Multiplatform Kotlin library

Last Updated 31 July 2020
In this tutorial, we will build a small library available from the worlds of JVM, JS, and Native.

You will learn step-by-step how to create a multiplatform library which can be used from any other common code (for example, shared with Android and iOS). You will also learn how to write tests which will be executed on all platforms and use an efficient implementation provided by a specific platform.

What are we building?

Our goal is to build a small multiplatform library to demonstrate the ability to share the code between the platforms and its benefits. In order to have a small implementation to focus on the multiplatform machinery, we will write a library which converts raw data (strings and byte arrays) to the Base64 format which can be used on JVM, JS, and any available Kotlin/Native platform.

On JVM implementation will use java.util.Base64 which is known to be extremely efficient because JVM is aware of this particular class and compiles it in a special way.

On JS we will use the native Buffer API and on Kotlin/Native we will write our own implementation.

We will cover this functionality with common tests and then publish the final library to Maven.

Setting up the local environment

We will be using IntelliJ IDEA Community Edition for this tutorial, though using Ultimate edition is possible as well.

Install the Kotlin Plugin 1.4.x or higher in the IDE. You can check the Kotlin version in Tools | Kotlin | Configure Plugin Updates.

Native part of this project is written using Mac OS X, but don't worry if you are using another platform, the platform affects only directory names in this particular tutorial.

Creating a project

  1. In IntelliJ IDEA, select File | New | Project.
  2. In the panel on the left, select Kotlin.
  3. Enter a project name and select Library under Multiplatform as the project template.

    Select a project template

  4. Click Next, and on the next screen click Finish.

A multiplatform sample library is now created and imported into IntelliJ IDEA.

Go to any .kt file and rename the package with the IntelliJ IDEA action Refactor | Rename action to org.jetbrains.base64

Let's just check everything is right with the project so far, the project structure should be:

└── src
    ├── commonMain
    │   └── kotlin
    ├── commonTest
    │   └── kotlin
    ├── jsMain
    │   └── kotlin
    ├── jsTest
    │   └── kotlin
    ├── jvmMain
    │   └── kotlin
    ├── jvmTest
    │   └── kotlin
    ├── macosMain
    │   └── kotlin
    └── macosTest
        └── kotlin

And the kotlin folder should contain an org.jetbrains.base64 subfolder.

Common part

Now we need to define the classes and interfaces we want to implement. Create the file Base64.kt in the commonMain/kotlin/jetbrains/base64 folder.

Core primitive will be the Base64Encoder interface which knows how to convert bytes to bytes in Base64 format:

interface Base64Encoder {
    fun encode(src: ByteArray): ByteArray

But the common code should somehow get an instance of this interface, for that purpose we define the factory object Base64Factory:

expect object Base64Factory {
    fun createEncoder(): Base64Encoder

Our factory is marked with the expect keyword. expect is a mechanism to define a requirement, which every platform should provide in order for the common part to work properly. So on each platform we should provide the actual Base64Factory which knows how to create the platform-specific encoder. Learn more about platform-specific declarations.

Platform-specific implementations

Now it is time to provide an actual implementation of Base64Factory for every platform.


We are starting with an implementation for the JVM. Let's create a file Base64.kt in jvmMain/kotlin/jetbrains/base64 folder and provide a simple implementation, which delegates to java.util.Base64:

actual object Base64Factory {
    actual fun createEncoder(): Base64Encoder = JvmBase64Encoder

object JvmBase64Encoder : Base64Encoder {
    override fun encode(src: ByteArray): ByteArray = Base64.getEncoder().encode(src)

Pretty simple, isn't it? We have provided a platform-specific implementation, but used a straightforward delegation to an implementation someone else has written!


Our JS implementation will be very similar to the JVM one. We create a file Base64.kt in jsMain/kotlin/jetbrains/base64 and provide an implementation which delegates to NodeJS Buffer API:

actual object Base64Factory {
    actual fun createEncoder(): Base64Encoder = JsBase64Encoder

object JsBase64Encoder : Base64Encoder {
    override fun encode(src: ByteArray): ByteArray {
        val buffer = js("Buffer").from(src)
        val string = buffer.toString("base64") as String
        return ByteArray(string.length) { string[it].toByte() }


On the generic Native platform we don't have the luxury to use someone else's implementation, so we will have to write one ourselves. It's pretty straightforward and follows Base64 format description without any optimizations:

private val BASE64_ALPHABET: String = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
private val BASE64_MASK: Byte = 0x3f
private val BASE64_PAD: Char = '='
private val BASE64_INVERSE_ALPHABET = IntArray(256) {

private fun Int.toBase64(): Char = BASE64_ALPHABET[this]

actual object Base64Factory {
    actual fun createEncoder(): Base64Encoder = NativeBase64Encoder

object NativeBase64Encoder : Base64Encoder {
    override fun encode(src: ByteArray): ByteArray {
            fun ByteArray.getOrZero(index: Int): Int = if (index >= size) 0 else get(index).toInt()
            // 4n / 3 is expected Base64 payload
            val result = ArrayList<Byte>(4 * src.size / 3) 
            var index = 0
            while (index < src.size) {
                val symbolsLeft = src.size - index
                val padSize = if (symbolsLeft >= 3) 0 else (3 - symbolsLeft) * 8 / 6
                val chunk = (src.getOrZero(index) shl 16) or (src.getOrZero(index + 1) shl 8) or src.getOrZero(index + 2)
                index += 3
                for (i in 3 downTo padSize) {
                val char = (chunk shr (6 * i)) and BASE64_MASK.toInt()
                // Fill the pad with '='
                repeat(padSize) { result.add(BASE64_PAD.toByte()) }
            return result.toByteArray()

Now we have implementations on all the platforms and it is time to for testing of our library.


To make the library complete we should write some tests, but we have three independent implementations and it is a waste of time to write duplicate tests for each one.

The good thing about common code is that it can be covered with common tests, which later are compiled and executed on every platform.

Let's create the class Base64Test in commonTest/kotlin/jetbrains/base64 folder and write the basic tests for Base64.

But as you remember, our API converts byte arrays to byte arrays in a different format and it is not easy to test byte arrays. So before we start writing a test, let's add the method encodeToString with a default implementation to our Base64Encoder interface:

interface Base64Encoder {
    fun encode(src: ByteArray): ByteArray

    fun encodeToString(src: ByteArray): String {
        val encoded = encode(src)
        return buildString(encoded.size) {
            encoded.forEach { append(it.toChar()) }

Note that the implementation on every platform can encode byte arrays to a string. If we want, we can provide a more efficient implementation for this method, for example, let's specialize it on the JVM:

object JvmBase64Encoder : Base64Encoder {
    override fun encode(src: ByteArray): ByteArray = Base64.getEncoder().encode(src)
    override fun encodeToString(src: ByteArray): String = Base64.getEncoder().encodeToString(src)

Default implementations with optional more specialized overrides is another bonus of the multiplatform library. Now, when we have a string-based API, we can cover it with basic tests:

class Base64Test {
    fun testEncodeToString() {
        checkEncodeToString("Kotlin is awesome", "S290bGluIGlzIGF3ZXNvbWU=")

    fun testPaddedStrings() {
        checkEncodeToString("", "")
        checkEncodeToString("1", "MQ==")
        checkEncodeToString("22", "MjI=")
        checkEncodeToString("333", "MzMz")
        checkEncodeToString("4444", "NDQ0NA==")

    private fun checkEncodeToString(input: String, expectedOutput: String) {
        assertEquals(expectedOutput, Base64Factory.createEncoder().encodeToString(input.asciiToByteArray()))

    private fun String.asciiToByteArray() = ByteArray(length) {

Use Gradle 6.0 and above to generate wrapper (gradle wrapper) in the project root directory to generate gradlew, gradlew.bat, and gradle/wrapper/gradle-wrapper.jar.

Execute ./gradlew check and you will see that the tests are run three times, on JVM, on JS, and on Native!

If we want, we can add tests to a specific platform, then it will be executed only as part of these platform tests.

For example, we can add UTF-16 tests on JVM. Just follow the same steps as before, but create file in jvmTest/kotlin/jetbrains/base64:

class Base64JvmTest {
    fun testNonAsciiString() {
        val utf8String = "Gödel"
        val actual = Base64Factory.createEncoder().encodeToString(utf8String.toByteArray())
        assertEquals("R8O2ZGVs", actual)

This test will be automatically executed on the JVM target in addition to the common part.

Publishing the library to Maven

Our first multiplatform library is almost ready. The last step is to publish it, so other projects can then depend on our library.

Use the maven-publish Gradle plugin.

Don't forget to specify the group and version of your library along with the plugin in build.gradle:

apply plugin: 'maven-publish'
group 'org.jetbrains.base64'
version '1.0.0'

Now check it with the command ./gradlew publishToMavenLocal and you should see a successful build.

That's it, our library is now successfully published and any Kotlin project can depend on it, whether it is another common library, JVM, JS, or Native application.

In this tutorial we have:

  • Created a multiplatform library with platform-specific implementations.
  • Provided default implementation for the common part and specialized it on JVM.
  • Wrote common tests which are executed on every platform.
  • Published the final library to the Maven repository.

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Licensed under the Apache License, Version 2.0.