Kotlin

Bazelle includes a custom Kotlin extension (gazelle-kotlin) that generates kt_jvm_library and kt_jvm_test rules compatible with rules_kotlin.

Note

Kotlin is disabled by default. Enable it with # gazelle:kotlin_enabled true in your BUILD file.

Generated Rules

Source Location	Generated Rule
src/main/kotlin/**/*.kt	kt_jvm_library
src/main/kotlin/**/*.kts	kt_jvm_library
src/test/kotlin/**/*.kt	kt_jvm_test
src/test/kotlin/**/*.kts	kt_jvm_test

Caution

Kotlin requires a Maven-style directory layout (src/main/kotlin, src/test/kotlin). Other layouts are not currently supported.

Configuration

Enable Kotlin

# In your root BUILD.bazel
# gazelle:kotlin_enabled true

Available Directives

# Enable/disable Kotlin extension
# gazelle:kotlin_enabled true

# Parser backend: "heuristic" (default), "treesitter", or "hybrid"
# gazelle:kotlin_parser_backend heuristic

# Enable/disable FQN scanning (enabled by default)
# gazelle:kotlin_fqn_scanning true

# Custom macros (optional)
# gazelle:kotlin_library_macro kt_jvm_library
# gazelle:kotlin_test_macro kt_jvm_test

# Visibility for generated targets
# gazelle:kotlin_visibility //visibility:public

# Custom load path for macros
# gazelle:kotlin_load //my/macros:defs.bzl

Project Structure

Kotlin requires the Maven-style layout:

your-project/
├── BUILD.bazel           # With kotlin_enabled directive
└── src/
    ├── main/
    │   └── kotlin/
    │       └── com/
    │           └── example/
    │               └── Greeter.kt    # → kt_jvm_library
    └── test/
        └── kotlin/
            └── com/
                └── example/
                    └── GreeterTest.kt # → kt_jvm_test

Example

Given this Kotlin source file:

src/main/kotlin/com/example/Greeter.kt

package com.example

class Greeter {
    fun hello(name: String): String {
        return "Hello, $name!"
    }
}

Bazelle generates:

src/main/kotlin/com/example/BUILD.bazel

load("@rules_kotlin//kotlin:jvm.bzl", "kt_jvm_library")

kt_jvm_library(
    name = "example",
    srcs = glob(["**/*.kt", "**/*.kts"]),
    visibility = ["//visibility:public"],
)

Test Generation

Test sources in src/test/kotlin generate kt_jvm_test rules:

src/test/kotlin/com/example/GreeterTest.kt

package com.example

import org.junit.Test
import kotlin.test.assertEquals

class GreeterTest {
    @Test
    fun testHello() {
        val greeter = Greeter()
        assertEquals("Hello, World!", greeter.hello("World"))
    }
}

Generates:

src/test/kotlin/com/example/BUILD.bazel

load("@rules_kotlin//kotlin:jvm.bzl", "kt_jvm_test")

kt_jvm_test(
    name = "example_test",
    srcs = glob(["**/*.kt", "**/*.kts"]),
    deps = ["//src/main/kotlin/com/example"],
)

Parser Backends

The Kotlin extension supports multiple parsing strategies:

Backend	Description
heuristic	Fast regex-based parsing (default)
treesitter	Full AST parsing via tree-sitter
hybrid	Heuristic with tree-sitter fallback

# Use tree-sitter for more accurate parsing
# gazelle:kotlin_parser_backend treesitter

Dependencies

rules_kotlin Setup

Configure rules_kotlin in your MODULE.bazel:

MODULE.bazel

bazel_dep(name = "rules_kotlin", version = "2.2.2")

Limitations

Current limitations of the Kotlin extension:

• Maven-style layout required - Only src/main/kotlin and src/test/kotlin are recognized
• No kt_jvm_binary generation - Binaries must be manually configured
• No automatic Maven dependency resolution - External deps need manual configuration
• Multiplatform Kotlin (KMP) not supported - JVM only
• Kotlin/JS and Kotlin/Native not supported
• Annotation processors (kapt) require manual configuration