Coroutine Basics

What Is a Coroutine?

A coroutine is a lightweight unit of concurrent work that can be paused and resumed without blocking an underlying operating system thread, letting many coroutines share a small pool of real threads.

Warning: This example uses kotlinx.coroutines, which is not part of the plain Kotlin standard library. It requires the kotlinx-coroutines-core dependency (added via Gradle/Maven) to compile and run.

Example: What Is a Coroutine?

import kotlinx.coroutines.*

fun main() = runBlocking {
    println("Starting a coroutine")
    launch {
        println("Hello from a coroutine!")
    }
    println("Coroutine launched")
}

runBlocking as an Entry Point

runBlocking { } creates a coroutine scope and blocks the calling thread until everything inside completes, which is why it is commonly used to wrap main() in coroutine examples.

Note: Avoid runBlocking inside real application code paths; it is best suited for main() functions and tests.

Warning: This example uses kotlinx.coroutines, which is not part of the plain Kotlin standard library. It requires the kotlinx-coroutines-core dependency (added via Gradle/Maven) to compile and run.

Example: runBlocking as an Entry Point

import kotlinx.coroutines.*

fun main() = runBlocking {
    delay(100)
    println("Finished after a delay, inside runBlocking")
}

Coroutines Are Not Threads

Unlike threads, which are relatively expensive operating system resources, coroutines are cheap and lightweight -- you can run tens of thousands of them concurrently on a handful of actual threads.

Warning: This example uses kotlinx.coroutines, which is not part of the plain Kotlin standard library. It requires the kotlinx-coroutines-core dependency (added via Gradle/Maven) to compile and run.

Example: Coroutines Are Not Threads

import kotlinx.coroutines.*

fun main() = runBlocking {
    repeat(5) { i ->
        launch {
            println("Coroutine $i running")
        }
    }
}

Why kotlinx.coroutines Is a Library, Not the Standard Library

The core coroutine building blocks (suspend functions) are part of the Kotlin language, but the practical tools like launch, async, and Dispatchers come from the separate kotlinx.coroutines library, added to a project via Gradle or Maven.

Warning: This example uses kotlinx.coroutines, which is not part of the plain Kotlin standard library. It requires the kotlinx-coroutines-core dependency (added via Gradle/Maven) to compile and run.

Example: Why kotlinx.coroutines Is a Library, Not the Standard Library

import kotlinx.coroutines.*

fun main() = runBlocking {
    println("This program depends on kotlinx-coroutines-core")
}