runBlocking

Learn coroutine main with runBlocking.

runBlocking

runBlocking is the bridge from the blocking world to the suspending world. It blocks the calling thread while running coroutines inside and does not return until they all finish. Example: fun main() = runBlocking { println(compute()) }.

runBlocking is especially useful in main and tests. In production code, where blocking coroutines is undesirable, an appropriate scope is preferred over runBlocking. But at an entry point or in a test it is the easiest way to call suspend functions. Using runBlocking in main is a common pattern.

Using runBlocking

Inside runBlocking suspend functions are called directly. Example: fun main() = runBlocking { val result = compute(); println("Result: $result") }. runBlocking does not return until compute finishes.

When multiple suspend functions are called sequentially each is awaited; the total time is the sum of waits. For parallel work use async; you will see it in the next lesson. runBlocking is mainly used as a bridge in main or in @Test methods.

Exercise Call suspend with runBlocking

Add import kotlinx.coroutines.*. Define suspend fun compute(): Int { delay(100); return 10 }. Write fun main() = runBlocking { val result = compute(); println("Result: $result") }.

Show example solution

import kotlinx.coroutines.*

suspend fun compute(): Int {
    delay(100)
    return 10
}

fun main() = runBlocking {
    val result = compute()
    println("Result: $result")
}

Test yourself

What does runBlocking do?
- Does not start a coroutine
- Only calls suspend
- Never finishes
- Blocks the thread and runs coroutines
Show answer

Blocks the thread and runs coroutines — runBlocking blocks the thread while running coroutines.

Complete the runBlocking declaration

fun main() = ____ { println("Ready") }

Show answer

fun main() = runBlocking { println("Ready") }

Match the runBlocking concepts

runBlocking

Thread blocks
main()

Entry point
bridge

Bridge
test

Common use

Next lesson: async/await

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