Kotlin Async vs Launch

Question

Kotlin_version = '1.2.41'

I am pretty new to Kotlin. I would like to know what's the difference between async and launch. Especially in the following code

import kotlinx.coroutines.experimental.async
import kotlinx.coroutines.experimental.awaitAll
import kotlinx.coroutines.experimental.delay
import kotlinx.coroutines.experimental.launch
import kotlinx.coroutines.experimental.runBlocking

fun main(args: Array<String>) {
    runBlocking {
        (20..30).forEach {
            launch{
                println("main before" + it)
                val outer = it
                delay(1000L)
                val lists = (1..10)
                        .map { async{anotherMethod(outer, it)}}
                println("main after------------------Awaiting" + it)
                lists.awaitAll()
                println("Done awaiting -main after-----------------" + it)

            }


        }

        println("Hello,") // main thread continues here immediately
    }
}

.

suspend fun anotherMethod (outer: Int,index: Int){
    println("inner-b4----" + outer + "--" + index)
    delay(3000L)
    println("inner-After----" + outer + "--" + index)
}

Vs

fun main(args: Array<String>) {
    runBlocking {
        (20..30).forEach {
            async{
                println("main before" + it)
                val outer = it
                delay(1000L)
                val lists = (1..10)
                        .map { async{anotherMethod(outer, it)}}
                println("main after------------------Awaiting" + it)
                lists.awaitAll()
                println("Done awaiting -main after-----------------" + it)

            }


        }

        println("Hello,") // main thread continues here immediately
    }
}

suspend fun anotherMethod (outer: Int,index: Int){
    println("inner-b4----" + outer + "--" + index)
    delay(3000L)
    println("inner-After----" + outer + "--" + index)
}

score 10 · Accepted Answer · answered Oct 15 '18 at 16:45

10

async does return a Deferred<>, while launch does only return a Job, both start a new coroutine. So it depends if you require a return value or not.

In your first example the launch does not return a value - the last println only produces an Unit. If you use async as in the second example, the overall result will be the same, but you create some useless Deferred<Unit> objects.

answered Oct 15 '18 at 16:45

Rene

5,122
12
17

Thanks for the explanation. Yes, I am getting it now :) – so-random-dude Oct 15 '18 at 16:46
1

There's also difference in exception handling. `async` is safer to use - if anything inside the block throws an exception, You will receive it in the result. However exceptions in `launch` will be re-thrown (by default) and crash your application. – Pawel Oct 15 '18 at 19:08
2

Make sure not to take the above as advice to use `async` instead of `launch`. You can provide an exception handler to `launch` to make it safe. – Marko Topolnik Oct 15 '18 at 20:19

Kotlin Async vs Launch

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