Why does this program not terminate? `suspend fun...

# coroutines

[Aaron Stacy]

Why does this program not terminate?

suspend fun main() = withContext(Executors.newSingleThreadExecutor().asCoroutineDispatcher()) {}

Even if I call cancel() from within the withContext block, I see an exception logged, but the program just hangs.

[octylFractal]

probably the thread never gets spun down, and it's not a daemon thread, so JVM can't exit

[Dico]

Read API docs for newSingleThreadExecutor about whether thread is daemon or not

[Thiyagu]

use Dispatcher.close() method.https://gist.github.com/thiyagu06/9813da5a5b12ad91ce0ad8b26043d867

[Aaron Stacy]

Thanks @Thiyagu! close() worked

[Aaron Stacy]

Interestingly, the thread is created with a default thread factory, which according to https://github.com/openjdk-mirror/jdk7u-jdk/blob/f4d80957e89a19a29bb9f9807d2a28351ed7f7df/src/share/classes/java/util/concurrent/ThreadPoolExecutor.java#L103 suggests it's not a daemon thread.

[Aaron Stacy]

OK, confusingly, if I supply my own factory that creates a daemon thread, it exists without extra action. This aligns with that ThreadPoolExecutor javadoc saying that by default, the thread is not a daemon, but it's surprising because I'd think a "daemon" would need to be explicitly exited.

[Dico]

No it's the other way around.

[Aaron Stacy]

Ah, https://docs.oracle.com/javase/7/docs/api/java/lang/Thread.html#setDaemon(boolean) welp there you go. Thank you everyone for your help!

[Thiyagu]

You can always provide your ThreadFactory implementation to the executor.

[louiscad]

The code from the original snippet should be replaced by fun main() = runBlocking { ... } to it doesn't waste a bunch of threads (namely, the JVM main thread and the threads from Dispatchers.Default).

[Aaron Stacy]

Ah cool, thank you Louis!