java - 为什么总是 threadPoolExecutor.getActiveCount() <= MaximumPoolSize/2？

Question

我们有一个任务队列，其初始化如下所示:

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

maximumPoolSize 的值为 200。在工作时，队列会获得大量线程(超过一千个)，但 threadPoolExecutor.getActiveCount() 返回的值始终小于或等于 100。例如，值 threadPoolExecutor.getActiveCount() 和 queue.size() 记录如下:

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

结果我们得到以下日志:

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.

文档说 threadPoolExecutor.getActiveCount() 方法

returns the approximate number of threads that are actively executing tasks

但是为什么这个近似值的最大阈值总是maximumPoolSize/2？应该是这样吗？

附注我无法重现这种情况:在这种情况下，我的 PC 上总是有 200 个 Activity 线程。但上面的日志不是来 self 的电脑。它可以取决于处理器/虚拟核心的数量吗？

我还在 The Grey Blog 中发现了一段有趣的代码:

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

但最终我很困惑，因为 Runtime.getRuntime().availableProcessors() 的值在我的电脑上是 8。

Answer 1

您确定变量 maximumPoolSize 设置为 200。在我这边工作得很好。

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());
        }
    }
}

输出

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.