnable task, int maxRunCount, long period, TimeUnit unit, ScheduledExecutorService executor) {
    new FixedExecutionRunnable(task, maxRunCount).runNTimes(executor, period, unit);
}

class FixedExecutionRunnable implements Runnable {
    private final AtomicInteger runCount = new AtomicInteger();
    private final Runnable delegate;
    private volatile ScheduledFuture<?> self;
    private final int maxRunCount;

    public FixedExecutionRunnable(Runnable delegate, int maxRunCount) {
        this.delegate = delegate;
        this.maxRunCount = maxRunCount;
    }

    @Override
    public void run() {
        delegate.run();
        if(runCount.incrementAndGet() == maxRunCount) {
            boolean interrupted = false;
            try {
                while(self == null) {
                    try {
                        Thread.sleep(1);
                    } catch (InterruptedException e) {
                        interrupted = true;
                    }
                }
                self.cancel(false);
            } finally {
                if(interrupted) {
                    Thread.currentThread().interrupt();
                }
            }
        }
    }

    public void runNTimes(ScheduledExecutorService executor, long period, TimeUnit unit) {
        self = executor.scheduleAtFixedRate(this, 0, period, unit);
    }
}

Answer 2

引用自 API 描述 (ScheduledExecutorService.scheduleWithFixedDelay）：

创建并执行一个周期性操作，该操作在给定的初始延迟后首先启用，然后在一次执行终止和下一次执行开始之间具有给定的延迟。 如果任务的任何执行遇到异常，则后续执行将被抑制。否则，任务只会通过取消或终止执行器来终止。

因此，最简单的事情就是“只是抛出异常”（尽管这被认为是不好的做法）：

static class MyTask implements Runnable {

    private int runs = 0;

    @Override
    public void run() {
        System.out.println(runs);
        if (++runs >= 20)
            throw new RuntimeException();
    }
}

public static void main(String[] args) {
    ScheduledExecutorService s = Executors.newSingleThreadScheduledExecutor();
    s.scheduleWithFixedDelay(new MyTask(), 0, 100, TimeUnit.MILLISECONDS);
}

Answer 3

到目前为止，sbridges 解决方案似乎是最干净的解决方案，除了您提到的之外，它将处理执行次数的责任留给了 Runnable 本身。它不应该关心这个，相反，重复应该是处理调度的类的参数。为了实现这一目标，我建议采用以下设计，为 Runnables 引入一个新的执行器类。该类提供了两种用于调度任务的公共方法，它们是标准的 Runnables，具有有限或无限重复。如果需要，可以传递相同的 Runnable 来进行有限和无限调度（这对于扩展 Runnable 类以提供有限重复的所有建议解决方案都是不可能的）。取消有限重复的处理完全封装在调度程序类中：

class MaxNScheduler
{

  public enum ScheduleType 
  {
     FixedRate, FixedDelay
  }

  private ScheduledExecutorService executorService =
     Executors.newSingleThreadScheduledExecutor();

  public ScheduledFuture<?> scheduleInfinitely(Runnable task, ScheduleType type, 
    long initialDelay, long period, TimeUnit unit)
  {
    return scheduleNTimes(task, -1, type, initialDelay, period, unit);
  }

  /** schedule with count repetitions */
  public ScheduledFuture<?> scheduleNTimes(Runnable task, int repetitions, 
    ScheduleType type, long initialDelay, long period, TimeUnit unit) 
  {
    RunnableWrapper wrapper = new RunnableWrapper(task, repetitions);
    ScheduledFuture<?> future;
    if(type == ScheduleType.FixedDelay)
      future = executorService.scheduleWithFixedDelay(wrapper, 
         initialDelay, period, TimeUnit.MILLISECONDS);
    else
      future = executorService.scheduleAtFixedRate(wrapper, 
         initialDelay, period, TimeUnit.MILLISECONDS);
    synchronized(wrapper)
    {
       wrapper.self = future;
       wrapper.notify(); // notify wrapper that it nows about it's future (pun intended)
    }
    return future;
  }

  private static class RunnableWrapper implements Runnable 
  {
    private final Runnable realRunnable;
    private int repetitions = -1;
    ScheduledFuture<?> self = null;

    RunnableWrapper(Runnable realRunnable, int repetitions) 
    {
      this.realRunnable = realRunnable;
      this.repetitions = repetitions;
    }

    private boolean isInfinite() { return repetitions < 0; }
    private boolean isFinished() { return repetitions == 0; }

    @Override
    public void run()
    {
      if(!isFinished()) // guard for calls to run when it should be cancelled already
      {
        realRunnable.run();

        if(!isInfinite())
        {
          repetitions--;
          if(isFinished())
          {
            synchronized(this) // need to wait until self is actually set
            {
              if(self == null)
              {
                 try { wait(); } catch(Exception e) { /* should not happen... */ }
              }
              self.cancel(false); // cancel gracefully (not throwing InterruptedException)
            }
          }
        }
      }
    }
  }

}

公平地说，管理重复的逻辑仍然是 a Runnable，但它是 < code>Runnable 完全位于 MaxNScheduler 内部，而为调度传递的 Runnable 任务不必关心调度的性质。此外，如果需要，可以通过在每次执行 RunnableWrapper.run 时提供一些回调，轻松地将这个问题转移到调度程序中。这会使代码稍微复杂化，并且需要保留一些 RunnableWrapper 映射和相应的重复，这就是为什么我选择将计数器保留在 RunnableWrapper 类中。

设置 self 时，我还在包装器上添加了一些同步。从理论上讲，这是需要的，当执行完成时， self 可能尚未被分配（这是一个相当理论的场景，但只可能重复 1 次）。

取消会被优雅地处理，不会抛出 InterruptedException，并且如果在执行取消之前安排了另一轮，则 RunnableWrapper 不会调用底层的 Runnable< /代码>。

Answer 4

这是我的建议（我相信它可以处理问题中提到的所有情况）：

public class RepeatedScheduled implements Runnable {

    private int repeatCounter = -1;
    private boolean infinite;

    private ScheduledExecutorService ses;
    private long initialDelay;
    private long delay;
    private TimeUnit unit;

    private final Runnable command;
    private Future<?> control;

    public RepeatedScheduled(ScheduledExecutorService ses, Runnable command,
        long initialDelay, long delay, TimeUnit unit) {

        this.ses = ses;
        this.initialDelay = initialDelay;
        this.delay = delay;
        this.unit = unit;

        this.command = command;
        this.infinite = true;

    }

    public RepeatedScheduled(ScheduledExecutorService ses, Runnable command,
        long initialDelay, long delay, TimeUnit unit, int maxExecutions) {

        this(ses, command, initialDelay, delay, unit);
        this.repeatCounter = maxExecutions;
        this.infinite = false;

    }

    public Future<?> submit() {

        // We submit this, not the received command
        this.control = this.ses.scheduleWithFixedDelay(this,
            this.initialDelay, this.delay, this.unit);

        return this.control;

    }

    @Override
    public synchronized void run() {

        if ( !this.infinite ) {
            if ( this.repeatCounter > 0 ) {
                this.command.run();
                this.repeatCounter--;
            } else {
                this.control.cancel(false);
            }
        } else {
            this.command.run();
        }

    }

}

此外，它允许外部方停止 submit()Future 中的所有内容> 方法。

用法：

Runnable MyRunnable = ...;
// Repeat 20 times
RepeatedScheduled rs = new RepeatedScheduled(
    MySes, MyRunnable, 33, 44, TimeUnit.SECONDS, 20);
Future<?> MyControl = rs.submit();
...

Answer 5

对于轮询直到某个超时的用例，我们可以使用 Future.get() 采用更简单的解决方案。

/* Define task */
public class Poll implements Runnable {
    @Override
    public void run() {
        // Polling logic
    }
}

/* Create executor service */
ScheduledExecutorService executorService = Executors.newScheduledThreadPool(5);

/* Schedule task - poll every 500ms */
ScheduledFuture<?> future = executorService.scheduleAtFixedRate(new Poll(), 0, 500, TimeUnit.MILLISECONDS);

/* Wait till 60 sec timeout */
try {
    future.get(60, TimeUnit.SECONDS);
} catch (TimeoutException e) {
    scheduledFuture.cancel(false);
    // Take action on timeout
}

Answer 6

你的第一种方法似乎没问题。您可以通过将 mode 对象传递给其构造函数（或传递 -1 作为其必须运行的最大次数）来组合两种类型的可运行对象，并使用此模式来确定是否必须取消可运行对象or not ：

private class DoSomethingNTimesTask implements Runnable{
    private int count = 0;
    private final int limit;

    /**
     * Constructor for no limit
     */
    private DoSomethingNTimesTask() {
        this(-1);
    }

    /**
     * Constructor allowing to set a limit
     * @param limit the limit (negative number for no limit)
     */
    private DoSomethingNTimesTask(int limit) {
        this.limit = limit;
    }

    @Override
    public void run(){
        doSomething();
        count++;
        if(limit >= 0 && count > limit){
            // Cancel the scheduling
        }
    }
}

您必须将计划的未来传递给您的任务才能使其自行取消，否则您可能会引发异常。

Answer 7

我一直在寻找完全相同的功能并选择了 org.springframework.scheduling.Trigger。

以下是完整测试的工作示例（抱歉，如果代码过多）
应用程序上下文.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
 xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xmlns:task="http://www.springframework.org/schema/task"
 xmlns:util="http://www.springframework.org/schema/util"
 xmlns:context="http://www.springframework.org/schema/context"
 xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
        http://www.springframework.org/schema/context/ http://www.springframework.org/schema/context/spring-context.xsd
        http://www.springframework.org/schema/util/ http://www.springframework.org/schema/util/spring-util.xsd
        http://www.springframework.org/schema/task http://www.springframework.org/schema/task/spring-task-3.0.xsd">

    <bean id="blockingTasksScheduler" class="org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler">
        <property name="poolSize" value="10" />
    </bean>

    <task:scheduler id="deftaskScheduler" pool-size="10" />

</beans>

JAVA

package com.alz.springTests.schedulerTest;

import java.time.LocalDateTime;
import java.time.ZoneId;
import java.time.temporal.ChronoUnit;
import java.util.Date;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.ThreadLocalRandom;
import java.util.concurrent.atomic.AtomicInteger;

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
import org.springframework.scheduling.TaskScheduler;
import org.springframework.scheduling.Trigger;
import org.springframework.scheduling.TriggerContext;
import org.springframework.scheduling.concurrent.ThreadPoolTaskScheduler;

public class ScheduledTest {

    private static ApplicationContext applicationContext;
    private static TaskScheduler taskScheduler;

    private static final class SelfCancelableTask implements Runnable, Trigger {
        Date creationTime = new Date();
        AtomicInteger counter = new AtomicInteger(0);
        private volatile boolean shouldStop = false;
        private int repeatInterval = 3; //seconds

        @Override
        public void run() {
            log("task: run started");

            // simulate "doing job" started
            int sleepTimeMs = ThreadLocalRandom.current().nextInt(500, 2000+1);
            log("will sleep " + sleepTimeMs + " ms");
            try {
                Thread.sleep(sleepTimeMs);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            // "doing job" finished

            int i = counter.incrementAndGet();
            if (i > 5) { //cancel myself
                logErr("Attempts exceeded, will mark as shouldStop");
                shouldStop = true;

            } else {
                log("task: executing cycle #"+i);
            }
        }

        @Override
        public Date nextExecutionTime(TriggerContext triggerContext) {
            log("nextExecutionTime: triggerContext.lastActualExecutionTime() " + triggerContext.lastActualExecutionTime());
            log("nextExecutionTime: triggerContext.lastCompletionTime() " + triggerContext.lastCompletionTime());
            log("nextExecutionTime: triggerContext.lastScheduledExecutionTime() " + triggerContext.lastScheduledExecutionTime());

            if (shouldStop) 
                return null;

            if (triggerContext.lastCompletionTime() == null) {
                LocalDateTime ldt = creationTime.toInstant().atZone(ZoneId.systemDefault()).toLocalDateTime().plus(repeatInterval, ChronoUnit.SECONDS);
                return Date.from(ldt.atZone(ZoneId.systemDefault()).toInstant());
            } else {
                LocalDateTime ldt = triggerContext.lastCompletionTime().toInstant().atZone(ZoneId.systemDefault()).toLocalDateTime().plus(repeatInterval, ChronoUnit.SECONDS);
                return Date.from(ldt.atZone(ZoneId.systemDefault()).toInstant());               
            }

        }

    }

    private static void log(String log) {
        System.out.printf("%s [%s] %s\r\n", LocalDateTime.now(), Thread.currentThread(), log);
    }

    private static void logErr(String log) {
        System.err.printf("%s [%s] %s\r\n", LocalDateTime.now(), Thread.currentThread(), log);
    }

    public static void main(String[] args) {

        log("main: Stated...");

        applicationContext = new ClassPathXmlApplicationContext("applicationContext.xml");

        taskScheduler = (TaskScheduler) applicationContext.getBean("blockingTasksScheduler");

        ScheduledThreadPoolExecutor scheduledThreadPoolExecutor = ((ThreadPoolTaskScheduler)taskScheduler).getScheduledThreadPoolExecutor();

        SelfCancelableTask selfCancelableTask = new SelfCancelableTask();
        taskScheduler.schedule(selfCancelableTask, selfCancelableTask);


        int waitAttempts = 0;
        while (waitAttempts < 30) {
            log("scheduledPool pending tasks: " + scheduledThreadPoolExecutor.getQueue().size());

            try {
                Thread.sleep(1*1000);
            } catch (InterruptedException e) {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }

            waitAttempts++;

        }

        log("main: Done!");


    }

}