Overview

swift4j is a library for calling Swift code from Java and Kotlin. You annotate Swift classes, structs, and enums with @jvm, run a code generator, and get proxy classes that let any JVM language use your Swift code as if it were a native Java library.

It works wherever a JVM runs: Android, macOS, Linux, and Windows.

How it works

swift4j has two parts:

@jvm macro — applied to Swift types at compile time. The macro expands to JNI registration code that wires each exported method to a native C entry point inside the compiled Swift library.

generate-java-bridging plugin — an SPM command plugin that reads your Swift source files and generates Kotlin/Java proxy classes. Each proxy holds a pointer to the underlying Swift object and delegates method calls to the native stubs.

The workflow:

1. Annotate Swift types with @jvm
2. Run swift package plugin generate-java-bridging to generate Kotlin/Java proxy classes
3. Build your Swift package as a dynamic library
4. In your JVM project, include the generated classes and load the library with System.loadLibrary()

What can be exported

• Classes, structs, and enums
• Instance and static methods
• Stored and computed properties
• Closures and callbacks
• async methods (mapped to CompletableFuture)
• @Observable classes (with optional Android ViewModel generation)

See Using swift4j for details on each.