Compatibility guide for Kotlin Multiplatform

This guide summarizes incompatible changes you might encounter while developing projects with Kotlin Multiplatform.

Version compatibility

When configuring your project, check the compatibility of a particular Kotlin version (the version of Kotlin Multiplatform Gradle plugin) with available Gradle, Xcode, and Android Gradle plugin versions:

Kotlin version	Gradle	Android Gradle plugin	Xcode
1.9.20	7.5 and later	7.4.2–8.2	15.0

New approach to auto-generated targets

What's changed?

Target accessors auto-generated by Gradle are no longer available inside the kotlin.targets block. Use the findByName("targetName") method instead.

Note that such accessors are still available in the kotlin.targets case, for example, kotlin.targets.linuxX64.

What's the best practice now?

Before	Now
kotlin { targets { configure(['windows', 'linux']) { } } }	kotlin { targets { configure([findByName('windows'), findByName('linux')]) { } } }

When do the changes take effect?

In Kotlin 1.7.20, an error is introduced when using target accessors in the kotlin.targets block.

For more information, see the corresponding issue in YouTrack.

Changes in Gradle input and output compile tasks

What's changed?

Kotlin compile tasks no longer inherit the Gradle AbstractCompile task that has the sourceCompatibility and targetCompatibility inputs, making them unavailable in Kotlin users' scripts.

Other breaking changes in compile tasks:

What's the best practice now?

Before	Now
The `SourceTask.stableSources` input is no longer available.	Use the `sources` input instead. Also, the `setSource()` methods are still available.
The `sourceFilesExtensions` input was removed.	Compile tasks still implement the `PatternFilterable` interface. Use its methods for filtering Kotlin sources.
The `Gradle destinationDir: File` output was deprecated.	Use the `destinationDirectory: DirectoryProperty` output instead.
The `classpath` property of the `KotlinCompile` task is deprecated.	All compile tasks now use the `libraries` input for a list of libraries required for compilation.

When do the changes take effect?

In Kotlin 1.7.20, inputs are not available, the output is replaced, and the classpath property is deprecated.

For more information, see the corresponding issue in YouTrack.

New configuration names for dependencies on the compilation

What's changed?

Compilation configurations created by the Kotlin Multiplatform Gradle Plugin received new names.

A target in the Kotlin Multiplatform project has two default compilations, main and test. Each of these compilations has its own default source set, for example, jvmMain and jvmTest. Previously the configuration names for the test compilation and its default source set were the same, which might lead to a name clash resulting in issues when a configuration marked with platform-specific attributes is included in another configuration.

Now compilation configurations have an extra Compilation postfix, while projects and plugins that use old hard-coded configuration names no longer compile.

Configuration names for dependencies on the corresponding source set stay the same.

What's the best practice now?

	Before	Now
Dependencies of the `jvmMain` compilation	jvm<Scope>	jvmCompilation<Scope>
Dependencies of the `jvmMain` compilation	dependencies { add("jvmImplementation", "foo.bar.baz:1.2.3") }	dependencies { add("jvmCompilationImplementation", "foo.bar.baz:1.2.3") }
Dependencies of the `jvmMain` source set	jvmMain<Scope>
Dependencies of the `jvmTest` compilation	jvmTest<Scope>	jvmTestCompilation<Scope>
Dependencies of the `jvmTest` source set	jvmTest<Scope>

The available scopes are Api, Implementation, CompileOnly, and RuntimeOnly.

When do the changes take effect?

In Kotlin 1.8.0, an error is introduced when using old configuration names in hard-coded strings.

For more information, see the corresponding issue in YouTrack.

Deprecated Gradle properties for hierarchical structure support

What's changed?

Throughout its evolution, Kotlin was gradually introducing the support for hierarchical structure, in multiplatform projects, an ability to have intermediate source sets between the common source set commonMain and any platform-specific one, for example, jvmMain.

For the transition period, while the toolchain wasn't stable enough, a couple of Gradle properties were introduced, allowing granular opt-ins and opt-outs.

Since Kotlin 1.6.20, the hierarchical project structure support has been enabled by default. However, these properties were kept for opting out in case of blocking issues. After processing all the feedback, we're now starting to phase out those properties completely.

The following properties are now deprecated:

kotlin.internal.mpp.hierarchicalStructureByDefault
kotlin.mpp.enableCompatibilityMetadataVariant
kotlin.mpp.hierarchicalStructureSupport
kotlin.mpp.enableGranularSourceSetsMetadata
kotlin.native.enableDependencyPropagation

What's the best practice now?

Remove these properties from your gradle.properties and local.properties files.
Avoid setting them programmatically in the Gradle build scripts or your Gradle plugins.
In case deprecated properties are set by some third-party Gradle plugin used in your build, ask the plugin maintainers not to set these properties.

As the default behavior of the Kotlin toolchain doesn't include such properties since 1.6.20, we don't expect any serious impact from removing them. Most possible consequences will be visible immediately after the project rebuild.

If you're a library author and want to be extra safe, check that consumers can work with your library.

When do the changes take effect?

Here's the planned deprecation cycle:

1.8.20: report a warning when these properties are used
1.9.20: raise this warning to an error
2.0: remove these properties; the Kotlin Gradle plugin ignores their usages

In the unlikely case you face some problems after removing these properties, create an issue in YouTrack.

Deprecated support of multiplatform libraries published in the legacy mode

What's changed?

Previously, we have deprecated the legacy mode in Kotlin Multiplatform projects preventing the publication of "legacy" binaries and encouraged you to migrate your projects to the hierarchical structure.

To continue phasing out "legacy" binaries from the ecosystem, starting with Kotlin 1.9.0, the use of legacy libraries is also discouraged. If your project uses dependencies on legacy libraries, you'll see the following warning:

The dependency group:artifact:1.0 was published in the legacy mode. Support for such dependencies will be removed in the future

What's the best practice now?

If you use multiplatform libraries, most of them have already migrated to the "hierarchical structure" mode, so you only need to update the library version. See the documentation of the respective libraries for details.

If the library doesn't support non-legacy binaries yet, you can contact the maintainers and tell them about this compatibility issue.

If you're a library author, update the Kotlin Gradle plugin to the latest version and ensure you've fixed the deprecated Gradle properties.

The Kotlin team is eager to help the ecosystem migrate, so if you face any issues, don't hesitate to create an issue in YouTrack.

When do the changes take effect?

Here's the planned deprecation cycle:

1.9: introduce a deprecation warning for dependencies on legacy libraries
2.0: raise the warning for dependencies on legacy libraries to an error
>2.0: remove support for dependencies on legacy libraries; using such dependencies can cause build failures

Deprecated API for adding Kotlin source sets directly to the Kotlin compilation

What's changed?

The access to KotlinCompilation.source has been deprecated. A code like this produces a deprecation warning:

kotlin {
    jvm()
    js()
    ios()
    
    sourceSets {
        val commonMain by getting
        val myCustomIntermediateSourceSet by creating {
            dependsOn(commonMain)
        }
        
        targets["jvm"].compilations["main"].source(myCustomIntermediateSourceSet)
    }
}

What's the best practice now?

To replace KotlinCompilation.source(someSourceSet), add the dependsOn relation from the default source set of the KotlinCompilation to someSourceSet. We recommend referring to the source directly using by getting, which is shorter and more readable. However, you can also use KotlinCompilation.defaultSourceSet.dependsOn(someSourceSet), which is applicable in all cases.

You can change the code above in one of the following ways:

kotlin {
    jvm()
    js()
    ios()

    sourceSets {
        val commonMain by getting
        val myCustomIntermediateSourceSet by creating {
            dependsOn(commonMain)
        }
        
        // Option #1. Shorter and more readable, use it when possible. 
        // Usually, the name of the default source set 
        // is a simple concatenation of the target name and the compilation name:
        val jvmMain by getting {
            dependsOn(myCustomIntermediateSourceSet)
        }
        
        // Option #2. Generic solution, use it if your build script requires a more advanced approach:
        targets["jvm"].compilations["main"].defaultSourceSet.dependsOn(myCustomIntermediateSourceSet)
    }
}

When do the changes take effect?

Here's the planned deprecation cycle:

1.9.0: introduce a deprecation warning when KotlinComplation.source is used
1.9.20: raise this warning to an error
>1.9.20: remove KotlinComplation.source from the Kotlin Gradle plugin, attempts to use it lead to "unresolved reference" errors during the buildscript compilation

Migration from kotlin-js Gradle plugin to kotlin-multiplatform Gradle plugin

What's changed?

Starting with Kotlin 1.9.0, the kotlin-js Gradle plugin is deprecated. Basically, it duplicated the functionality of the kotlin-multiplatform plugin with the js() target and shared the same implementation under the hood. Such overlap created confusion and increased maintenance load on the Kotlin team. We encourage you to migrate to the kotlin-multiplatform Gradle plugin with the js() target instead.

What's the best practice now?

Remove the kotlin-js Gradle plugin from your project and apply kotlin-multiplatform in the settings.gradle.kts file if you're using the pluginManagement block:
// settings.gradle.kts: pluginManagement { plugins { // Remove the following line: kotlin("js") version "1.9.0" } repositories { // ... } }
// settings.gradle.kts: pluginManagement { plugins { // Add the following line instead: kotlin("multiplatform") version "1.9.0" } repositories { // ... } }
In case you're using a different way of applying plugins, see the Gradle documentation for migration instructions.
Move your source files from the main and test folders to the jsMain and jsTest folders in the same directory.
Adjust dependency declarations:
- We recommend using the sourceSets block and configuring dependencies of respective source sets, jsMain for production dependencies and jsTest for test dependencies. See Adding dependencies for more details.
- However, if you want to declare your dependencies in a top-level block, change declarations from api("group:artifact:1.0") to add("jsMainApi", "group:artifact:1.0") and so on.
  In this case, make sure that the top-level dependencies block comes after the kotlin block. Otherwise, you'll get an error "Configuration not found".
You can change the code in your build.gradle.kts file in one of the following ways:
// build.gradle.kts: plugins { kotlin("js") version "1.9.0" } dependencies { testImplementation(kotlin("test")) implementation("org.jetbrains.kotlinx:kotlinx-html:0.8.0") } kotlin { js { // ... } }
// build.gradle.kts: plugins { kotlin("multiplatform") version "1.9.0" } kotlin { js { // ... } // Option #1. Declare dependencies in the sourceSets block: sourceSets { val jsMain by getting { dependencies { // No need for the js prefix here, you can just copy and paste it from the top-level block implementation("org.jetbrains.kotlinx:kotlinx-html:0.8.0") } } } } dependencies { // Option #2. Add the js prefix to the dependency declaration: add("jsTestImplementation", kotlin("test")) }
The DSL provided by the Kotlin Gradle plugin inside the kotlin block remains unchanged in most cases. However, if you were referring to low-level Gradle entities, like tasks and configurations, by names, you now need to adjust them, usually by adding the js prefix. For example, you can find the browserTest task under the name jsBrowserTest.

When do the changes take effect?

In 1.9.0, the use of the kotlin-js Gradle plugin produces a deprecation warning.

Rename of android target to androidTarget

What's changed?

We continue our efforts to stabilize Kotlin Multiplatform. An essential step in this way is to provide first-class support for the Android target. In the future, this support will be provided via a separate plugin, developed by the Android team from Google.

To open the way for the new solution from Google, we're renaming the android block to androidTarget in the current Kotlin DSL in 1.9.0. This is a temporary change that is necessary to free the short android name for the upcoming DSL from Google.

What's the best practice now?

Rename all the occurrences of the android block to androidTarget. When the new plugin for the Android target support is available, migrate to the DSL from Google. It will be the preferred option to work with Android in Kotlin Multiplatform projects.

When do the changes take effect?

In Kotlin 1.9.0, a deprecation warning is introduced when the android name is used in Kotlin Multiplatform projects.

Declaring several similar targets

What's changed?

We discourage declaring several similar targets in a single Gradle project. For example:

kotlin {
    jvm("jvmKtor")
    jvm("jvmOkHttp") // Not recommended and produces a deprecation warning
}

One popular case is having two related pieces of code together. For example, you might want to use jvm("jvmKtor") and jvm("jvmOkHttp") in your :shared Gradle project to implement networking using the Ktor or OkHttp libraries:

// shared/build.gradle.kts:
kotlin {
    jvm("jvmKtor") {
        attributes.attribute(/* ... */)
    }
    jvm("jvmOkHttp") {
        attributes.attribute(/* ... */)
    }

    sourceSets {
        val commonMain by getting
        val commonJvmMain by sourceSets.creating {
            dependsOn(commonMain)
            dependencies {
                // Shared dependencies
            }
        }
        val jvmKtorMain by getting {
            dependsOn(commonJvmMain)
            dependencies {
                // Ktor dependencies
            }
        }
        val jvmOkHttpMain by getting {
            dependsOn(commonJvmMain)
            dependencies {
                // OkHttp dependencies
            }
        }
    }
}

The implementation comes with non-trivial configuration complexity:

You have to set up Gradle attributes on the :shared side and each consumer's side. Otherwise, Gradle can't resolve dependencies in such projects because without additional information it's not clear whether the consumer should receive the Ktor-based or the OkHttp-based implementation.
You have to set up the commonJvmMain source set manually.
The configuration involves a handful of low-level Gradle and Kotlin Gradle plugin abstractions and APIs.

What's the best practice now?

The configuration is complex because Ktor-based and OkHttp-based implementations are in the same Gradle project. In many cases, it's possible to extract those parts into separate Gradle projects. Here's a general outline of such as a refactoring:

Replace two duplicated targets from the original project with a single target. If you had a shared source set between these targets, move its sources and configuration to the default source set of the newly created target:
// shared/build.gradle.kts: kotlin { jvm() sourceSets { jvmMain { // Copy the configuration of jvmCommonMain here } } }
Add two new Gradle projects, usually by calling include in your settings.gradle.kts file. For example:
include(":okhttp-impl") include(":ktor-impl")
Configure each new Gradle project:
- Most likely, you don't need to apply the kotlin("multiplatform") plugin, as these projects compile only to one target. In this example, you can apply kotlin("jvm").
- Move the content of original target-specific source sets to their respective projects, for example, from jvmKtorMain to ktor-impl/src.
- Copy the configuration of source sets: dependencies, compiler options, and so on.
- Add a dependency from the new Gradle project to the original project.
// ktor-impl/build.gradle.kts: plugins { kotlin("jvm") } dependencies { project(":shared") // Add dependency on the original project // Copy dependencies of jvmKtorMain here } kotlin { compilerOptions { // Copy compiler options of jvmKtorMain here } }

While this approach requires more work on the initial setup, it doesn't use any low-level entities of Gradle and the Kotlin Gradle plugin, making it easier to use and maintain the resulting build.

When do the changes take effect?

Here's the planned deprecation cycle:

1.9.20: introduce a deprecation warning when multiple similar targets are used in Kotlin Multiplatform projects
2.0: report an error in such cases, causing the build to fail

Deprecated jvmWithJava preset

What's changed?

targetPresets.jvmWithJava is deprecated, and its usage is discouraged.

What's the best practice now?

Use jvm { withJava() } target instead. Note that after switching to jvm { withJava() }, you'll need to adjust the paths to source directories with .java sources.

For example, if you use a jvm target with the default name "jvm":

Before	Now
`src/main/java`	`src/jvmMain/java`
`src/test/java`	`src/jvmTest/java`

When do the changes take effect?

Here's the planned deprecation cycle:

1.3.40: introduce a warning when targetPresets.jvmWithJava is used
1.9.20: raise this warning to an error
>1.9.20: remove targetPresets.jvmWithJava API; attempts to use it lead to the buildscript compilation failure

Deprecated legacy Android source set layout

What's changed?

The new Android source set layout is used by default since Kotlin 1.9.0. Support for the legacy layout is deprecated, and the use of the kotlin.mpp.androidSourceSetLayoutVersion Gradle property now triggers a deprecation diagnostic.

When do the changes take effect?

Here's the planned deprecation cycle:

<=1.9.0: report a warning when kotlin.mpp.androidSourceSetLayoutVersion=1 is used; the warning can be suppressed with kotlin.mpp.androidSourceSetLayoutVersion1.nowarn=true Gradle property
1.9.20: raise this warning to an error; the error cannot be suppressed
>1.9.20: remove support for kotlin.mpp.androidSourceSetLayoutVersion=1; the Kotlin Gradle plugin ignores the property

Deprecated commonMain and commonTest with custom dependsOn

What's changed?

The commonMain and commonTest source sets usually represent the roots of the main and test source set hierarchies, respectively. However, it was possible to override that by manually configuring dependsOn relations of these source sets.

Maintaining such configuration requires extra effort and knowledge about multiplatform build internals. Additionally, it decreases code readability and reusability of the code because you need to read the particular buildscript to be sure whether commonMain is the root of the main source set hierarchy.

Therefore, accessing dependsOn on commonMain and commonTest is now deprecated.

What's the best practice now?

Suppose you need to migrate to 1.9.20 the customCommonMain source set that uses commonMain.dependsOn(customCommonMain). In most cases, customCommonMain participates in the same compilations as commonMain, so you can merge customCommonMain into commonMain:

Copy sources of customCommonMain into commonMain.
Add all dependencies of customCommonMain to commonMain.
Add all compiler option settings of customCommonMain to commonMain.

In rare cases, customCommonMain might be participating in more compilations than commonMain. Such a configuration requires additional low-level configuration of the build script. If you're not sure if that's your use case, it most likely isn't.

If it is your use case, "swap" these two source sets by moving the sources and settings of customCommonMain to commonMain and vice versa.

When do the changes take effect?

Here's the planned deprecation cycle:

1.9.0: report a warning when dependsOn is used in commonMain
>=1.9.20: report an error when dependsOn is used in commonMain or commonTest

Deprecated target presets API

What's changed?

In the very early development stages, Kotlin Multiplatform introduced an API for working with so-called target presets. Each target preset essentially represented a factory for Kotlin Multiplatform targets. This API turned out to be largely redundant, as DSL functions like jvm() or iosSimulatorArm64() cover the same use cases while being much more straightforward and concise.

To reduce the confusion and provide clearer guidelines, all presets-related APIs are now deprecated and will be removed from the public API of the Kotlin Gradle plugin in future releases. This includes:

The presets property in org.jetbrains.kotlin.gradle.dsl.KotlinMultiplatformExtension
The org.jetbrains.kotlin.gradle.plugin.KotlinTargetPreset interface and all its inheritors
The fromPreset overloads

What's the best practice now?

Use respective Kotlin targets instead, for example:

Before	Now
kotlin { targets { fromPreset(presets.iosArm64, 'ios') } }	kotlin { iosArm64() }

When do the changes take effect?

Here's the planned deprecation cycle:

1.9.20: report a warning on any usages of the presets-related API
2.0: raise this warning to an error
>2.0: remove the presets-related API from the public API of the Kotlin Gradle plugin; sources that still use it fail with "unresolved reference" errors, and binaries (for example, Gradle plugins) might fail with linkage errors unless recompiled against the latest versions of the Kotlin Gradle plugin

New approach to forward declarations

What's changed?

The JetBrains team has revamped the approach to forward declarations in Kotlin to make their behavior more predictable and prepare this functionality for the upcoming Kotlin 2.0 release. From now on:

You can only import forward declarations using the cnames or objcnames packages.
You need to explicitly make a cast to and from the corresponding C and Objective-C forward declaration.

What's the best practice now?

Consider a C library with a library.package that declares a cstructName forward declaration. Previously, it was possible to import it directly from the library with import library.package.cstructName. Now, you can only use a special forward declaration package for that: import cnames.structs.cstructName. The same is true for objcnames.
Consider two objcinterop libraries, one that uses objcnames.protocols.ForwardDeclaredProtocolProtocol and the other that has an actual definition:
// First objcinterop library #import <Foundation/Foundation.h> @protocol ForwardDeclaredProtocol; NSString* consumeProtocol(id<ForwardDeclaredProtocol> s) { return [NSString stringWithUTF8String:"Protocol"]; }
// Second objcinterop library // Header: #import <Foundation/Foundation.h> @protocol ForwardDeclaredProtocol @end // Implementation: @implementation ForwardDeclaredProtocolImpl : NSObject @end; id<ForwardDeclaredProtocol> produceProtocol() { return [ForwardDeclaredProtocolImpl new]; }
Previously, it was possible to transfer objects between them seamlessly. Now, an explicit as cast is required for the forward declaration:
// Kotlin code: fun test() { consumeProtocol(produceProtocol() as objcnames.protocols.ForwardDeclaredProtocolProtocol) }
The casting to objcnames.protocols.ForwardDeclaredProtocolProtocol is only allowed from the corresponding real class. Otherwise, you'll get an error.

When do the changes take effect?

Starting with Kotlin 1.9.20, you need to explicitly make a cast to and from the corresponding C and Objective-C forward declarations. Also, it's now only possible to import forward declarations by using special packages.

Last modified: 15 December 2023