Libraries

Active contributors: Filipp Zhinkin, Ilya Gorbunov, Sergej Jaskiewicz

The libraries/ directory hosts everything that ships with the Kotlin distribution but is not the compiler itself: the standard library, reflection, the test framework, the script engine, the Gradle/Maven plugins, the Build Tools API, kotlinx-metadata, the ABI-validation tooling, and a long tail of build-time helpers. Several different build conventions (Gradle Kotlin DSL, Gradle Groovy DSL, Maven) coexist here for historical reasons.

Pages in this section

• Standard library — libraries/stdlib/
• kotlin-reflect — libraries/reflect/
• kotlin-test — libraries/kotlin.test/
• Scripting — libraries/scripting/
• See also: tools/ — the Gradle/Maven/JPS plugin family

Top-level layout

libraries/
├── stdlib/                    Kotlin standard library (multiplatform)
├── reflect/                   kotlin-reflect (full reflection on JVM)
├── kotlin.test/               kotlin-test framework
├── scripting/                 Kotlin scripting host and definitions
├── kotlinx-metadata/          Read/write Kotlin class metadata
├── tools/                     KGP, Maven plugin, BTAPI, daemon, build helpers
├── examples/                  Example projects
├── lib/                       Static helper libraries
├── kotlin-dom-api-compat/     DOM API compatibility shim for legacy JS
├── ReadMe.md
├── pom.xml                    Maven parent POM (some libs build with Maven)
├── mvnw, mvnw.cmd             Maven Wrapper
└── maven-settings.xml

What is the standard library?

The standard library is multiplatform from day one: a common/ source set defines the cross-platform API (with expect declarations), and per-target source sets (jvm/, js/, native-wasm/, wasm/) provide the actual implementations. There is no single canonical "stdlib jar" — there is a kotlin-stdlib for each target plus a multiplatform variant that ties them together via klibs.

See stdlib.md for the deep dive.

Reflection

Reflection on the JVM is in libraries/reflect/. It depends on stdlib and uses Kotlin's metadata format (encoded in JVM annotations on class files) to recover Kotlin-specific information at runtime. JS, Native, and Wasm have lighter reflection capabilities baked into stdlib.

Test framework

libraries/kotlin.test/ ships kotlin-test and the per-target adapter modules (kotlin-test-junit, kotlin-test-junit5, kotlin-test-testng, kotlin-test-js, ...). These give Kotlin code a uniform assertEquals / assertTrue API that works across all targets.

Scripting

libraries/scripting/ is the implementation behind kotlinc -script, .kts Gradle build scripts, and Kotlin notebooks. It defines the script-host API, the script-definition format, and the JSR-223 / Kotlin Main KTS launcher.

kotlinx-metadata

libraries/kotlinx-metadata/ is a small library that reads and writes the metadata format embedded in JVM and Kotlin/JS class files. It's used by tools that need to inspect or manipulate Kotlin declarations without invoking the full compiler.

ABI validation and binary compatibility

libraries/tools/binary-compatibility-validator/ and libraries/tools/abi-validation/ provide the tooling that verifies public API stability. Kotlin's standard library and many of its tools commit .api dumps; CI fails the build if a dump diverges from the source.

Examples

libraries/examples/ contains small projects used as examples in documentation and as smoke tests for the build.

Maven and Gradle coexistence

Most of libraries/ builds with Gradle, but the Maven plugin family (anything matching libraries/kotlin-maven-*/) builds with Maven via libraries/pom.xml and the wrapper at libraries/mvnw. The Gradle build invokes Maven for these subprojects; the Maven build also runs standalone for plugin development.

Where to start

If you're contributing to:

• stdlib — libraries/stdlib/. Pay attention to expect/actual parity across targets.
• kotlin-test — libraries/kotlin.test/. New asserts go in the common module first.
• kotlin-reflect — libraries/reflect/. Be careful around lazy initialization.
• scripting — libraries/scripting/. The kotlin-main-kts/ jar is a popular distribution.
• build tools (KGP, Maven plugin, BTAPI) — libraries/tools/ (see tools section).
• ABI validation — libraries/tools/abi-validation/ and libraries/tools/binary-compatibility-validator/.

See tools/index.md for the build-system integrations.