Bazel Kotlin Rules

Releases

For more information about release and changelogs please see Changelog or refer to the Github Releases page.

rules_kotlin Bazel Compatibility

• HEAD: 5, 6
• Release 1.7: 4, 5, 6

Overview

rules_kotlin supports the basic paradigm of *_binary, *_library, *_test of other Bazel language rules. It also supports jvm, android, and js flavors, with the prefix kt_jvm and kt_js, and kt_android typically applied to the rules.

Support for kotlin's -Xfriend-paths via the associates= attribute in the jvm allow access to internal members.

Also, kt_jvm_* rules support the following standard java_* rules attributes:

• data
• resource_jars
• runtime_deps
• resources
• resources_strip_prefix
• exports

Android rules also support custom_package for R.java generation, manifest=, resource_files, etc.

Other features:

• Persistent worker support.
• Mixed-Mode compilation (compile Java and Kotlin in one pass).
• Configurable Kotlinc distribution and version
• Configurable Toolchain
• Support for all recent major Kotlin releases.

Javascript is reported to work, but is not as well maintained (at present)

Documentation

Generated API documentation is available at https://bazelbuild.github.io/rules_kotlin/kotlin.

Quick Guide

WORKSPACE

In the project's WORKSPACE, declare the external repository and initialize the toolchains, like this:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

rules_kotlin_version = "1.7.1"
rules_kotlin_sha = "fd92a98bd8a8f0e1cdcb490b93f5acef1f1727ed992571232d33de42395ca9b3"
http_archive(
    name = "io_bazel_rules_kotlin",
    urls = ["https://github.com/bazelbuild/rules_kotlin/releases/download/v%s/rules_kotlin_release.tgz" % rules_kotlin_version],
    sha256 = rules_kotlin_sha,
)

load("@io_bazel_rules_kotlin//kotlin:repositories.bzl", "kotlin_repositories")
kotlin_repositories() # if you want the default. Otherwise see custom kotlinc distribution below

load("@io_bazel_rules_kotlin//kotlin:core.bzl", "kt_register_toolchains")
kt_register_toolchains() # to use the default toolchain, otherwise see toolchains below

BUILD files

In your project's BUILD files, load the Kotlin rules and use them like so:

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

kt_jvm_library(
    name = "package_name",
    srcs = glob(["*.kt"]),
    deps = [
        "//path/to/dependency",
    ],
)

Custom toolchain

To enable a custom toolchain (to configure language level, etc.) do the following. In a <workspace>/BUILD.bazel file define the following:

load("@io_bazel_rules_kotlin//kotlin:core.bzl", "define_kt_toolchain")

define_kt_toolchain(
    name = "kotlin_toolchain",
    api_version = KOTLIN_LANGUAGE_LEVEL,  # "1.1", "1.2", "1.3", "1.4", "1.5" "1.6", or "1.7"
    jvm_target = JAVA_LANGUAGE_LEVEL, # "1.6", "1.8", "9", "10", "11", "12", "13", "15", "16", or "17"
    language_version = KOTLIN_LANGUAGE_LEVEL,  # "1.1", "1.2", "1.3", "1.4", "1.5" "1.6", or "1.7"
)

and then in your WORKSPACE file, instead of kt_register_toolchains() do

register_toolchains("//:kotlin_toolchain")

Custom kotlinc distribution (and version)

To choose a different kotlinc distribution (1.3 and 1.4 variants supported), do the following in your WORKSPACE file (or import from a .bzl file:

load("@io_bazel_rules_kotlin//kotlin:repositories.bzl", "kotlin_repositories", "kotlinc_version")

kotlin_repositories(
    compiler_release = kotlinc_version(
        release = "1.6.21", # just the numeric version
        sha256 = "632166fed89f3f430482f5aa07f2e20b923b72ef688c8f5a7df3aa1502c6d8ba"
    )
)

Third party dependencies

(e.g. Maven artifacts)

Third party (external) artifacts can be brought in with systems such as rules_jvm_external or bazel_maven_repository or bazel-deps, but make sure the version you use doesn't naively use java_import, as this will cause bazel to make an interface-only (ijar), or ABI jar, and the native ijar tool does not know about kotlin metadata with respect to inlined functions, and will remove method bodies inappropriately. Recent versions of rules_jvm_external and bazel_maven_repository are known to work with Kotlin.

Development Setup Guide

As of 1.5.0, to use the rules directly from the rules_kotlin workspace (i.e. not the release artifact) require the use of release_archive repository. This repository will build and configure the current workspace to use rules_kotlin in the same manner as the released binary artifact.

In the project's WORKSPACE, change the setup:

# Use local check-out of repo rules (or a commit-archive from github via http_archive or git_repository)
local_repository(
    name = "io_bazel_rules_kotlin",
    path = "../path/to/rules_kotlin_clone/",
)

load("@io_bazel_rules_kotlin//kotlin:dependencies.bzl", "kt_download_local_dev_dependencies")

kt_download_local_dev_dependencies()

load("@io_bazel_rules_kotlin//kotlin:repositories.bzl", "kotlin_repositories", "versions")

kotlin_repositories()

load("@io_bazel_rules_kotlin//kotlin:core.bzl", "kt_register_toolchains")

kt_register_toolchains()

To use rules_kotlin from head without cloning the repository, (caveat emptor, of course), change the rules to this:

# Download master or specific revisions
http_archive(
    name = "io_bazel_rules_kotlin",
    strip_prefix = "rules_kotlin-master",
    urls = ["https://github.com/bazelbuild/rules_kotlin/archive/refs/heads/master.zip"],
)

load("@io_bazel_rules_kotlin//kotlin:repositories.bzl", "kotlin_repositories")

kotlin_repositories()

load("@io_bazel_rules_kotlin//kotlin:core.bzl", "kt_register_toolchains")

kt_register_toolchains()

Debugging native actions

To attach debugger and step through native action code when using local checkout of rules_kotlin repo :

1. Open rules_kotlin project in Android Studio, using existing .bazelproject file as project view.
2. In terminal, build the kotlin target you want to debug, using the subcommand option, ex: bazel build //lib/mylib:main_kt -s. You can also use bazel aquery to get this info.
3. Locate the subcommand for the kotlin action you want to debug, let's say KotlinCompile. Note: If you don't see it, target rebuild may have been skipped (in this case touch one of the source .kt file to trigger rebuild).
4. Export REPOSITORY_NAME as specified in action env, ex : export REPOSITORY_NAME=io_bazel_rules_kotlin
5. Copy the command line, ex : bazel-out/darwin_arm64-opt-exec-2B5CBBC6/bin/external/io_bazel_rules_kotlin/src/main/kotlin/build '--flagfile=bazel-out/darwin_arm64-fastbuild/bin/lib/mylib/main_kt-kt.jar-0.params'
6. Change directory into the execRoot, normally bazel-MYPROJECT, available via bazel info | grep execution_root.
7. Add --debug=5005 to command line to make the action wait for a debugger to attach, ex: bazel-out/darwin_arm64-opt-exec-2B5CBBC6/bin/external/io_bazel_rules_kotlin/src/main/kotlin/build --debug=5005 '--flagfile=bazel-out/darwin_arm64-fastbuild/bin/lib/mylib/main_kt-kt.jar-0.params'. Note: if command invokes java toolchain directly, use -agentlib:jdwp=transport=dt_socket,server=y,suspend=y,address=5005 instead.
8. You should see in output that action is waiting for debugger. Use a default Remote JVM Debug configuration in Android Studio, set breakpoint in kotlin action java/kt code, and attach debugger. Breakpoints should be hit.

Kotlin and Java compiler flags

The kt_kotlinc_options and kt_javac_options rules allows passing compiler flags to kotlinc and javac.

Note: Not all compiler flags are supported in all language versions. When this happens, the rules will fail.

For example you can define global compiler flags by doing:

load("@io_bazel_rules_kotlin//kotlin:core.bzl", "kt_kotlinc_options", "kt_javac_options", "define_kt_toolchain")

kt_kotlinc_options(
    name = "kt_kotlinc_options",
    kotlinc_opts = ["-Xno-param-assertions"],
    jvm_target = "1.8",
)

kt_javac_options(
    name = "kt_javac_options",
    javac_opts = ["-nowarn"],
)

define_kt_toolchain(
    name = "kotlin_toolchain",
    kotlinc_options = "//:kt_kotlinc_options",
    javac_options = "//:kt_javac_options",
)

You can optionally override compiler flags at the target level by providing an alternative set of kt_kotlinc_options or kt_javac_options in your target definitions.

Compiler flags that are passed to the rule definitions will be taken over the toolchain definition.

Example:

load("@io_bazel_rules_kotlin//kotlin:core.bzl", "kt_kotlinc_options", "kt_javac_options")
load("@io_bazel_rules_kotlin//kotlin:jvm.bzl", "kt_jvm_library")

kt_kotlinc_options(
    name = "kt_kotlinc_options_for_package_name",
    kotlinc_opts = [
     "-Xno-param-assertions",
     "-Xopt-in=kotlin.Experimental",
     "-Xopt-in=kotlin.ExperimentalStdlibApi",
    ],
)

kt_javac_options(
    name = "kt_javac_options_for_package_name",
    javac_opts = ["-nowarn"],
)

kt_jvm_library(
    name = "package_name",
    srcs = glob(["*.kt"]),
    kotlinc_opts = "//:kt_kotlinc_options_for_package_name",
    javac_opts = "//:kt_javac_options_for_package_name",
    deps = ["//path/to/dependency"],
)

Additionally, you can add options for both tracing and timing of the bazel build using the kt_trace and kt_timings flags, for example:

• bazel build --define=kt_trace=1
• bazel build --define=kt_timings=1

kt_trace=1 will allow you to inspect the full kotlinc commandline invocation, while kt_timings=1 will report the high level time taken for each step.

KSP (Kotlin Symbol Processing)

KSP is officially supported as of rules_kotlin 1.8 and can be declared using the new kt_ksp_plugin rule.

A few things to note:

• As of now rules_kotlin only supports KSP plugins that generate Kotlin code.
• KSP is not yet thread safe and will likely fail if you are using it in a build that has multiplex workers enabled. To work around this add the following flag to your Bazelrc:

  build --experimental_worker_max_multiplex_instances=KotlinKsp=1
  

load("@io_bazel_rules_kotlin//kotlin:core.bzl", "kt_ksp_plugin")
load("@io_bazel_rules_kotlin//kotlin:jvm.bzl", "kt_jvm_library")

kt_ksp_plugin(
    name = "moshi-kotlin-codegen",
    processor_class = "com.squareup.moshi.kotlin.codegen.ksp.JsonClassSymbolProcessorProvider",
    deps = [
        "@maven//:com_squareup_moshi_moshi",
        "@maven//:com_squareup_moshi_moshi_kotlin",
        "@maven//:com_squareup_moshi_moshi_kotlin_codegen",
    ],
)

kt_jvm_library(
    name = "lib",
    srcs = glob(["*.kt"]),
    plugins = ["//:moshi-kotlin-codegen"],
)

Kotlin compiler plugins

The kt_compiler_plugin rule allows running Kotlin compiler plugins, such as no-arg, sam-with-receiver and allopen.

For example, you can add allopen to your project like this:

load("@io_bazel_rules_kotlin//kotlin:core.bzl", "kt_compiler_plugin")
load("@io_bazel_rules_kotlin//kotlin:jvm.bzl", "kt_jvm_library")

kt_compiler_plugin(
    name = "open_for_testing_plugin",
    id = "org.jetbrains.kotlin.allopen",
    options = {
        "annotation": "plugin.allopen.OpenForTesting",
    },
    deps = [
        "@com_github_jetbrains_kotlin//:allopen-compiler-plugin",
    ],
)

kt_jvm_library(
    name = "user",
    srcs = ["User.kt"], # The User class is annotated with OpenForTesting
    plugins = [
        ":open_for_testing_plugin",
    ],
    deps = [
        ":open_for_testing", # This contains the annotation (plugin.allopen.OpenForTesting)
    ],
)

Full examples of using compiler plugins can be found here.

Examples

Examples can be found in the examples directory, including usage with Android, Dagger, Node-JS, Kotlin compiler plugins, etc.

History

These rules were initially forked from pubref/rules_kotlin, and then re-forked from bazelbuild/rules_kotlin. They were merged back into this repository in October, 2019.

License

This project is licensed under the Apache 2.0 license, as are all contributions

Contributing

See the CONTRIBUTING doc for information about how to contribute to this project.