newFixedThreadPool 可重用固定线程数的线程池

灰太狼 2024-03-22 19:35 50阅读 0赞

### newFixedThreadPool ###

public static ExecutorService newFixedThreadPool(int nThreads) {
        
            return new ThreadPoolExecutor(nThreads, 
            							  nThreads,
                                          0L, 
                                          TimeUnit.MILLISECONDS,
                                          new LinkedBlockingQueue<Runnable>());
        }

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   <th align="left">FixedThreadPool被称为<code>可重用固定线程数的线程池</code>。</th> 
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   <td align="left">FixedThreadPool的corePoolSize和maximumPoolSize都被设置为创建FixedThreadPool时指定的参数<code>nThreads</code>。</td> 
  </tr> 
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   <td align="left">当线程池中的线程数大于corePoolSize时，keepAliveTime为多余的空闲线程等待新任务的最长时间，超过这个时间后多余的线程将被终止。<strong>这里把keepAliveTime设置为0L，意味着多余的空闲线程会被立即终止</strong>。</td> 
  </tr> 
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   <td align="left"></td> 
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   <td align="left">FixedThreadPool的execute()方法的运行示意图如下图所示。</td> 
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   <td align="left"></td> 
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![在这里插入图片描述][06f13b458f1a4883afca49347dd6d337.png_pic_center]

<table> 
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   <th align="left"></th> 
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   <td align="left">1）如果当前运行的线程数少于corePoolSize，则创建新线程来执行任务。</td> 
  </tr> 
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   <td align="left">2）在线程池完成预热之后（<code>当前运行的线程数等于corePoolSize</code>），将任务加入LinkedBlockingQueue。</td> 
  </tr> 
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   <td align="left">3）线程执行完<code>1</code>中的任务后，会在循环中反复从LinkedBlockingQueue获取任务来执行。</td> 
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   <td align="left"></td> 
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   <td align="left">FixedThreadPool使用无界队列<code>LinkedBlockingQueue</code>作为线程池的工作队列（队列的容量为Integer.MAX_VALUE）。使用无界队列作为工作队列会对线程池带来如下<code>影响</code>。</td> 
  </tr> 
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   <td align="left">1）当线程池中的线程数达到corePoolSize后，新任务将在无界队列中等待，因此线程池中的线程数不会超过corePoolSize。</td> 
  </tr> 
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   <td align="left">2）由于<code>1</code>，使用无界队列时maximumPoolSize将是一个无效参数。</td> 
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   <td align="left">3）由于<code>1</code>和<code>2</code>，使用无界队列时keepAliveTime将是一个无效参数。</td> 
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   <td align="left">4）由于使用无界队列，运行中的FixedThreadPool（未执行方法shutdown()或shutdownNow()）不会拒绝任务（不会调用RejectedExecutionHandler.rejectedExecution方法）。</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();
            }
        }
    }

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

### 固定线程数量线程池 ###

public class FixThreadPool {
        
        public static void main(String args[]) throws InterruptedException, ExecutionException {
        
    
            System.out.println(Thread.currentThread().getName() + "线程: Start at: " + new Date());
    
            ExecutorService exec = Executors.newFixedThreadPool(3);
    
            for (int i = 1; i < 10; i++) {
        
                System.out.println("添加了第" + i + "个任务类");
                Thread.sleep(2);
                exec.execute(new Task("线程名字" + i));
            }
    
            exec.shutdown();
    
            System.out.println(Thread.currentThread().getName() + "线程: Finished all threads at：" + new Date() + ". isTerminated 判断次数 ");
        }
    }

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

### 执行结果分析 ###

<table> 
 <thead> 
  <tr> 
   <th align="left">执行结果</th> 
  </tr> 
 </thead> 
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  <tr> 
   <td align="left">main线程: Start at: Fri May 19 21:29:50 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第1个任务类</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第2个任务类</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第3个任务类</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第4个任务类</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第5个任务类</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 线程名字1 Start Time = Fri May 19 21:29:50 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-2 → 线程名字2 Start Time = Fri May 19 21:29:50 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-3 → 线程名字3 Start Time = Fri May 19 21:29:50 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第6个任务类</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第7个任务类</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第8个任务类</td> 
  </tr> 
  <tr> 
   <td align="left">添加了第9个任务类</td> 
  </tr> 
  <tr> 
   <td align="left">main线程: Finished all threads at：Fri May 19 21:29:50 CST 2023. isTerminated 判断次数</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-2 → 线程名字2 End Time = Fri May 19 21:29:53 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-3 → 线程名字3 End Time = Fri May 19 21:29:53 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-3 → 线程名字5 Start Time = Fri May 19 21:29:53 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-2 → 线程名字4 Start Time = Fri May 19 21:29:53 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 线程名字1 End Time = Fri May 19 21:29:53 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 线程名字6 Start Time = Fri May 19 21:29:53 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-3 → 线程名字5 End Time = Fri May 19 21:29:56 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-3 → 线程名字7 Start Time = Fri May 19 21:29:56 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 线程名字6 End Time = Fri May 19 21:29:56 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 线程名字8 Start Time = Fri May 19 21:29:56 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-2 → 线程名字4 End Time = Fri May 19 21:29:56 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-2 → 线程名字9 Start Time = Fri May 19 21:29:56 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-2 → 线程名字9 End Time = Fri May 19 21:29:59 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-3 → 线程名字7 End Time = Fri May 19 21:29:59 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left">pool-1-thread-1 → 线程名字8 End Time = Fri May 19 21:29:59 CST 2023</td> 
  </tr> 
  <tr> 
   <td align="left"></td> 
  </tr> 
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   <td align="left"><strong>从控制台结果可以看出：</strong></td> 
  </tr> 
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   <td align="left">线程池创建<code>3</code>个线程来 执行这<code>9</code>个任务，线程池大小为<code>3</code>，也就说同一时刻最多只有<code>3</code>个线程能运行</td> 
  </tr> 
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   <td align="left">主线程的执行与线程池里的线程分开，有可能主线程结束了，但是线程池还在运行</td> 
  </tr> 
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   <td align="left">放入线程池的线程并不一定会按其放入的先后而顺序执行</td> 
  </tr> 
 </tbody> 
</table>

[06f13b458f1a4883afca49347dd6d337.png_pic_center]: https://image.dandelioncloud.cn/pgy_files/images/2024/03/18/34396dacedc6478dbffd0161c9817798.png
[Executor]: https://blog.csdn.net/weixin_37646636/article/details/131028935