创建并发布多平台库——教程
In this tutorial, you will learn how to create a multiplatform library for JVM, JS, and Native platforms, write common tests for all platforms, and publish the library to a local Maven repository.
This library converts raw data – strings and byte arrays – to the Base64 format. It can be used on Kotlin/JVM, Kotlin/JS, and any available Kotlin/Native platform.
You will use different ways to implement the conversion to the Base64 format on different platforms:
- For JVM – the
java.util.Base64
class. - For JS – the
btoa()
function. - For Kotlin/Native – your own implementation.
You will also test your code using common tests, and then publish the library to your local Maven repository.
搭建环境
You can complete this tutorial on any operating system. Download and install the latest version of IntelliJ IDEA with the latest Kotlin plugin.
创建一个项目
- In IntelliJ IDEA, select File | New | Project.
- In the left-hand panel, select Kotlin Multiplatform.
Enter a project name, then in the Multiplatform section select Library as the project template.
By default, your project will use Gradle with Kotlin DSL as the build system.
- Specify the JDK, which is required for developing Kotlin projects.
- Click Next and then Finish.
For more complex projects, you might need to add more modules and targets:
Configure target settings, such as the target JVM version and test framework.

If necessary, specify dependencies between modules:

The wizard will create a sample multiplatform library with the following structure:
Write cross-platform code
Define the classes and interfaces you are going to implement in the common code.
- In the
commonMain/kotlin
directory, create theorg.jetbrains.base64
package. - Create the
Base64.kt
file in the new package. Define the
Base64Encoder
interface that converts bytes to theBase64
format:package org.jetbrains.base64 interface Base64Encoder { fun encode(src: ByteArray): ByteArray }
Define the
Base64Factory
object to provide an instance of theBase64Encoder
interface to the common code:expect object Base64Factory { fun createEncoder(): Base64Encoder }
The factory object is marked with the expect
keyword in the cross-platform code.
For each platform, you should provide an actual
implementation of the Base64Factory
object with the platform-specific encoder.
Learn more about platform-specific implementations.
提供平台相关实现
Now you will create the actual
implementations of the Base64Factory
object for each platform:
JVM
- In the
jvmMain/kotlin
directory, create theorg.jetbrains.base64
package. - Create the
Base64.kt
file in the new package. Provide a simple implementation of the
Base64Factory
object that delegates to thejava.util.Base64
class:IDEA inspections help create
actual
implementations for anexpect
declaration.package org.jetbrains.base64 import java.util.* actual object Base64Factory { actual fun createEncoder(): Base64Encoder = JvmBase64Encoder } object JvmBase64Encoder : Base64Encoder { override fun encode(src: ByteArray): ByteArray = Base64.getEncoder().encode(src) }
Pretty simple, right? You've provided a platform-specific implementation by using a straightforward delegation to a third-party implementation.
JS
The JS implementation will be very similar to the JVM one.
- In the
jsMain/kotlin
directory, create theorg.jetbrains.base64
package. - Create the
Base64.kt
file in the new package. Provide a simple implementation of the
Base64Factory
object that delegates to thebtoa()
function.package org.jetbrains.base64 import kotlinx.browser.window actual object Base64Factory { actual fun createEncoder(): Base64Encoder = JsBase64Encoder } object JsBase64Encoder : Base64Encoder { override fun encode(src: ByteArray): ByteArray { val string = src.decodeToString() val encodedString = window.btoa(string) return encodedString.encodeToByteArray() } }
Native
Unfortunately, there is no third-party implementation available for all Kotlin/Native targets, so you need to write it yourself.
- In the
nativeMain/kotlin
directory, create theorg.jetbrains.base64
package. - Create the
Base64.kt
file in the new package. Provide your own implementation for the
Base64Factory
object:package org.jetbrains.base64 private val BASE64_ALPHABET: String = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/" private val BASE64_MASK: Byte = 0x3f private val BASE64_PAD: Char = '=' private val BASE64_INVERSE_ALPHABET = IntArray(256) { BASE64_ALPHABET.indexOf(it.toChar()) } 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 是预期的 Base64 有效载荷 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() result.add(char.toBase64().code.toByte()) } // 使用 '=' 填充 repeat(padSize) { result.add(BASE64_PAD.code.toByte()) } } return result.toByteArray() } }
Test your library
Now when you have actual
implementations of the Base64Factory
object for all platforms, it's time to test your multiplatform library.
To save time on testing, you can write common tests that will be executed on all platforms instead of testing each platform separately.
Prerequisites
Before writing tests, add the encodeToString
method with the default implementation to the Base64Encoder
interface, which is defined in commonMain/kotlin/org/jetbrains/base64/Base64.kt
.
This implementation converts byte arrays to strings, which are much easier to test.
interface Base64Encoder {
fun encode(src: ByteArray): ByteArray
fun encodeToString(src: ByteArray): String {
val encoded = encode(src)
return buildString(encoded.size) {
encoded.forEach { append(it.toInt().toChar()) }
}
}
}
You can also provide a more efficient implementation of this method for a specific platform, for example, for JVM in jvmMain/kotlin/org/jetbrains/base64/Base64.kt
:
object JvmBase64Encoder : Base64Encoder {
override fun encode(src: ByteArray): ByteArray = Base64.getEncoder().encode(src)
override fun encodeToString(src: ByteArray): String = Base64.getEncoder().encodeToString(src)
}
One of the benefits of a multiplatform library is having a default implementation with optional platform-specific overrides.
Write common tests
Now you have a string-based API that you can cover with basic tests.
- In the
commonTest/kotlin
directory, create theorg.jetbrains.base64
package. - Create the
Base64Test.kt
file in the new package. Add tests to this file:
package org.jetbrains.base64 import kotlin.test.Test import kotlin.test.assertEquals class Base64Test { @Test fun testEncodeToString() { checkEncodeToString("Kotlin is awesome", "S290bGluIGlzIGF3ZXNvbWU=") } @Test 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) { get(it).code.toByte() } }
In the Terminal, execute the
check
Gradle task:./gradlew check
You can also run the
check
Gradle task by double-clicking it in the list of Gradle tasks.
The tests will run on all platforms (JVM, JS, and Native).
Add platform-specific tests
You can also add tests that will be run only for a specific platform. For example, you can add UTF-16 tests on JVM:
- In the
jvmTest/kotlin
directory, create theorg.jetbrains.base64
package. - Create the
Base64Test.kt
file in the new package. Add tests to this file:
package org.jetbrains.base64 import kotlin.test.Test import kotlin.test.assertEquals class Base64JvmTest { @Test fun testNonAsciiString() { val utf8String = "Gödel" val actual = Base64Factory.createEncoder().encodeToString(utf8String.toByteArray()) assertEquals("R8O2ZGVs", actual) } }
This test will automatically run on the JVM platform in addition to the common tests.
Publish your library to the local Maven repository
Your multiplatform library is ready for publishing so that you can use it in other projects.
To publish your library, use the maven-publish
Gradle plugin.
In the
build.gradle.kts
file, apply themaven-publish
plugin and specify the group and version of your library:plugins { kotlin("multiplatform") version "1.8.21" id("maven-publish") } group = "org.jetbrains.base64" version = "1.0.0"
In the Terminal, run the
publishToMavenLocal
Gradle task to publish your library to your local Maven repository:./gradlew publishToMavenLocal
You can also run the
publishToMavenLocal
Gradle task by double-clicking it in the list of Gradle tasks.
Your library will be published to the local Maven repository.
Publish your library to the external Maven Central repository
You can go public and release your multiplatform library to Maven Central, a remote repository where maven artifacts are stored and managed. This way, other developers will be able to find it and add as a dependency to their projects.
Register a Sonatype account and generate GPG keys
If this is your first library, or you used the sunset Bintray to do this before, you need first to register a Sonatype account.
You can use the GetStream article to create and set up your account. The Registering a Sonatype account section describes how to:
- Register a Sonatype Jira account.
- Create a new issue. You can use our issue as an example.
- Verify your domain ownership corresponding to the group ID you want to use to publish your artifacts.
Then, since artifacts published on Maven Central have to be signed, follow the Generating a GPG key pair section to:
- Generate a GPG key pair for signing your artifacts.
- Publish your public key.
- Export your private key.
When the Maven repository and signing keys for your library are ready, you can move on and set up your build to upload the library artifacts to a staging repository and then release them.
Set up publication
Now you need to instruct Gradle how to publish the library. Most of the work is already done by the maven-publish
and Kotlin
Gradle plugins, all the required publications are created automatically. You already know the result when the library is
published to a local Maven repository. To publish it to Maven Central, you need to take additional steps:
- Configure the public Maven repository URL and credentials.
- Provide a description and
javadocs
for all library components. - Sign publications.
You can handle all these tasks with Gradle scripts. Let's extract all the publication-related logic from the library
module build.script
, so you can easily reuse it for other modules in the future.
The most idiomatic and flexible way to do that is to use Gradle's precompiled script plugins. All the build logic will be provided as a precompiled script plugin and could be applied by plugin ID to every module of our library.
To implement this, move the publication logic to a separate Gradle project:
- Add a new Gradle project inside your library root project. For that, create a new folder named
convention-plugins
and create a new filebuild.gradle.kts
inside of it. Place the following code into the new
build.gradle.kts
file:plugins { `kotlin-dsl` // Is needed to turn our build logic written in Kotlin into the Gradle Plugin } repositories { gradlePluginPortal() // To use 'maven-publish' and 'signing' plugins in our own plugin }
In the
convention-plugins
directory, create asrc/main/kotlin/convention.publication.gradle.kts
file to store all the publication logic.Add all the required logic in the new file. Be sure to make changes to match your project configuration and where explicitly noted by angle brackets (i.e.
<replace-me>
):import org.gradle.api.publish.maven.MavenPublication import org.gradle.api.tasks.bundling.Jar import org.gradle.kotlin.dsl.`maven-publish` import org.gradle.kotlin.dsl.signing import java.util.* plugins { `maven-publish` signing } // Stub secrets to let the project sync and build without the publication values set up ext["signing.keyId"] = null ext["signing.password"] = null ext["signing.secretKeyRingFile"] = null ext["ossrhUsername"] = null ext["ossrhPassword"] = null // Grabbing secrets from local.properties file or from environment variables, which could be used on CI val secretPropsFile = project.rootProject.file("local.properties") if (secretPropsFile.exists()) { secretPropsFile.reader().use { Properties().apply { load(it) } }.onEach { (name, value) -> ext[name.toString()] = value } } else { ext["signing.keyId"] = System.getenv("SIGNING_KEY_ID") ext["signing.password"] = System.getenv("SIGNING_PASSWORD") ext["signing.secretKeyRingFile"] = System.getenv("SIGNING_SECRET_KEY_RING_FILE") ext["ossrhUsername"] = System.getenv("OSSRH_USERNAME") ext["ossrhPassword"] = System.getenv("OSSRH_PASSWORD") } val javadocJar by tasks.registering(Jar::class) { archiveClassifier.set("javadoc") } fun getExtraString(name: String) = ext[name]?.toString() publishing { // Configure maven central repository repositories { maven { name = "sonatype" setUrl("https://s01.oss.sonatype.org/service/local/staging/deploy/maven2/") credentials { username = getExtraString("ossrhUsername") password = getExtraString("ossrhPassword") } } } // Configure all publications publications.withType<MavenPublication> { // Stub javadoc.jar artifact artifact(javadocJar.get()) // Provide artifacts information requited by Maven Central pom { name.set("MPP Sample library") description.set("Sample Kotlin Multiplatform library (jvm + ios + js) test") url.set("https://github.com/<your-github-repo>/mpp-sample-lib") licenses { license { name.set("MIT") url.set("https://opensource.org/licenses/MIT") } } developers { developer { id.set("<your-github-profile>") name.set("<your-name>") email.set("<your-email>") } } scm { url.set("https://github.com/<your-github-repo>/mpp-sample-lib") } } } } // Signing artifacts. Signing.* extra properties values will be used signing { sign(publishing.publications) }
Applying just maven-publish
is enough for publishing to the local Maven repository, but not to Maven Central.
In the provided script, you get the credentials from local.properties
or environment variables,
do all the required configuration in the publishing
section, and sign your publications with the signing plugin.
Go back to your library project. To ask Gradle to prebuild your plugins, update the root
settings.gradle.kts
with the following:rootProject.name = "multiplatform-lib" // your project name includeBuild("convention-plugins")
Now, you can apply this logic in the library's
build.script
. In theplugins
section, replacemaven-publish
withconventional.publication
:plugins { kotlin("multiplatform") version "1.8.21" id("convention.publication") }
Create a
local.properties
file within your library's root directory with all the necessary credentials and make sure to add it to your.gitignore
:# The GPG key pair ID (last 8 digits of its fingerprint) signing.keyId=... # The passphrase of the key pair signing.password=... # Private key you exported earlier signing.secretKeyRingFile=... # Your credentials for the Jira account ossrhUsername=... ossrhPassword=...
Run
./gradlew clean
and sync the project.
New Gradle tasks related to the Sonatype repository should appear in the publishing group – that means that everything is ready for you to publish your library.
Publish your library to Maven Central
To upload your library to the Sonatype repository, run the following program:
./gradlew publishAllPublicationsToSonatypeRepository
The staging repository will be created, and all the artifacts for all publications will be uploaded to that repository. All it's left to do is to check that all the artifacts you wanted to upload have made it there and to press the release button.
These steps are described in the Your first release section. In short, you need to:
- Go to https://s01.oss.sonatype.org and log in using your credentials in Sonatype Jira.
- Find your repository in the Staging repositories section.
- Close it.
- Release the library.
- To activate the sync to Maven Central, go back to the Jira issue you created and leave a comment saying that you've released your first component. This step is only needed if it's your first release.
Soon your library will be available at https://repo1.maven.org/maven2, and other developers will be able to add it as a dependency. In a couple of hours, other developers will be able to find it using Maven Central Repository Search.
Add a dependency on the published library
You can add your library to other multiplatform projects as a dependency.
In the build.gradle.kts
file, add mavenLocal()
or MavenCentral()
(if the library was published
to the external repository) and add a dependency on your library:
repositories {
mavenCentral()
mavenLocal()
}
kotlin {
sourceSets {
val commonMain by getting {
dependencies {
implementation("org.jetbrains.base64:multiplatform-lib:1.0.0")
}
}
}
}
The implementation
dependency consists of:
- The group ID and version — specified earlier in the
build.gradle.kts
file - The artifact ID — by default, it's your project's name specified in the
settings.gradle.kts
file
For more details, see the Gradle documentation on the maven-publish
plugin.