runBlocking in Kotlin

Introduction

runBlocking is a fundamental concept every Kotlin developer should understand. Coroutines let you write asynchronous code that reads like synchronous code, without blocking threads or nesting callbacks.

RunBlocking bridges blocking code with coroutines. In this tutorial you will learn the syntax, walk through a complete example program, study the sample output, and review best practices so you can apply the concept confidently in your own projects.

Definition

• runBlocking bridges blocking code with coroutines.
• It blocks current thread until coroutine completes.
• Commonly used in main function for examples and tests.

Syntax

runBlocking { }

runBlocking in Kotlin Example Program in Kotlin

import kotlinx.coroutines.*

fun main(args: Array<String>) = runBlocking {
    println("Before delay")
    delay(100)
    println("After delay")
}

Sample Output

Before delay
After delay

When to use

Use coroutines for network requests, database queries, or any work that would block the main thread if done synchronously.

How it works

1. The program starts with a main function — the entry point that runs when you execute the file.

2. The println("Before delay") statement writes a line to the console — this produces part of the sample output below.

3. The println("After delay") statement writes a line to the console — this produces part of the sample output below.

4. RunBlocking bridges blocking code with coroutines.

5. Run the program in IntelliJ IDEA, Android Studio, or with the Kotlin command-line compiler (kotlinc / kotlin). Compare your console output with the sample output shown below.

Best Practices

• Never block the main thread — use suspend functions and appropriate dispatchers.
• Scope coroutines with coroutineScope or supervisorScope for structured cancellation.
• Use delay instead of Thread.sleep inside coroutines.

Common Mistakes

• Launching coroutines without a scope — leaks work after the UI is destroyed.
• Using GlobalScope in application code instead of a lifecycle-aware scope.
• Calling blocking APIs directly on Dispatchers.Main.

Key Points

• runBlocking bridges blocking code with coroutines.
• It blocks current thread until coroutine completes.
• Commonly used in main function for examples and tests.
• Test the example locally and verify the output matches the sample.
• Experiment by changing input values to see how behaviour changes.

Notes

• Add the kotlinx-coroutines-core dependency when running coroutine examples outside Android or IntelliJ.
• Semicolons at the end of statements are optional in Kotlin.