为什么缓存线程池会创建两个线程，为什么关闭它会改变这种情况？

Question

这是我的 java 代码。

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.ThreadFactory;

class ExceptionThread2 implements Runnable {
    @Override
    public void run() {
        Thread t = Thread.currentThread();
        System.out.println("run() by " + t);
        System.out.println("eh = " + t.getUncaughtExceptionHandler());
        throw new RuntimeException();
    }
}

class MyUncaughtExceptionHandler implements Thread.UncaughtExceptionHandler {
    @Override
    public void uncaughtException(Thread t, Throwable e) {
        System.out.println("caught " + e);
    }
}

class HandlerThreadFactory implements ThreadFactory {
    @Override
    public Thread newThread(Runnable r) {
        System.out.println(this + " creating new Thread ");
        Thread t = new Thread(r);
        System.out.println("created " + t);
        t.setUncaughtExceptionHandler(new MyUncaughtExceptionHandler());
        System.out.println("eh = " + t.getUncaughtExceptionHandler());
        return t;
    }
}

public class CaptureUncaughtException {
    public static void main(String[] args) {
        ExecutorService exec = Executors.newCachedThreadPool(new HandlerThreadFactory());
        exec.execute(new ExceptionThread2());
        //exec.shutdown();
    }
}

结果：

com.concurrent.example.HandlerThreadFactory@7f31245a creating new Thread
created Thread[Thread-0,5,main]
eh = com.concurrent.example.MyUncaughtExceptionHandler@6d6f6e28
run() by Thread[Thread-0,5,main]
eh = com.concurrent.example.MyUncaughtExceptionHandler@6d6f6e28
com.concurrent.example.HandlerThreadFactory@7f31245a creating new Thread
created Thread[Thread-1,5,main]
eh = com.concurrent.example.MyUncaughtExceptionHandler@2870fdbb
caught java.lang.RuntimeException

当我取消注释 //exec.shutdown() 时，结果不同。只创建了一个线程。

com.concurrent.example.HandlerThreadFactory@7f31245a creating new Thread
created Thread[Thread-0,5,main]
eh = com.concurrent.example.MyUncaughtExceptionHandler@6d6f6e28
run() by Thread[Thread-0,5,main]
eh = com.concurrent.example.MyUncaughtExceptionHandler@6d6f6e28
caught java.lang.RuntimeException

为什么？

Answer 1

当你这样做时：

exec.execute(new ExceptionThread2());

工作线程已创建并执行您的任务。

但是异常完成，抛出异常后死亡。

在这种情况下，线程池会创建另一个 worker 来替换死掉的 worker，因此您会看到两条 created 消息，但只有一条 run() 消息

这是执行您的任务的runWorker：

 * 1. We may start out with an initial task, in which case we
 * don't need to get the first one. Otherwise, as long as pool is
 * running, we get tasks from getTask. If it returns null then the
 * worker exits due to changed pool state or configuration
 * parameters.  Other exits result from exception throws in
 * external code, in which case completedAbruptly holds, which
 * usually leads processWorkerExit to replace this thread.

 final void runWorker(Worker w)

Other exits result from exception throws in external code, in which case completedAbruptly holds, which usually leads processWorkerExit to replace this thread

Answer 2

您的可运行程序使其线程崩溃。因此池将用一个新线程替换该线程（用于您可能提交的下一个任务）。

如果池已经被关闭，它不会那样做。

为什么缓存线程池会创建两个线程，为什么关闭它会改变这种情况？

Why does a cached thread pool create two threads, and why does shutting it down change that?

java

executorservice

threadpool

threadpoolexecutor