How to wait for all coroutines to finish?

Question

I'm launching a coroutine and I want it to finish before I resume execution of main thread.

My code simplified looks like this:

fun hello() {
    for (i in 0..100) {
        println("hello")
    }
}

fun main(args: Array<String>) {
    val job = GlobalScope.launch { hello() } //launch parallel 
    GlobalScope.launch { job.join() }  //try to wait for job to finish
    print("done")
}

The problem is, because job.join() needs to be within a coroutine, the main line of execution is deferred to "done", so the output looks like this:

donehello
hello
hello
hello

I want to wait for the job to complete, like using sync.WaitGroup in Go. So my output would deterministically look like this:

hello
hello
hello
hello
...
done

How do I accomplish this?

Answer 1

Actually for your sample the job.join() is the way to ensure that at this point it waits until the given job is finished. Unfortunately you packed it inside a GlobalScope.launch again, which just puts that waiting in a background thread. Therefore it reaches the done earlier then you expect and on my machine it didn't even print any hello (but it could).

I assume you used launch because join can only be called from a coroutine or another suspending function? For your example it would have been sufficed to just add suspend to main, e.g.:

suspend fun main() {
  val job = GlobalScope.launch { hello() }
  job.join()
  print("done")
}

or you could have used runBlocking and wrap the main with it, e.g.:

fun main() = runBlocking {
  val job = launch { hello() }      
  job.join()
  print("done")
}

Now the disclaimer... you may want to consult the following sources before continuing (if you didn't do already):

1. Coroutine basics
2. The reason to avoid GlobalScope by Roman Elizarov
3. Concurrent coroutines - Concurrency is not parallelism by @s1m0nw1