Skip to main content
Version: 2.20


Kotlin support for Pants.

Kotlin support is alpha stage

Kotlin support in Pants is still under active development, but currently supports compilation and testing. It has been tested with Kotlin v1.6.20.

Please share feedback for what you need to use Pants with your Kotlin project by either opening a GitHub issue or joining our Slack!

Example Kotlin repository

Check out to try a sample Pants project with Kotlin support.


Kotlin is a programming language from Jetbrains that runs on the JVM and certain other platforms. The Kotlin backend in Pants supports compilation, testing, and linting of Kotlin code for the JVM. (The other Kotlin platforms including Kotlin Multiplatform Mobile and Kotlin/JS are not currently supported, nor are there currently any plans to do so.)

Initial Setup

First, activate the Kotlin backend in pants.toml plus the ktlint backend if you would like to use ktlint for code formatting and linting:

backend_packages = [

# Activate the following backend if you want to use `ktlint` for code formatting and linting.

Setting up targets

Run pants tailor :: to generate BUILD files. This will create kotlin_sources targets in every directory containing library code, as well as kotlin_junit_tests targets for filenames that look like tests.

❯ pants tailor ::
Created src/jvm/org/pantsbuild/example/app/BUILD:
- Add kotlin_sources target app
Created src/jvm/org/pantsbuild/example/json/BUILD:
- Add kotlin_sources target json
Created src/jvm/org/pantsbuild/example/lib/BUILD:
- Add java_sources target lib

You can run pants list :: to see all targets in your project:

❯ pants list ::

Choosing JDK and Kotlin versions

Pants supports choosing the JDK and Kotlin versions per target in your repository. To reduce the amount of boilerplate required, however, most users set repository-wide defaults in pants.toml, and then only override them when necessary for particular targets.


JDKs used by Pants are automatically fetched using Coursier, and are chosen using the [jvm].jdk option to set a repository-wide default.

To override the default on a particular target, you can use the jdk= field. It can be useful to use the parametrize builtin with the jdk= field, particularly to run test targets under multiple JDKs.

Kotlin version

The Kotlin version to use is configured on a resolve-by-resolve basis (see the "Third-party dependencies" section below) using the [kotlin].version_for_resolve option. The default Kotlin version for your repository will thus be whichever Kotlin version is configured for the "default" resolve, which is configured by the [jvm].default_resolve option.

Each resolve must contain the following jars for the Kotlin runtime with the version matching the version specified for the resolve in the [kotlin].version_for_resolve option:

  • org.jetbrains.kotlin:kotlin-stdlib
  • org.jetbrains.kotlin:kotlin-reflect
  • org.jetbrains.kotlin:kotlin-script-runtime

To use multiple Kotlin versions in a repository, you would define multiple resolves, and then adjust the resolve field of any targets which should be used with the non-default_resolve resolve.

To cross-build a set of Kotlin targets for multiple Kotlin versions, you can use the parametrize builtin with the resolve= field of the target and its dependencies.

jvm_artifact targets for the Kotlin runtime must be explicitly defined.

The Kotlin backend currently requires that a jvm_artifact target for each Kotlin runtime jars be present in any resolve used for Kotlin. If any of the required jvm_artifact targets are missing, Pants will error. Pants will automatically inject a dependency on the runtime into Kotlin targets. (These targets may be automatically supplied by Pants in a future version, but that is not currently implemented.)


First-party dependencies

In many cases, the dependencies of your first-party code are automatically inferred via dependency inference based on import statements in the code. If you do need to declare additional dependencies for any reason, you can do so using Pants' syntax for declaring dependencies for targets.

Third-party dependencies and lockfiles

Third-party dependencies (i.e. those from repositories like Maven central) are also automatically inferred via dependency inference, but must first be declared once per repository as jvm_artifact targets:

# See the callout below for more information on the `packages` argument.

Pants requires use of a lockfile for third-party dependencies. After adding or editing jvm_artifact targets, you will need to update affected lockfiles by running pants generate-lockfiles. The default lockfile is located at 3rdparty/jvm/default.lock, but it can be relocated (as well as additional resolves declared) via the [jvm].resolves option.

Thirdparty symbols and the packages argument

To efficiently determine which symbols are provided by third-party code (i.e., without hitting the network in order to compute dependencies in the common case), Pants relies on a static mapping of which artifacts provide which symbols, and defaults to treating each jvm_artifact as providing symbols within its group.

The packages argument allows you to override which symbols a jvm_artifact provides. See the jvm_artifact docs for more information.

resource targets

To have your code load files as "resources":

  1. Add a resource or resources target with the relevant files in the source / sources field, respectively.
  2. Ensure that an appropriate source_root is detected for the resources target, in order to trim the relevant prefix from the filename to align with the layout of your JVM packages.
  3. Add that target to the dependencies field of the relevant JVM target (usually the one that uses the JVM APIs to load the resource).

For example:

# In order for the resource to be loadable as `org/pantsbuild/example/lib/hello.txt`,
# the `/src/jvm/ prefix needs to be stripped.
root_patterns = ["/src/*"]


Compile code

To manually check that sources compile, use pants check:

# Check a single file
❯ pants check src/jvm/org/pantsbuild/example/lib/ExampleLib.kt

# Check files located recursively under a directory
❯ pants check src/jvm::

# Check the whole repository
❯ pants check ::

Run tests

To run tests, use pants test:

# Run a single test file
❯ pants test tests/jvm/org/pantsbuild/example/lib/ExampleLibTest.kt

# Test all files in and under a directory
❯ pants test tests/jvm::

# Test the whole repository
❯ pants test ::

The Kotlin backend currently supports JUnit tests specified using the kotlin_junit_tests target type.

Setting environment variables

Test runs are hermetic, meaning that they are stripped of the parent pants process's environment variables. This is important for reproducibility, and it also increases cache hits.

To add any arbitrary environment variable back to the process, you can either add the environment variable to the specific tests with the extra_env_vars field on kotlin_junit_test / kotlin_junit_tests targets or to all your tests with the [test].extra_env_vars option. Generally, prefer the field extra_env_vars field so that more of your tests are hermetic.

With both [test].extra_env_vars and the extra_env_vars field, you can either hardcode a value or leave off a value to "allowlist" it and read from the parent pants process's environment.

extra_env_vars = ["VAR1", "VAR2=hardcoded_value"]

Lint and Format

ktlint can be enabled by adding the pants.backend.experimental.kotlin.lint.ktlint backend to backend_packages in the [GLOBAL] section of pants.toml.

Once enabled, lint and fmt will check and automatically reformat your code:

# Format this directory and all subdirectories
❯ pants fmt src/jvm::

# Check that the whole project is formatted
❯ pants lint ::

# Format all changed files
❯ pants --changed-since=HEAD fmt


The Kotlin backend is currently experimental since many features are not implemented including:

  • Kotlin modules. We would love to hear from Kotlin developers for advice on how modules are used and could be potentially supported by Pants.
  • Non-JVM backends including Kotlin Multiplatform Mobile and Kotlin/JS