Kotlin + Bazel Example

Bazel Configuration | Requirements | Windows-specific information

A minimal, professional template demonstrating how to configure Bazel to build, run and debug Kotlin code (using the IntelliJ IDEA IDE with the official Bazel plugin).

Project Structure

.
├── MODULE.bazel         # Bzlmod configuration
├── .bazelrc             # Bazel runtime configuration
├── .bazelversion        # Pinned Bazel version
├── BUILD.bazel          # Kotlin toolchain definition
└── src/
    ├── BUILD.bazel      # Build targets definition
    ├── main/
        ├── java/
        │   └── org/
        │       └── example/
        │           └── Main.java    # Entry point (necessary for MS Windows)
        └── kotlin/
            └── org/
                └── example/
                    └── FileUtil.kt  # Kotlin utility class

Bazel Configuration

This project demonstrates modern Bazel configuration with Bzlmod:

MODULE.bazel

• Uses Bzlmod for dependency management (Bazel's module system)
• Depends on rules_kotlin version 2.1.8 for Kotlin support
• Configures Maven repositories for external dependencies
• Registers a custom Kotlin toolchain

.bazelrc

• Sets Java language version to 17 and runtime to JDK 21
• Enables Windows-specific configuration with --legacy_external_runfiles
• Contains workarounds for known issues with rules_kotlin on MS Windows

BUILD.bazel (root)

• Defines Kotlin toolchain with language and API version 2.0
• Sets JVM target to 17
• Configures Kotlin compiler options

src/BUILD.bazel

• Defines three targets:
  • main_lib: Java library containing Main.java
  • utils_lib: Kotlin library containing FileUtil.kt
  • Main: Java binary with the main class org.example.Main
• Uses Java binary wrapper instead of kt_jvm_binary for Windows compatibility

Application Overview

The example demonstrates Java-Kotlin interoperability (due to known Windows-specific issues when it comes to debugging):

• Java entry point (Main.java) calls Kotlin utility class
• Kotlin utility (FileUtil.kt) provides file extension extraction functionality

Build & Run

Build the application:

bazel build //src:Main

Run the application:

bazel run //src:Main

Expected output:

The file extension is: .txt

Requirements

• Bazel 8.x (use bazelisk for version management)
  • For Windows, it is recommended to install Bazelisk through Chocolatey: choco install bazelisk
• Optional: IntelliJ IDEA with Bazel plugin

Windows-specific information

Running Bazel on Windows requires at the time of writing a few workarounds:

• Use Java binary wrapper instead of kt_jvm_binary
  • The kt_jvm_binary rule does not (at this moment) support debugging (with IntelliJ) on Windows
• Set --legacy_external_runfiles to avoid issues with runfiles
  • It is known that on Windows runfiles are not properly resolved and this error is thrown LAUNCHER ERROR: Rlocation failed on _main/external/rules_kotlin++rules_kotlin_extensions+com_github_jetbrains_kotlin_git/lib/annotations-13.0.jar, path doesn't exist in MANIFEST file

GitHub CI/CD

This repository contains a GitHub Actions workflow that builds the source code on Windows, Linux, and macOS. It should provide a good starting point for CI/CD pipelines utilizing Bazel with Kotlin. The workflow is not complete and must be extended to include testing and deployment steps that make sense for your project.

References

• Bazel Documentation
• Bazelisk
• Bzlmod Documentation
• Chocolatey

License

Licensed under the BSD 3-Clause License http://opensource.org/licenses/bsd-3-clause. This project may not be copied, modified, or distributed except according to those terms.