Code Generation

swift4j generates Kotlin/Java proxy classes from your Swift source files. There are two ways to run the generator: as an SPM command plugin (the primary workflow) or directly via the CLI tool.

SPM plugin

The generate-java-bridging command plugin ships with swift4j. Run it from your package directory:

swift package plugin generate-java-bridging --product <ProductName>

Replace <ProductName> with the name of the SPM product whose targets you want to export. The plugin processes all targets in the product recursively.

Output location

Generated files are written to:

.build/plugins/generate-java-bridging/outputs/<ProductName>/main/java/

The directory structure under main/java/ follows the Java package naming convention, using the target name as the package name (hyphens replaced with underscores):

main/java/
  MyLibrary/
    GreetingService.kt
    Greeting.kt
    Counter.kt

Copying Java sources

If your Swift targets include bundled .java files (such as the base classes shipped with swift4j itself), pass --copy-java-sources to include them in the output:

swift package plugin generate-java-bridging --product MyLibrary --copy-java-sources

Android ViewModel generation

For Android projects using @Observable types, pass --generate-android-viewmodels to generate ViewModel and ViewModelFactory subclasses:

swift package plugin generate-java-bridging \
    --product MyLibrary \
    --generate-android-viewmodels

See Observable for details.

Including generated files in your project

Android (Gradle)

Copy or symlink the generated main/java/ directory into your Android module’s source set. In build.gradle:

android {
    sourceSets {
        main {
            java.srcDirs += [
                "path/to/.build/plugins/generate-java-bridging/outputs/MyLibrary/main/java"
            ]
        }
    }
}

JVM (Gradle)

sourceSets {
    main {
        kotlin.srcDirs += [
            "path/to/.build/plugins/generate-java-bridging/outputs/MyLibrary/main/java"
        ]
    }
}

Java version

The generator defaults to Java 11 compatibility. Pass --java-version to target a different version:

swift package plugin generate-java-bridging --product MyLibrary --java-version 17

Re-running generation

Run the plugin again whenever you add, remove, or rename exported Swift types. The output directory is replaced entirely on each run — do not edit the generated files by hand.