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docs: add initial Kotlin docs (pantsbuild#16304)
Add an initial set of docs for the experimental Kotlin backend. [ci skip-rust] [ci skip-build-wheels]
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--- | ||
title: "Kotlin" | ||
slug: "kotlin" | ||
excerpt: "Kotlin support for Pants." | ||
hidden: false | ||
createdAt: "2022-07-26T02:02:59.962Z" | ||
updatedAt: "2022-07-26T02:02:59.962Z" | ||
--- | ||
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> 🚧 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](https://github.com/pantsbuild/pants/issues/new/choose) | ||
> or [joining our Slack](doc:community)! | ||
> 📘 Example Kotlin repository | ||
> | ||
> Check out [github.com/pantsbuild/example-kotlin](https://github.com/pantsbuild/example-kotlin) to try a | ||
> sample Pants project with Kotlin support. | ||
# Overview | ||
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[Kotlin](https://kotlinlang.org/) 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](https://kotlinlang.org/docs/server-overview.html). (The other Kotlin platforms including | ||
[Kotlin Multiplatform Mobile](https://kotlinlang.org/docs/multiplatform.html) and | ||
[Kotlin/JS](https://kotlinlang.org/docs/js-overview.html) are not currently supported, nor are there currently | ||
any plans to do so.) | ||
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# Initial Setup | ||
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First, activate the Kotlin backend in `pants.toml` plus the `ktlint` backend if you would like to use | ||
[`ktlint`](https://ktlint.github.io/) for code formatting and linting: | ||
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```toml pants.toml | ||
[GLOBAL] | ||
backend_packages = [ | ||
"pants.backend.experimental.kotlin", | ||
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# Activate the following backend if you want to use `ktlint` for code formatting and linting. | ||
"pants.backend.experimental.kotlin.lint.ktlint", | ||
] | ||
``` | ||
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## Choosing JDK and Kotlin versions | ||
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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. | ||
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### JDK | ||
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JDKs used by Pants are automatically fetched using [Coursier](https://get-coursier.io/), and are chosen using | ||
the [`[jvm].jdk` option](doc:reference-jvm#section-jdk) to set a repository-wide default. | ||
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To override the default on a particular target, you can use the [`jdk=` field](doc:reference-kotlin_source#codejdkcode). | ||
It can be useful to use the [`parametrize` builtin](doc:targets#parametrizing-targets) with the `jdk=` field, | ||
particularly to run test targets under multiple JDKs. | ||
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### Kotlin version | ||
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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](doc:reference-kotlin#section-version_for_resolve). 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](reference-jvm#section-default-resolve). | ||
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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: | ||
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- `org.jetbrains.kotlin:kotlin-stdlib` | ||
- `org.jetbrains.kotlin:kotlin-reflect` | ||
- `org.jetbrains.kotlin:kotlin-script-runtime` | ||
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To use multiple Kotlin versions in a repository, you would define multiple resolves, and then adjust | ||
the [`resolve` field](doc:reference-kotlin_junit_test#coderesolvecode) of any targets which should be used with the | ||
non-`default_resolve` resolve. | ||
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To cross-build a set of Kotlin targets for multiple Kotlin versions, you can use the | ||
[`parametrize` builtin](doc:targets#parametrizing-targets) with the `resolve=` field of the target and its dependencies. | ||
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> 🚧 `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.) | ||
## Dependencies | ||
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### First-party dependencies | ||
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In many cases, the dependencies of your first-party code are automatically inferred via | ||
[dependency inference](https://blog.pantsbuild.org/automatically-unlocking-concurrent-builds-and-fine-grained-caching-on-the-jvm-with-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](doc:targets). | ||
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### Third-party dependencies and lockfiles | ||
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Third-party dependencies (i.e. those from repositories like [Maven central](https://search.maven.org/)) are also | ||
automatically inferred via dependency inference, but must first be declared once per repository as | ||
[`jvm_artifact` targets](doc:reference-jvm_artifact): | ||
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```python BUILD | ||
jvm_artifact( | ||
group="com.google.guava", | ||
artifact="guava", | ||
version="31.0.1-jre", | ||
# See the callout below for more information on the `packages` argument. | ||
packages=["com.google.common.**"], | ||
) | ||
``` | ||
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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](doc:reference-jvm#section-resolves). | ||
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> 📘 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](doc:reference-jvm_artifact#codepackagescode) for more information. | ||
### `resource` targets | ||
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To have your code [load files as "resources"](https://docs.oracle.com/javase/8/docs/technotes/guides/lang/resources.html): | ||
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1. Add a `resource` or `resources` target with the relevant files in the `source` / `sources` field, respectively. | ||
2. Ensure that [an appropriate `source_root`](doc:source-roots) 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). | ||
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For example: | ||
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```toml pants.toml | ||
[source] | ||
# 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/*"] | ||
``` | ||
```python src/jvm/org/pantsbuild/example/lib/BUILD | ||
kotlin_sources(dependencies=[":hello"]) | ||
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resources(name="hello", sources=["hello.txt"]) | ||
``` | ||
```java src/jvm/org/pantsbuild/example/lib/Loader.java | ||
package org.pantsbuild.example.lib | ||
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import com.google.common.io.Resources | ||
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fun load() { | ||
... = Resources.getResource(Loader.class, "hello.txt") | ||
} | ||
``` | ||
```text src/jvm/org/pantsbuild/example/lib/hello.txt | ||
Hello world! | ||
``` | ||
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# Tasks | ||
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## Compile code | ||
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To manually check that sources compile, use `./pants check`: | ||
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``` | ||
# 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 :: | ||
``` | ||
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## Run tests | ||
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To run tests, use `./pants test`: | ||
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``` | ||
# 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 :: | ||
``` | ||
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The Kotlin backend currently supports JUnit tests specified using the `kotlin_junit_tests` target type. | ||
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## Lint and Format | ||
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[`ktlint`](https://ktlint.github.io/) can be enabled by adding the `pants.backend.experimental.kotlin.lint.ktlint` | ||
backend to `backend_packages` in the `[GLOBAL]` section of `pants.toml`. | ||
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Once enabled, `lint` and `fmt` will check and automatically reformat your code: | ||
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``` | ||
# 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 | ||
``` | ||
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# Caveats | ||
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The Kotlin backend is currently experimental since many features are not implemented including: | ||
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- 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](https://kotlinlang.org/docs/multiplatform.html) and | ||
[Kotlin/JS](https://kotlinlang.org/docs/js-overview.html) |