Welcome to the world of Kotlin and Bazel! If you’re looking to integrate Kotlin into your Bazel projects seamlessly, you’ve come to the right place. In this article, we’ll walk you through the essential Kotlin rules for Bazel, making the complex simple and digestible. Buckle up as we dive into the details!

Getting Started with Kotlin Rules in Bazel

Before we jump into the rules, let’s understand what Bazel and Kotlin are. Bazel is a build tool from Google designed for high-performance builds suitable for large-scale projects, while Kotlin is a modern programming language that runs on the Java Virtual Machine (JVM) and is also a great fit for Android development.

Essential Rules Overview

Here’s a quick snapshot of the Kotlin rules available in Bazel:

• kotlin_repositories: Loads the necessary workspace dependencies.
• kotlin_library: Builds a Java library from Kotlin source files.
• kotlin_binary: Builds a Java binary from Kotlin source files.
• kotlin_android_library: Builds an Android library from Kotlin source files.
• kotlin_test: Executes Kotlin tests.

Setting Up Your Workspace

To get things going, you need to set up your workspace for Bazel. Here’s what needs to be added to your WORKSPACE file:

git_repository(
    name = org_pubref_rules_kotlin,
    remote = https://github.com/pubref/rules_kotlin.git,
    tag = v0.5.0, # update as needed
)
load(@org_pubref_rules_kotlin//kotlin:rules.bzl, kotlin_repositories)
kotlin_repositories()

Understanding BUILD Rules

Next up, let’s explore how to define BUILD rules in your BUILD file. Imagine your BUILD file is a recipe where each rule is an ingredient that helps you create your software dish.

Defining a Kotlin Library

Here’s an example of how to define a Kotlin library:

load(@org_pubref_rules_kotlin//kotlin:rules.bzl, kotlin_library)

kotlin_library(
    name = my_kotlin_lib,
    srcs = [kotlin_source_file.kt],
    deps = [:some_other_kotlin_library_rule],
    java_deps = [:some_other_java_library_rule, @another_maven_jar]
)

Think of the `kotlin_library` as the foundation of your house. You’ll need it to build your other structures (e.g., an application or an Android library).

Creating a Kotlin Binary

To build a Kotlin binary, which is essentially an executable, you will structure it like so:

kotlin_binary(
    name = main_kt,
    main_class = my.project.MainKt,
    srcs = [main.kt],
    deps = [:my_kotlin_lib],
    java_deps = [:javalib]
)

Think of the `kotlin_binary` as the grand unveiling of your finished cake. It’s the final product that everyone gets to enjoy!

Building an Android Library

For Android development, use the `kotlin_android_library`:

kotlin_android_library(
    name = src,
    srcs = glob([src/**/*.kt]),
    custom_package = PACKAGE,
    manifest = MANIFEST,
    resource_files = glob([res/**]),
    java_deps = [
        @com_squareup_okhttp3_okhttp_jar,
        @com_squareup_okio_okio_jar,
    ],
    aar_deps = [
        @androidsdk//com.android.support:appcompat-v7:25.3.1,
    ],
)

This rule is like combining various layers of flavors in a cake—each layer (Kotlin source files, resources) contributes to the overall deliciousness of the library.

Running Tests

It is crucial to ensure that your code works as expected. Use the following for testing:

kotlin_test(
    name = main_kt_test,
    test_class = examples.helloworld.MainKtTest,
    srcs = [MainKtTest.kt],
    size = small,
    deps = [:rules],
    java_deps = [@junit4_jar],
)

Testing can be likened to tasting your cake before serving; it’s essential to catch any flaws!

Examples and Execution

To put this all into practice, clone the repository:

git clone https://github.com/pubref/rules_kotlin
cd rules_kotlin
bazel query ... --output label_kind
bazel run examples/helloworld:main_kt
bazel test examples/helloworld:main_test

Troubleshooting Common Issues

While setting up Kotlin rules for Bazel, you might run into some hiccups. Here are some troubleshooting ideas:

• Ensure you are using Bazel version 0.7.0 or higher with rules_kotlin 0.5.0.
• Verify your WORKSPACE and BUILD files for common syntax errors.
• Double-check your dependencies to ensure they are declared correctly.

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Conclusion

And there you have it! You are now equipped with the know-how to integrate Kotlin rules into your Bazel projects. The provided rules and commands should foster a fruitful development environment where Kotlin and Bazel can thrive together.

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