CODEWITHSUNDEEP
kotlinkotlin-coroutines
newbie_coder
newbie_coder

Reputation: 285

Why is this coroutine code not printing anything?

When runBlocking is used the lines are getting printed but in the following case its not printing anything

fun main(){
     GlobalScope.launch{
    
        println("function started") //printing the first line
        delay(1000)
        println("function ended")   //printing the last line

}}

Upvotes: 3

Views: 1178

Answers (1)

Joffrey
Joffrey

Reputation: 37730

You program terminates before the launched coroutine has a chance to start.

launch starts an asynchronous piece of code, so the program can keep going after calling launch without waiting for the body of launch to execute. There is nothing in your current code that waits for this coroutine to finish before terminating the program, so the program just ends.

When runBlocking is used the lines are getting printed

I'm assuming you're referring to running it this way:

fun main(): Unit = runBlocking { // this: CoroutineScope
    launch {
        println("function started") //printing the first line
        delay(1000)
        println("function ended")   //printing the last line
    }
}

runBlocking here defines a CoroutineScope that is available as this inside the lambda. The launch here is launched in that scope, which makes it a child coroutine of runBlocking.

By definition, runBlocking blocks the current thread (the main thread here) until all child coroutines are completed. This is why in that case the program waits for the coroutine to complete.

If you're just learning coroutines, you should really avoid using delicate APIs like GlobalScope. Instead, try to stick with structured concurrency. Structured concurrency is a way to structure your coroutines with parent-child relations so that you don't get surprised by coroutine leaks (like this one).

Upvotes: 6

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