为什么Thread.activeCount()计算的线程数比我的代码创建的线程多？

Question

我是Java开发的初学者，我试图在下面的代码中使用CountDownLatch编写一个多线程进程。但是ExecutorService并没有像它应该的那样工作。

ExecutorService线程数未按照代码中的定义工作。

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://stackoverflow.com/","https://youtube.com/","https://google.com/","https://microsoft.com/","https://yahoo.com/","https://stackoverflow.com/","https://youtube.com/",
                "https://google.com/","https://microsoft.com/","https://yahoo.com/","https://stackoverflow.com/","https://youtube.com/","https://google.com/","https://microsoft.com/","https://yahoo.com/","https://stackoverflow.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

我将线程数定义为50，但是当它工作时，Thread.activeCount()函数返回81。为什么？

Answer 1

您正在使用Thread.activeCount()，它返回(估计)调用者线程的ThreadGroup中的线程数量。

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

正如其他人所说，你不是唯一一个创建线程的人，具体地说，你也不是唯一一个在(我猜)你的主线程的ThreadGroup中创建线程的人。

您创建的executor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(出于教育和调试目的)查看jvm中所有线程的完整情况以及它们正在做什么(即它们的堆栈跟踪)，请尝试使用jstack (它应该捆绑在您选择的JDK中)。

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