为什么 Thread.activeCount() 计数的线程数比我的代码创建的线程数多？

Question

我是 Java 开发的初学者，我尝试在下面的代码中使用 CountDownLatch 编写多线程进程。但是 ExecutorService 没有正常工作。

ExecutorService 线程号未按照代码中定义的方式工作。

import java.net.URI;
import java.net.http.HttpClient;
import java.net.http.HttpRequest;
import java.net.http.HttpResponse;
import java.net.http.HttpResponse.BodyHandlers;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

public class HTTPrequester {
    
    private ExecutorService exs;
    private CountDownLatch latch;
    private int thread_num = 50;
    private String[] url_list;
    
    public static void main(String[] args) {
        HTTPrequester req = new HTTPrequester();
        req.action();
    }
    
    
    public void action() {
        url_list = new String[]{"https://google.com/","https://microsoft.com/","https://yahoo.com/","https://whosebug.com/","https://youtube.com/","https://google.com/","https://microsoft.com/","https://yahoo.com/","https://whosebug.com/","https://youtube.com/",
                "https://google.com/","https://microsoft.com/","https://yahoo.com/","https://whosebug.com/","https://youtube.com/","https://google.com/","https://microsoft.com/","https://yahoo.com/","https://whosebug.com/","https://youtube.com/"};
        exs = Executors.newFixedThreadPool(thread_num);
        latch = new CountDownLatch(url_list.length);
        
        for(String url: url_list) {
            exs.submit(new Runner(latch,url));
        }
        
    }
    public class Runner implements Runnable {
        
        private CountDownLatch latch;
        private String url;
        public Runner(CountDownLatch latch, String url) {
            this.latch = latch;
            this.url = url;
        }

        @Override
        public void run() {
            try {
                HttpClient client = HttpClient.newHttpClient();
                HttpRequest request = HttpRequest.newBuilder().uri(URI.create(url)).build();
                HttpResponse<String> response = client.send(request, BodyHandlers.ofString());
                System.out.println("Thread Count: "+Thread.activeCount()+" - Latch Count: "+latch.getCount());
            }catch(Exception e) {}
            latch.countDown();
        }
        
    }
    
}

控制台日志；

Thread Count: 81 - Latch Count: 20
Thread Count: 81 - Latch Count: 20
Thread Count: 81 - Latch Count: 20
Thread Count: 81 - Latch Count: 17
Thread Count: 81 - Latch Count: 16
Thread Count: 81 - Latch Count: 15
Thread Count: 81 - Latch Count: 14
Thread Count: 81 - Latch Count: 13
Thread Count: 81 - Latch Count: 12
Thread Count: 81 - Latch Count: 11
Thread Count: 81 - Latch Count: 10
Thread Count: 81 - Latch Count: 9
Thread Count: 81 - Latch Count: 8
Thread Count: 81 - Latch Count: 7
Thread Count: 81 - Latch Count: 6
Thread Count: 81 - Latch Count: 5
Thread Count: 81 - Latch Count: 4
Thread Count: 81 - Latch Count: 3
Thread Count: 81 - Latch Count: 2
Thread Count: 81 - Latch Count: 1

我定义线程数为50，但是当它起作用时Thread.activeCount()函数returns 81。为什么？

Answer 1

您正在使用 Thread.activeCount()，其中 returns（估计）调用者线程的线程组中的线程数。

这与查询有多少线程正在使用您的固定大小线程池不同（您已经知道答案，因为您已经创建了它）。

正如其他人所说，您不是唯一创建线程的人，具体来说，您不是唯一在（我猜）主线程的线程组中创建线程的人。

您创建的执行程序将创建属于您实际实例化它的线程的 ThreadGroup 的线程，因此您的计数将始终高于您的线程池大小。

如果您想查看（出于教育和调试目的）您的 jvm 中所有线程的完整图片以及它们在做什么（即它们的堆栈跟踪），请尝试使用 jstack（它应该捆绑在您的 JDK 的选择）。

https://docs.oracle.com/en/java/javase/11/tools/jstack.html

为什么 Thread.activeCount() 计数的线程数比我的代码创建的线程数多？

Why does Thread.activeCount() count more threads than my code creates?

java

concurrency

multithreading