java - ForkJoinPool中的辅助线程是守护程序线程吗？

Question

我正在从Java - The Complete Reference书中阅读有关Fork/Join Framework的信息。它说ForkJoinPool使用守护程序线程:

ForkJoinPool uses daemon threads. A daemon thread is automatically terminated when all user threads have terminated. Thus, there is no need to explicitly shut down a ForkJoinPool. However, with the exception of the common pool, it is possible to do so by calling shutdown( ). The shutdown( ) method has no effect on the common pool.

这是否意味着所有ForkJoinWorkerThread都是守护程序线程？

由于守护进程线程是low priority threads，那么我们不应该将ForkJoinPool用于重要任务吗？

如果工作线程不是守护程序线程，那么shutdown()是否等待工作线程完成？

Answer 1

1.

jshell> ForkJoinPool.commonPool().execute(() ->  System.out.println(Thread.currentThread().getClass() + " isDaemon?  " + Thread.currentThread().isDaemon()))
class java.util.concurrent.ForkJoinWorkerThread isDaemon?  true

jshell>

一个:是的，它们是守护程序线程。
2.

jshell> new Thread(() -> System.out.println("isDaemon? " + Thread.currentThread().isDaemon() + " priority:" +  Thread.currentThread().getPriority())).start()
isDaemon? false priority:5

jshell> ForkJoinPool.commonPool().execute(() -> System.out.println("isDaemon? " + Thread.currentThread().isDaemon() + " priority:" +  Thread.currentThread().getPriority()))
isDaemon? true priority:5

:默认情况下，ForkJoinPool创建的线程与其他任何线程具有相同的优先级。
3.
来自ForkJoinPool＃commonPool的javadoc

Returns the common pool instance. This pool is statically constructed; its run state is unaffected by attempts to shutdown() or shutdownNow(). However this pool and any ongoing processing are automatically terminated upon program System.exit(int). Any program that relies on asynchronous task processing to complete before program termination should invoke commonPool().awaitQuiescence, before exit.

:ForkJoinPool忽略关闭，但是应用程序可以调用 awaitQuiescence 以确保所有任务均已完成。