Configure compilations

Kotlin multiplatform projects use compilations for producing artifacts. Each target can have one or more compilations, for example, for production and test purposes.

For each target, default compilations include:

main and test compilations for JVM, JS, and Native targets.
A compilation per Android build variant, for Android targets.

If you need to compile something other than production code and unit tests, for example, integration or performance tests, you can create a custom compilation.

You can configure how artifacts are produced in:

All compilations in your project at once.
Compilations for one target since one target can have multiple compilations.
A specific compilation.

See the list of compilation parameters and compiler options available for all or specific targets.

Configure all compilations

This example configures a compiler option that is common across all targets:

kotlin {
    compilerOptions {
        allWarningsAsErrors.set(true)
    }
}

kotlin {
    compilerOptions {
        allWarningsAsErrors = true
    }
}

Configure compilations for one target

kotlin {
    jvm {
        compilerOptions {
            jvmTarget.set(JvmTarget.JVM_1_8)
        }
    }
}

kotlin {
    jvm {
        compilerOptions {
            jvmTarget = JvmTarget.JVM_1_8
        }
    }
}

Configure one compilation

kotlin {
    jvm {
        val main by compilations.getting {
            compileTaskProvider.configure {
                compilerOptions {
                    jvmTarget.set(JvmTarget.JVM_1_8)
                }
            }
        }
    }
}

kotlin {
    jvm {
        compilations.main {
            compileTaskProvider.configure {
                compilerOptions {
                    jvmTarget = JvmTarget.JVM_1_8
                }
            }
        }
    }
}

Create a custom compilation

If you need to compile something other than production code and unit tests, for example, integration or performance tests, create a custom compilation.

For example, to create a custom compilation for integration tests of the jvm() target, add a new item to the compilations collection.

kotlin {
    jvm() {
        compilations {
            val main by getting
            
            val integrationTest by compilations.creating {
                defaultSourceSet {
                    dependencies {
                        // Compile against the main compilation's compile classpath and outputs:
                        implementation(main.compileDependencyFiles + main.output.classesDirs)
                        implementation(kotlin("test-junit"))
                        /* ... */
                    }
                }
                
                // Create a test task to run the tests produced by this compilation:
                tasks.register<Test>("integrationTest") {
                    // Run the tests with the classpath containing the compile dependencies (including 'main'),
                    // runtime dependencies, and the outputs of this compilation:
                    classpath = compileDependencyFiles + runtimeDependencyFiles + output.allOutputs
                    
                    // Run only the tests from this compilation's outputs:
                    testClassesDirs = output.classesDirs
                }
            }
        }
    }
}

kotlin {
    jvm() {
        compilations.create('integrationTest') {
            defaultSourceSet {
                dependencies {
                    def main = compilations.main
                    // Compile against the main compilation's compile classpath and outputs:
                    implementation(main.compileDependencyFiles + main.output.classesDirs)
                    implementation kotlin('test-junit')
                    /* ... */
                }
            }
           
            // Create a test task to run the tests produced by this compilation:
            tasks.register('jvmIntegrationTest', Test) {
                // Run the tests with the classpath containing the compile dependencies (including 'main'),
                // runtime dependencies, and the outputs of this compilation:
                classpath = compileDependencyFiles + runtimeDependencyFiles + output.allOutputs
                
                // Run only the tests from this compilation's outputs:
                testClassesDirs = output.classesDirs
            }
        }
    }
}

You also need to create a custom compilation in other cases, for example, if you want to combine compilations for different JVM versions in your final artifact, or you have already set up source sets in Gradle and want to migrate to a multiplatform project.

Compilation for JVM

When you declare the jvm target in your multiplatform project, the Kotlin Multiplatform plugin automatically creates Java sources sets and includes them in the compilations of the JVM target.

The common source sets can't include Java resources, so you should place them in the corresponding child directories of your multiplatform project. For example:

Currently, the Kotlin Multiplatform plugin replaces some tasks configured by the Java plugin:

JAR task: instead of a standard jar, it uses a target-specific task based on the artifact's name, for example, jvmJar for the jvm() target declaration and desktopJar for jvm("desktop").
Test task: instead of a standard test, it uses a target-specific task based on the artifact's name, for example, jvmTest.
Resource processing: instead of *ProcessResources tasks, resources are handled by the corresponding compilation tasks.

These tasks are created automatically when the target is declared. However, you can manually define the JAR task and configure it if necessary:

// Shared module's `build.gradle.kts` file
plugins {
    kotlin("multiplatform") version "2.1.20"
}

kotlin {
    // Specify the JVM target
    jvm {
        // Add the task for JAR generation
        tasks.named<Jar>(artifactsTaskName).configure {
            // Configure the task
        }
    }

    sourceSets {
        jvmMain {
            dependencies {
                // Add JVM-specific dependencies
            }
        }
    }
}

// Shared module's `build.gradle` file
plugins {
    id 'org.jetbrains.kotlin.multiplatform' version '2.1.20'
}

kotlin {
    // Specify the JVM target
    jvm {
        // Add the task for JAR generation
        tasks.named<Jar>(artifactsTaskName).configure {
            // Configure the task
        }
    }

    sourceSets {
        jvmMain {
            dependencies {
                // Add JVM-specific dependencies
            }
        }
    }
}

This target is published by the Kotlin Multiplatform plugin and doesn't require steps that are specific to the Java plugin.

Configure interop with native languages

Kotlin provides interoperability with native languages and DSL to configure this for a specific compilation.

Native language	Supported platforms	Comments
C	All platforms, except for WebAssembly
Objective-C	Apple platforms (macOS, iOS, watchOS, tvOS)
Swift via Objective-C	Apple platforms (macOS, iOS, watchOS, tvOS)	Kotlin can use only Swift declarations marked with the `@objc` attribute.

A compilation can interact with several native libraries. Configure interoperability with available properties in the definition file or in the cinterops block of your build file:

kotlin {
    linuxX64 { // Replace with a target you need.
        compilations.getByName("main") {
            val myInterop by cinterops.creating {
                // Def-file describing the native API.
                // The default path is src/nativeInterop/cinterop/<interop-name>.def
                definitionFile.set(project.file("def-file.def"))
                
                // Package to place the Kotlin API generated.
                packageName("org.sample")
                
                // Options to be passed to compiler by cinterop tool.
                compilerOpts("-Ipath/to/headers")
              
                // Directories to look for headers.
                includeDirs.apply {
                    // Directories for header search (an equivalent of the -I<path> compiler option).
                    allHeaders("path1", "path2")
                    
                    // Additional directories to search headers listed in the 'headerFilter' def-file option.
                    // -headerFilterAdditionalSearchPrefix command line option equivalent.
                    headerFilterOnly("path1", "path2")
                }
                // A shortcut for includeDirs.allHeaders.
                includeDirs("include/directory", "another/directory")
            }
            
            val anotherInterop by cinterops.creating { /* ... */ }
        }
    }
}

kotlin {
    linuxX64 { // Replace with a target you need.
        compilations.main {
            cinterops {
                myInterop {
                    // Def-file describing the native API.
                    // The default path is src/nativeInterop/cinterop/<interop-name>.def
                    definitionFile = project.file("def-file.def")
                    
                    // Package to place the Kotlin API generated.
                    packageName 'org.sample'
                    
                    // Options to be passed to compiler by cinterop tool.
                    compilerOpts '-Ipath/to/headers'
                    
                    // Directories for header search (an eqivalent of the -I<path> compiler option).
                    includeDirs.allHeaders("path1", "path2")
                    
                    // Additional directories to search headers listed in the 'headerFilter' def-file option.
                    // -headerFilterAdditionalSearchPrefix command line option equivalent.
                    includeDirs.headerFilterOnly("path1", "path2")
                    
                    // A shortcut for includeDirs.allHeaders.
                    includeDirs("include/directory", "another/directory")
                }
                
                anotherInterop { /* ... */ }
            }
        }
    }
}

Compilation for Android

The compilations created for an Android target by default are tied to Android build variants: for each build variant, a Kotlin compilation is created under the same name.

Then, for each Android source set compiled for each of the variants, a Kotlin source set is created under that source set name prepended by the target name, like the Kotlin source set androidDebug for an Android source set debug and the Kotlin target named androidTarget. These Kotlin source sets are added to the variants' compilations accordingly.

The default source set commonMain is added to each production (application or library) variant's compilation. The commonTest source set is similarly added to the compilations of unit test and instrumented test variants.

Annotation processing with kapt is also supported, but due to current limitations it requires that the Android target is created before the kapt dependencies are configured, which needs to be done in a top-level dependencies {} block rather than within Kotlin source set dependencies.

kotlin {
    androidTarget { /* ... */ }
}

dependencies {
    kapt("com.my.annotation:processor:1.0.0")
}

Compilation of the source set hierarchy

Kotlin can build a source set hierarchy with the dependsOn relation.

If the source set jvmMain depends on a source set commonMain then:

Whenever jvmMain is compiled for a certain target, commonMain takes part in that compilation as well and is also compiled into the same target binary form, such as JVM class files.
Sources of jvmMain 'see' the declarations of commonMain, including internal declarations, and also see the dependencies of commonMain, even those specified as implementation dependencies.
jvmMain can contain platform-specific implementations for the expected declarations of commonMain.
The resources of commonMain are always processed and copied along with the resources of jvmMain.
The language settings of jvmMain and commonMain should be consistent.

Language settings are checked for consistency in the following ways:

jvmMain should set a languageVersion that is greater than or equal to that of commonMain.
jvmMain should enable all unstable language features that commonMain enables (there's no such requirement for bugfix features).
jvmMain should use all experimental annotations that commonMain uses.
apiVersion, bugfix language features, and progressiveMode can be set arbitrarily.

Configure Isolated Projects feature in Gradle

Gradle provides the Isolated Projects feature, which improves build performance by "isolating" individual projects from each other. The feature separates the build scripts and plugins between projects, allowing them to run safely in parallel.

To enable this feature, follow Gradle's instructions to set the system property.

For more information about the Isolated Projects feature, see Gradle's documentation.

Last modified: 09 April 2025