周期性线程池 newScheduledThreadPool

川长思鸟来 2024-03-22 19:47 7阅读 0赞

## newScheduledThreadPool ##

public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
        
        return new ScheduledThreadPoolExecutor(corePoolSize);
    }
        
    public ScheduledThreadPoolExecutor(int corePoolSize) {
        
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS, new DelayedWorkQueue());
    }

<table> 
 <thead> 
  <tr> 
   <th align="left">ScheduledThreadPoolExecutor继承自ThreadPoolExecutor。它主要用来在给定的<code>延迟</code>之后运行任务，或者<code>定期执行任务</code>。ScheduledThreadPoolExecutor的功能与Timer类似，但ScheduledThreadPoolExecutor功能更强大、更灵活。Timer对应的是<code>单个后台线程</code>，而ScheduledThreadPoolExecutor可以在构造函数中指定<code>多个</code>对应的后台线程数。</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">Creates a thread pool that can schedule commands to run after a given <a href="https://dict.youdao.com/w/delay/#keyfrom=dict2.top" rel="nofollow">delay</a>, or to execute <a href="https://dict.youdao.com/w/periodically/#keyfrom=dict2.top" rel="nofollow">periodically</a>.</td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"><code>DelayQueue</code>是一个无界队列，所以ThreadPoolExecutor的<code>maximumPoolSize</code>在Scheduled-ThreadPoolExecutor中没有什么意义（设置maximumPoolSize的大小没有什么效果）。</td> 
  </tr> 
 </tbody> 
</table>

<table> 
 <thead> 
  <tr> 
   <th align="left"><code>ScheduledThreadPoolExecutor</code>的执行主要分为两大部分。</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left">1）当调用ScheduledThreadPoolExecutor的<code>scheduleAtFixedRate()</code>方法或者<code>scheduleWithFixedDelay()</code>方法时，会向ScheduledThreadPoolExecutor的<code>DelayQueue</code><strong>添加</strong>一个实现了<code>RunnableScheduledFutur</code>接口的<code>ScheduledFutureTask</code>。</td> 
  </tr> 
  <tr> 
   <td align="left">2）线程池中的线程从DelayQueue中获取<code>ScheduledFutureTask</code>，然后执行任务。</td> 
  </tr> 
 </tbody> 
</table>

ScheduledThreadPoolExecutor的实现

<table> 
 <thead> 
  <tr> 
   <th align="left">ScheduledThreadPoolExecutor会把待调度的任务（<code>ScheduledFutureTask</code>）放到一个<code>DelayQueue</code>中。</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"><strong><code>ScheduledFutureTask</code></strong> 主要包含3个成员变量，如下。</td> 
  </tr> 
  <tr> 
   <td align="left">·long型成员变量time，表示这个任务将要被执行的<code>具体时间</code>。</td> 
  </tr> 
  <tr> 
   <td align="left">·long型成员变量sequenceNumber，表示这个任务被添加到ScheduledThreadPoolExecutor中的<code>序号</code>。</td> 
  </tr> 
  <tr> 
   <td align="left">·long型成员变量period，表示任务执行的<code>间隔周期</code>。</td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"><strong><code>DelayQueue</code></strong> 封装了一个 <strong><code>PriorityQueue</code></strong> ，这个PriorityQueue会对队列中的Scheduled-FutureTask进行排序。<strong>排序时，<code>time</code>小的排在前面（时间早的任务将被先执行）。如果两个ScheduledFutureTask的time相同，就比较<code>sequenceNumber</code>，sequenceNumber小的排在前面（也就是说，如果两个任务的执行时间相同，那么先提交的任务将被先执行）</strong>。</td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">下图是ScheduledThreadPoolExecutor中的<code>线程1</code>执行某个周期任务的4个步骤。</td> 
  </tr> 
 </tbody> 
</table>

![在这里插入图片描述][523072b97d634548bd23c05c10cfbca8.png_pic_center]

<table> 
 <thead> 
  <tr> 
   <th align="left"></th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left">1）线程1从DelayQueue中<code>获取已到期</code>的ScheduledFutureTask（DelayQueue.take()）。到期任务是指ScheduledFutureTask的time大于等于当前时间。</td> 
  </tr> 
  <tr> 
   <td align="left">2）线程1<code>执行</code>这个ScheduledFutureTask。</td> 
  </tr> 
  <tr> 
   <td align="left">3）线程1<code>修改</code>ScheduledFutureTask的<code>time</code>变量为下次将要被执行的时间。</td> 
  </tr> 
  <tr> 
   <td align="left">4）线程1把这个修改time之后的ScheduledFutureTask<code>放回</code>DelayQueue中（<code>Delay-Queue.add()</code>）。</td> 
  </tr> 
 </tbody> 
</table>

获取任务的过程   
![在这里插入图片描述][ef0a97a8a38e4fb28b8400ac52c8c2d3.png_pic_center]

<table> 
 <thead> 
  <tr> 
   <th align="left">获取任务分为3大步骤</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left">1）获取Lock。</td> 
  </tr> 
  <tr> 
   <td align="left">2）获取周期任务。</td> 
  </tr> 
  <tr> 
   <td align="left">·如果PriorityQueue为空，<code>当前线程到Condition中等待</code>；否则执行下面的2.2。</td> 
  </tr> 
  <tr> 
   <td align="left">·如果PriorityQueue的头元素的time时间比当前时间大，<code>到Condition中等待到time时间</code>；否则执行下面的2.3。</td> 
  </tr> 
  <tr> 
   <td align="left">·<code>获取</code>PriorityQueue的头元素（2.3.1）；如果PriorityQueue不为空，则<code>唤醒</code>在Condition中等待的所有线程（2.3.2）。</td> 
  </tr> 
  <tr> 
   <td align="left">3）释放Lock。</td> 
  </tr> 
  <tr> 
   <td align="left">ScheduledThreadPoolExecutor在一个循环中执行步骤2，直到线程从PriorityQueue获取到一个元素之后（执行2.3.1之后），<code>才会退出无限循环</code>（结束步骤2）。</td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
  </tr> 
 </tbody> 
</table>

把ScheduledFutureTask放入DelayQueue中的过程

![在这里插入图片描述][068f585e89e44ea1bfb0bd0daba83c7f.png_pic_center]

<table> 
 <thead> 
  <tr> 
   <th align="left">添加任务分为3大步骤。</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left">1）获取Lock。</td> 
  </tr> 
  <tr> 
   <td align="left">2）添加任务。</td> 
  </tr> 
  <tr> 
   <td align="left">·向PriorityQueue添加任务。</td> 
  </tr> 
  <tr> 
   <td align="left">·如果在上面2.1中<code>添加</code>的任务是PriorityQueue的头元素，<code>唤醒</code>在Condition中等待的所有线程。</td> 
  </tr> 
  <tr> 
   <td align="left">3）释放Lock。</td> 
  </tr> 
 </tbody> 
</table>

\-----------------------------------------------------------------------------读书笔记摘自 书名：Java并发编程的艺术 作者：方腾飞；魏鹏；程晓明

### 常见用途 ###

参考：[Executor框架的成员][Executor]

### 线程任务 ###

public class Task implements Runnable {
        
        private final String name;
    
        public Task(String name) {
        
            this.name = name;
        }
    
        @Override
        public void run() {
        
            System.out.println(Thread.currentThread().getName() + " → " + name + " Start Time = " + new Date());
            processCommand();
            System.out.println(Thread.currentThread().getName() + " → " + name + " End   Time = " + new Date());
        }
    
        private void processCommand() {
        
            try {
        
                Thread.sleep(3000);
            } catch (InterruptedException e) {
        
                e.printStackTrace();
            }
        }
    }

### 1. `schedule(new Task(), 10, TimeUnit.SECONDS)` ###

public class ScheduledThreadPool {
        
        public static void main(String args[]) throws InterruptedException, ExecutionException {
        
    
            System.out.println(Thread.currentThread().getName() + "线程: Start at: " + new Date());
    
            ScheduledThreadPoolExecutor exec = (ScheduledThreadPoolExecutor) Executors.newScheduledThreadPool(5);
    
            for (int i = 0; i < 10; i++) {
        
                System.out.println("添加了第" + i + "个线程任务 " + new Date());
                exec.schedule(new Task("线程名字" + i), 10, TimeUnit.SECONDS);// 延迟10秒执行
            }
    
            exec.shutdown();
    
            System.out.println(Thread.currentThread().getName() + "线程: 打卡" + new Date());
    
            while (!exec.isTerminated()) {
        
                // wait for all tasks to finish
            }
    
            System.out.println(Thread.currentThread().getName() + "线程: Finished all threads at：" + new Date());
        }
    }

--------------------

#### 执行结果分析 ####

<table> 
 <thead> 
  <tr> 
   <th align="left">执行结果</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left">main线程: Start at: Sat May 20 12:51:32 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第0个线程任务 Sat May 20 12:51:32 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第1个线程任务 Sat May 20 12:51:32 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第2个线程任务 Sat May 20 12:51:32 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第3个线程任务 Sat May 20 12:51:32 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第4个线程任务 Sat May 20 12:51:32 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第5个线程任务 Sat May 20 12:51:32 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第6个线程任务 Sat May 20 12:51:32 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第7个线程任务 Sat May 20 12:51:32 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第8个线程任务 Sat May 20 12:51:32 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第9个线程任务 Sat May 20 12:51:32 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">main线程: 打卡Sat May 20 12:51:32 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-5 → 线程名字2 Start Time = Sat May 20 12:51:42 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-3 → 线程名字1 Start Time = Sat May 20 12:51:42 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-4 → 线程名字3 Start Time = Sat May 20 12:51:42 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-2 → 线程名字4 Start Time = Sat May 20 12:51:42 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 线程名字0 Start Time = Sat May 20 12:51:42 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-3 → 线程名字1 End Time = Sat May 20 12:51:45 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-5 → 线程名字2 End Time = Sat May 20 12:51:45 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-3 → 线程名字5 Start Time = Sat May 20 12:51:45 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-5 → 线程名字6 Start Time = Sat May 20 12:51:45 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-4 → 线程名字3 End Time = Sat May 20 12:51:45 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-4 → 线程名字7 Start Time = Sat May 20 12:51:45 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-2 → 线程名字4 End Time = Sat May 20 12:51:45 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-2 → 线程名字8 Start Time = Sat May 20 12:51:45 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 线程名字0 End Time = Sat May 20 12:51:45 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 线程名字9 Start Time = Sat May 20 12:51:45 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-3 → 线程名字5 End Time = Sat May 20 12:51:48 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-5 → 线程名字6 End Time = Sat May 20 12:51:48 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-4 → 线程名字7 End Time = Sat May 20 12:51:48 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-2 → 线程名字8 End Time = Sat May 20 12:51:48 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 线程名字9 End Time = Sat May 20 12:51:48 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">main线程: Finished all threads at：Sat May 20 12:51:48 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"><strong>从控制台结果可以看出：</strong></td> 
  </tr> 
  <tr> 
   <td align="left"><code>5</code>个线程去执行<code>10</code>个线程任务，线程任务放到线程池后<strong>延迟</strong><code>10</code>秒执行</td> 
  </tr> 
 </tbody> 
</table>

--------------------

<table> 
 <thead> 
  <tr> 
   <th align="left"><code>scheduleAtFixedRate()</code></th> 
   <th align="left"><code>scheduleWithFixedDelay()</code></th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left">以固定的 <strong><code>频率</code></strong> 执行</td> 
   <td align="left">以固定的 <strong><code>延时</code></strong> 执行</td> 
  </tr> 
  <tr> 
   <td align="left"><code>period</code>（周期）指的是两次成功执行之间的时间。<br>上一个任务开始的时间计时，一个period后，检测上一个任务是否执行完毕，<br>如果上一个任务执行完毕，则当前任务立即执行，<br>如果上一个任务没有执行完毕，则需要等上一个任务执行完毕后立即执行。</td> 
   <td align="left"><code>delay</code>（延时）指的是一次执行终止和下一次执行开始之间的延迟。</td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
   <td align="left"></td> 
  </tr> 
 </tbody> 
</table>

### 2. `scheduleWithFixedDelay(new Task(), 2, 4, TimeUnit.SECONDS)` 初始延时 2s， 固定延时4s ###

public ScheduledFuture<?> scheduleWithFixedDelay(Runnable command,
                                                     long initialDelay,
                                                     long delay,
                                                     TimeUnit unit);

<table> 
 <thead> 
  <tr> 
   <th align="left">参数</th> 
   <th align="left">含义</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left"><strong><code>command</code></strong></td> 
   <td align="left">the task to execute</td> 
  </tr> 
  <tr> 
   <td align="left"><strong><code>initialDelay</code></strong></td> 
   <td align="left">the time to delay first execution</td> 
  </tr> 
  <tr> 
   <td align="left"><strong><code>delay</code></strong></td> 
   <td align="left">the delay between the termination of one execution and the <a href="https://dict.youdao.com/w/commencement/#keyfrom=dict2.top" rel="nofollow">commencement</a> of the next</td> 
  </tr> 
  <tr> 
   <td align="left"><strong><code>unit</code></strong></td> 
   <td align="left">the time unit of the initialDelay and delay parameters</td> 
  </tr> 
 </tbody> 
</table>

public class FixedDelay {
        
        public static void main(String args[]) throws InterruptedException, ExecutionException {
        
            way1();
            way2();
        }
    
        private static void way2() {
        
            System.out.println(Thread.currentThread().getName() + "线程: Start at: " + new Date());
    
            ScheduledThreadPoolExecutor scheduledExecutorService = new ScheduledThreadPoolExecutor(1);
    
            scheduledExecutorService.scheduleWithFixedDelay(() -> {
        
                System.out.println(Thread.currentThread().getName() + " → " + " Start Time = " + new Date());
                try {
        
                    sleep(3000);
                } catch (InterruptedException e) {
        
                    throw new RuntimeException(e);
                }
                System.out.println(Thread.currentThread().getName() + " → " + " End   Time = " + new Date());
            }, 1, 4, TimeUnit.SECONDS);  // 延迟 1s，周期 4s
        }
    
        private static void way1() {
        
            System.out.println(Thread.currentThread().getName() + "线程: Start at: " + new Date());
    
            ScheduledExecutorService scheduledExecutorService = Executors.newScheduledThreadPool(2);
    
            scheduledExecutorService.scheduleWithFixedDelay(new Task("任务"), 1, 4, TimeUnit.SECONDS);  // 延迟 1s, 周期 4s
        }
    }

#### 执行结果分析 ####

<table> 
 <thead> 
  <tr> 
   <th align="left">执行结果</th> 
   <th align="left">分析</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left">main线程: Start at: Sat May 20 18:54:29 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 18:54:30 CST 2023</td> 
   <td align="left">延迟 <code>1</code> s</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 End Time = Sat May 20 18:54:33 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 18:54:37 CST 2023</td> 
   <td align="left">等待 <code>4</code> s 再开始</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 End Time = Sat May 20 18:54:40 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 18:54:44 CST 2023</td> 
   <td align="left">等待 <code>4</code> s 再开始</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 End Time = Sat May 20 18:54:47 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 18:54:51 CST 2023</td> 
   <td align="left">等待 <code>4</code> s 再开始</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 End Time = Sat May 20 18:54:54 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
 </tbody> 
</table>

--------------------

### 3. `scheduleAtFixedRate(new Task(), 1, 4, TimeUnit.SECONDS)` 初始延时 1s， 周期4s ###

#### 执行结果分析 ####

<table> 
 <thead> 
  <tr> 
   <th align="left">执行结果</th> 
   <th align="left">分析</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left"><strong>前置条件</strong></td> 
   <td align="left"><strong>任务用时 <code>3</code> 秒， period = <code>4</code> 秒</strong></td> 
  </tr> 
  <tr> 
   <td align="left">main线程: Starting at: Sat May 20 19:31:13 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 19:31:14 CST 2023</td> 
   <td align="left">延时 <code>1</code> 秒</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 End Time = Sat May 20 19:31:17 CST 2023</td> 
   <td align="left">任务用时 <code>3</code> 秒</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 19:31:18 CST 2023</td> 
   <td align="left"><code>period</code>（周期） <code>4</code> 秒</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 End Time = Sat May 20 19:31:21 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 19:31:22 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 End Time = Sat May 20 19:31:25 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"><strong><code>任务用时</code> <code>&lt;</code> <code>period</code>（周期） ，等待时长为：<code>period</code></strong></td> 
   <td align="left"><strong>需要等待<code>period</code>再开始下一个</strong></td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"><strong>前置条件</strong></td> 
   <td align="left"><strong>任务用时 <code>3</code> 秒， period = <code>3</code> 秒</strong></td> 
  </tr> 
  <tr> 
   <td align="left">main线程: Starting at: Sat May 20 19:35:33 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 19:35:34 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 End Time = Sat May 20 19:35:37 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 19:35:37 CST 2023</td> 
   <td align="left"><code>period</code>（周期） <code>3</code> 秒</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 End Time = Sat May 20 19:35:40 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 19:35:40 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 End Time = Sat May 20 19:35:43 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 19:35:43 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"><strong><code>任务用时</code> <code>=</code> <code>period</code>（周期） ，等待时长为：<code>period</code></strong></td> 
   <td align="left"><strong>需要等待<code>period</code>再开始下一个</strong></td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"><strong>前置条件</strong></td> 
   <td align="left">任务用时 <code>3</code> 秒， period = <code>2</code> 秒</td> 
  </tr> 
  <tr> 
   <td align="left">main线程: Starting at: Sat May 20 19:37:03 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 19:37:04 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 End Time = Sat May 20 19:37:07 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 Start Time = Sat May 20 19:37:07 CST 2023</td> 
   <td align="left"><code>period</code>（周期） <code>2</code> 秒 无效，以任务用时为准，故结束就开始</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 任务 End Time = Sat May 20 19:37:10 CST 2023</td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
   <td align="left"></td> 
  </tr> 
  <tr> 
   <td align="left"><strong><code>任务用时</code> <code>&gt;</code> <code>period</code>（周期） <code>4</code> 秒 ，等待时长为：<code>任务用时</code></strong></td> 
   <td align="left"><strong>等任务结束后直接开始执行下一个</strong></td> 
  </tr> 
 </tbody> 
</table>

--------------------

#### 定时任务☞每天晚上9点执行一次 ####

public class FixedRateDemo {
        
        public static void main(String args[]) throws InterruptedException, ExecutionException, ParseException {
        
            System.out.println(Thread.currentThread().getName() + "线程: Starting at: " + new Date());
    
            ScheduledExecutorService executor = Executors.newScheduledThreadPool(1);
            // long oneDay = 24 * 60 * 60 * 1000=86400000; // 1天24小时作为周期
            // long initDelay = getTimeMillis("21:00:00") - System.currentTimeMillis(); // 当前时间
    
            long oneDayPeriod = 10000; // 正常1天=24*60*60*1000=86400000ms,这里用5s来模拟
    //        long initDelay = getTimeMillis("21:00:00") - getTimeMillis("20:59:57"); // 假设模拟此时当天的20:59:57 → 3000ms
            long initDelay = getTimeMillis("21:00:00") - getTimeMillis("21:00:03"); // 假设模拟此时当天的21:00:03 →-3000ms
    
            initDelay = initDelay > 0 ? initDelay : oneDayPeriod + initDelay;
    
            executor.scheduleAtFixedRate(new Task("定时任务"), initDelay, oneDayPeriod, TimeUnit.MILLISECONDS);
        }
    
    
        /**
         * 获取指定时间对应的毫秒数
         */
        private static long getTimeMillis(String time) throws ParseException {
        
            DateFormat dateFormat = new SimpleDateFormat("yy-MM-dd HH:mm:ss");
            DateFormat dayFormat = new SimpleDateFormat("yy-MM-dd");
            Date curDate = dateFormat.parse(dayFormat.format(new Date()) + " " + time);
            return curDate.getTime();
        }
    }

#### 执行结果分析 ####

<table> 
 <thead> 
  <tr> 
   <th align="left">执行结果</th> 
   <th align="left">模拟数据</th> 
   <th>分析</th> 
  </tr> 
 </thead> 
 <tbody> 
  <tr> 
   <td align="left"><strong>前置条件：模拟<code>20:59:57</code>开始执行定时任务，<code>21:00:00</code>正式执行，周期<code>10</code>秒</strong></td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left">main线程: Starting at: Sat May 20 20:27:54 CST 2023</td> 
   <td align="left"><strong><code>20:59:57</code></strong></td> 
   <td>开始执行时间</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 Start Time = Sat May 20 20:27:57 CST 2023</td> 
   <td align="left"><strong><code>21:00:00</code></strong></td> 
   <td><strong>当天</strong>到<code>9</code>点开始执行</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 End Time = Sat May 20 20:28:00 CST 2023</td> 
   <td align="left"><strong><code>21:00:03</code></strong></td> 
   <td>任务执行完毕</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 Start Time = Sat May 20 20:28:07 CST 2023</td> 
   <td align="left"><strong><code>21:00:00</code></strong></td> 
   <td>第<code>2</code>天<code>9</code>点重新执行</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 End Time = Sat May 20 20:28:10 CST 2023</td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 Start Time = Sat May 20 20:28:17 CST 2023</td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 End Time = Sat May 20 20:28:20 CST 2023</td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 Start Time = Sat May 20 20:28:27 CST 2023</td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 End Time = Sat May 20 20:28:30 CST 2023</td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 Start Time = Sat May 20 20:28:37 CST 2023</td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left"><strong>前置条件：模拟<code>21:00:03</code>开始执行定时任务，<code>21:00:00</code>正式执行，周期<code>10</code>秒</strong></td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left">main线程: Starting at: Sat May 20 20:41:50 CST 2023</td> 
   <td align="left"><strong><code>21:00:03</code></strong></td> 
   <td>开始执行时间</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 Start Time = Sat May 20 20:41:57 CST 2023</td> 
   <td align="left"><strong><code>21:00:00</code></strong></td> 
   <td>第<code>2</code>天<code>9</code>点开始执行</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 End Time = Sat May 20 20:42:00 CST 2023</td> 
   <td align="left"><strong><code>21:00:03</code></strong></td> 
   <td>任务执行完毕</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 Start Time = Sat May 20 20:42:07 CST 2023</td> 
   <td align="left"><strong><code>21:00:00</code></strong></td> 
   <td>第<code>3</code>天<code>9</code>点重新执行</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 End Time = Sat May 20 20:42:10 CST 2023</td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 Start Time = Sat May 20 20:42:17 CST 2023</td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 End Time = Sat May 20 20:42:20 CST 2023</td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 Start Time = Sat May 20 20:42:27 CST 2023</td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 定时任务 End Time = Sat May 20 20:42:30 CST 2023</td> 
   <td align="left"></td> 
   <td></td> 
  </tr> 
 </tbody> 
</table>

[523072b97d634548bd23c05c10cfbca8.png_pic_center]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/18/9ccc22ed12274921b6b599cc172346c0.png
[ef0a97a8a38e4fb28b8400ac52c8c2d3.png_pic_center]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/18/f7cda1ffa57549be82cd43cfa0ca0cba.png
[068f585e89e44ea1bfb0bd0daba83c7f.png_pic_center]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/18/ca7d611577f0490ca521b52034938dcb.png
[Executor]: https://blog.csdn.net/weixin_37646636/article/details/131028935