Kotlin Concurrency: Threads, Coroutines, and async/await for Beginners

Kotlin vs Python: Who Wins? The Battle for Concurrency

If you are reading this material on PythonLib, you are most likely already familiar with Python and its GIL (Global Interpreter Lock), which turns multithreading into a headache. You write threading, hope for a speed boost, but get… synchronous code with a hint of locks. Sound familiar?

Welcome to the world of Kotlin! This is a language that decided: "Let's make concurrency convenient, fast, and painless." In this article, we will break down three approaches to concurrency in Kotlin: classic threads, modern coroutines, and the powerful duo async/await. You will learn how to write asynchronous code that doesn't blow your mind, and why Kotlin is an excellent choice for backend and Android development.

1. Threads in Kotlin: Is an Old Friend Better Than Two New Ones?

Let's start with the basics. Threads in Kotlin are the same threads as in Java. Kotlin runs on the JVM, so you have access to all the power of java.lang.Thread. But let's be honest: threads are like driving a car without airbags. It works, but it's scary.

How to Create a Thread in Kotlin?

The simplest way:

fun main() {\n    thread {\n        println("Hello from thread: ${Thread.currentThread().name}")\n    }\n    println("Main thread: ${Thread.currentThread().name}")\n}

Looks familiar, right? But there's a catch: threads are heavy. Each thread is a separate system resource. If you create 10,000 threads, your program will either crash with an OutOfMemoryError or run as slowly as an old computer with Windows 95.

Problems with Threads:

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• High memory consumption — each thread reserves a stack (~1 MB).
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• Complex synchronization — locks, deadlocks, race conditions. Remember synchronized in Java and weep.
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• Context switching — the OS spends time switching between threads.
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For small projects, threads will do. But if you are building a high-load service or an application with thousands of parallel tasks, it's time to look towards coroutines.

2. Coroutines in Kotlin: Light as a Feather

Coroutines are "threads on steroids." They are not tied to system threads and can execute on a single thread, switching between tasks without OS involvement. Sounds complicated? In practice, it's simple and elegant.

Your First Coroutine

To work with coroutines, you will need the kotlinx-coroutines-core dependency. But if you are using Kotlin 1.3+, it is already built-in.

import kotlinx.coroutines.*\n\nfun main() = runBlocking {\n    launch {\n        delay(1000L)\n        println("World after delay!")\n    }\n    println("Hello,")\n}

What is happening here?

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• runBlocking — blocks the main thread until all coroutines inside it complete. Fine for learning, use with caution in production.
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• launch — launches a new coroutine. It does not block the thread, but simply suspends on delay.
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• delay — an analogue of Thread.sleep(), but non-blocking. The coroutine simply "sleeps" and frees the thread for other tasks.
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Result: first "Hello," is printed, and after a second — "World after delay!". Without locks, without extra threads.

Why are Coroutines Better than Threads?

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• Lightweight — a million coroutines? Easy! They don't create separate threads.
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• Suspension — a coroutine can stop at any moment and continue later.
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• Structured concurrency — coroutines live within a scope and are automatically cancelled if the scope completes.
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Dispatchers