Does anyone know why GlobalScope Dispatchers.Default contexted coroutines were implemented using Dae...

# coroutines

[Stephen Edwards]

Does anyone know why GlobalScope Dispatchers.Default contexted coroutines were implemented using Daemon Threads?

[Stephen Edwards]

@Niklas Gürtler.

fun coroutinesAreSometimesDispatchedOntoDaemonThreads() = runBlocking<Unit> {
  println("Main. Thread: ${Thread.currentThread()}. isDaemon? ${Thread.currentThread().isDaemon}")

  val scope = CoroutineScope(Dispatchers.Default)
  scope.launch {
    println("Coroutine. Thread: ${Thread.currentThread()}. isDaemon? ${Thread.currentThread().isDaemon}")
  }
}

Prints:

Main. Thread: Thread[main,5,main]. isDaemon? false
Coroutine. Thread: Thread[DefaultDispatcher-worker-1,5,main]. isDaemon? true

[Stephen Edwards]

Is this actually related to Dispatchers.Default and only ends up happenign because GlobalScope uses that dispatcher when not otherwise specified?

[Stephen Edwards]

CC @elizarov is there a reason Dispatchers.Default coroutines use daemon threads?

[Niklas Gürtler]

The Default dispatcher is special, it uses some worker thread... Not sure what the exact purpose is, probably to do long-running calculations or something like that

[Zach Klippenstein (he/him) [MOD]]

It’s special in that it shares threads with <http://Dispatchers.IO|Dispatchers.IO>, not because it uses worker threads. You can create a CoroutineDispatcher from any regular java thread pool.

As for the purpose of the default dispatcher, I think that’s pretty well-documented in the official kotlin coroutine docs. This question is specifically about why the default dispatcher’s thread pool threads are daemon threads, instead of just regular threads. Daemon threads are a JVM concept, see the java docs for more.