爲什麼總是threadPoolExecutor.getActiveCount（）<= maximumPoolSize/2？

Question

我們的任務隊列和其正開始看起來像：

LinkedBlockingQueue<Runnable> queue = new LinkedBlockingQueue<Runnable>(); 
ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(maximumPoolSize, maximumPoolSize, 50000L, TimeUnit.MILLISECONDS, queue); 

的maximumPoolSize值是200工作時的隊列中獲得了大量的線程（上千），但threadPoolExecutor.getActiveCount()返回的值總是小於或等於100。例如，值threadPoolExecutor.getActiveCount()和queue.size()被記錄如下：

logger.debug("Active threads: " + threadPoolExecutor.getActiveCount() + ". The queue has " + queue.size() + " threads."); 

並且作爲結果，我們得到以下日誌：

Active threads: 1. The queue has 0 threads. 
Active threads: 2. The queue has 0 threads. 
Active threads: 3. The queue has 0 threads. 
Active threads: 4. The queue has 0 threads. 
Active threads: 5. The queue has 0 threads. 
... 
Active threads: 86. The queue has 0 threads. 
Active threads: 87. The queue has 1 threads. 
Active threads: 88. The queue has 1 threads. 
Active threads: 89. The queue has 1 threads. 
Active threads: 90. The queue has 1 threads. 
... 
Active threads: 99. The queue has 1 threads. 
Active threads: 100. The queue has 1 threads. 
Active threads: 100. The queue has 2 threads. 
Active threads: 100. The queue has 3 threads. 
Active threads: 100. The queue has 4 threads. 
... 
Active threads: 100. The queue has 1874 threads. 
Active threads: 100. The queue has 1875 threads. 
Active threads: 100. The queue has 1876 threads. 
Active threads: 100. The queue has 1877 threads. 
Active threads: 100. The queue has 1878 threads. 
Active threads: 100. The queue has 1879 threads. 
Active threads: 100. The queue has 1880 threads. 
Active threads: 100. The queue has 1881 threads. 
Active threads: 100. The queue has 1882 threads. 
Active threads: 100. The queue has 1883 threads. 

個

的文檔說threadPoolExecutor.getActiveCount()方法

返回正在積極執行 任務

但是線程的大致數量，爲什麼總是這樣一個近似值maximumPoolSize/2的最大閾值？它應該是這樣嗎？

P.S.我無法重現這種情況：在我的PC上，在這種情況下總是有200個活動線程。但上述日誌不是來自我的電腦。它可以取決於處理器/虛擬內核的數量嗎？

我也發現有趣的一段代碼在The Grey Blog：

int cpus = Runtime.getRuntime().availableProcessors(); 
int maxThreads = cpus * scaleFactor; 
maxThreads = (maxThreads > 0 ? maxThreads : 1); 

但最終我很困惑，因爲Runtime.getRuntime().availableProcessors()值爲8我的電腦上。

Answer 1

您確定變量maximumPoolSize設置爲200。它對我的工作很好。

import java.util.concurrent.LinkedBlockingQueue; 
import java.util.concurrent.ThreadPoolExecutor; 
import java.util.concurrent.TimeUnit; 

public class SO35570728 { 

    private class Task implements Runnable{ 

     @Override 
     public void run() { 
      System.out.println(Thread.currentThread().getName() + " started"); 
      try { 
       Thread.sleep(5000); 
      } catch (InterruptedException e) { 
       e.printStackTrace(); 
      } 
      System.out.println(Thread.currentThread().getName() + " finished"); 
     } 
    } 

    public static void main(String[] args) { 
     LinkedBlockingQueue<Runnable> queue = new LinkedBlockingQueue<Runnable>(); 
     ThreadPoolExecutor threadPoolExecutor = new ThreadPoolExecutor(200, 200, 50000L, TimeUnit.MILLISECONDS, queue); 

     for(int i= 0 ; i < 100000 ; i++){ 
      System.out.println("Active threads: " + threadPoolExecutor.getActiveCount() + ". The queue has " + queue.size() + " threads."); 
      threadPoolExecutor.submit(new SO35570728().new Task()); 
     } 
    } 
} 

輸出

Active threads: 0. The queue has 0 threads. 
Active threads: 1. The queue has 0 threads. 
Active threads: 2. The queue has 0 threads. 
Active threads: 3. The queue has 0 threads. 
... 
pool-1-thread-1 started 
pool-1-thread-2 started 
pool-1-thread-3 started 
... 
Active threads: 200. The queue has 99793 threads. 
Active threads: 200. The queue has 99794 threads. 
Active threads: 200. The queue has 99795 threads. 
Active threads: 200. The queue has 99796 threads. 
Active threads: 200. The queue has 99797 threads. 
Active threads: 200. The queue has 99798 threads. 
Active threads: 200. The queue has 99799 threads.