Why are threads messing up? - kotlin

Question

i started learning kotlin Coroutines. But I am facing a situation like this.
fun main(){
println("${Thread.currentThread().name} thread started")
GlobalScope.launch {
println("${Thread.currentThread().name} thread started")
Thread.sleep(1000)
println("${Thread.currentThread().name} thread finished")
}
GlobalScope.launch {
println("${Thread.currentThread().name} thread started")
delay(1000)
println("${Thread.currentThread().name} thread finished")
}
GlobalScope.launch {
println("${Thread.currentThread().name} thread started")
delay(1000)
println("${Thread.currentThread().name} thread finished")
}
runBlocking {
delay(1000)
}
println("${Thread.currentThread().name} thread finished")
}
the output of this code:
main thread started
DefaultDispatcher-worker-1 thread started
DefaultDispatcher-worker-2 thread started
DefaultDispatcher-worker-3 thread started
DefaultDispatcher-worker-1 thread finished
DefaultDispatcher-worker-1 thread finished
DefaultDispatcher-worker-2 thread finished
main thread finished
Why "DefaultDispatcher-worker-1 thread finished" is running 2 times.
I guess it's just the threads' names are changing, but I'm wondering why ?

Answer 1

Coroutines provide no guarantee that the whole coroutine will run on the same thread for its whole duration. Thread management is handled by the dispatchers you use. Dispatchers.Default uses a thread pool and can swap threads each time it resumes from suspension, as it does after each delay() call. This is intended behavior, as it can reduce the total number of spawned Thread objects.