Kotlin autoformatter

This is an automated and opinionated tool for reformatting Kotlin source files. Source files are reformatted so that the code fits within a specified column limit (by default, 100 columns, as specified by the Google Kotlin style guide) and line breaks occur at reasonable points.

This tool is still work in progress, but can already handle a wide variety of cases.

Running the tool

The tool can be run as a standalone program, a simple Gradle plugin, or embedded in another program as a library.

When run on a file, the formatter replaces the existing file content. Use a revision control system such as Git to ensure that the original is not lost in case of bugs in the tool.

Standalone

The Gradle target installDist installs the formatter under build/install/kotlin-auto-formatter. To run it, use

./build/install/kotlin-auto-format/bin/kotlin-auto-format <file to be formatted>

As Gradle plugin

This formatter comes with a Gradle plugin adding the following tasks to your project:

• formatKotlinSource, which runs the formatter on all source files.
• formatKotlinScript, which runs the formatter on all Kotlin script files, e.g. build.gradle.kts.
• formatKotlin, which runs both formatKotlinSource and formatKotlinScript.

To use the plugin, add the following to your build.gradle.kts:

plugins {
    id("tech.formatter-kt.formatter") version "0.7.9"
}

Or, if you are using the Groovy DSL, in build.gradle:

plugins {
    id 'tech.formatter-kt.formatter' version '0.7.9'
}

The task formatKotlinSource depends on one of the Kotlin plugins having been previously applied in order to locate the project's Kotlin source files. If the task has the status NO-SOURCE despite the existence of Kotlin source files, check that the Kotlin plugin is applied before the Kotlin auto-formatter plugin.

As a library

It is possible to invoke the formatter directly from other Java/Kotlin code. See the class KotlinFormatter for documentation.

Configuration

Currently, it is only possible to configure the formatter when it is embedded as a library. See KotlinFormatter for details.

Preserving newlines

The formatter contains experimental support for preserving newlines in blocks of code when requested. The formatter normally aggressively removes syntactically unnecessary newlines when no line break is needed to maintain the 100 column limit. This can cause some DSLs to be rendered in a less readable manner. To instruct the formatter to preserve newlines, add the following comment:

// ktformat: start-preserve-newlines

To restore default behaviour, add the following comment:

// ktformat: end-preserve-newlines

Comparison with other tools

Kotlin autoformatter vs. ktlint

• Can autoformat to fit in a (configurable) column limit; ktlint complains about a violated column limit but cannot reformat to fit.
• Aggressively reformats ala Google Java formatter, whereas ktlint reformats only lines which violate linting rules.
• Does not have all of the rules of ktlint, e.g. replacing ${variable} with $variable in a string template.

For the most part, it should be fine to use both ktlint and this formatter in the same project. There is one point where this formatter's output is incompatible with the default settings in ktlint. Namely, ktlint will complain about the newline before the lambda expression in the following:

aFunctionCall(someArgument, someOtherArgument)
    { ...a lambda argument }

If the column limit is right after the closing right parenthesis on the first line, then the above is exactly how this tool will format the code. The recommended solution here, should you be using ktlint and run into this, is to disable the corresponding ktlint rule by adding the line

disabled_rules=curly-spacing

to your project's .editorconfig file.

Kotlin autoformatter vs. ktfmt

• Output of Kotlin autoformatter is compliant with Kotlin coding conventions; ktfmt output is not.
• Indentation amount and column limit are configurable in Kotlin autoformatter.
• Runs on JDK 8+; ktfmt requires JDK 11+.

Kotlin autoformatter vs. IntelliJ

• IntelliJ autoformatter can autoformat individual sections of a file, but doesn't always produce desirable output.
• The Kotlin autoformatter can only format full files (unless used as a library) but produces generally better output.

Upcoming work

• Creation of an IntelliJ plugin.
• Allow configuration via command line arguments and Gradle plugin.
• Configuration of the maximum line length from .editorconfig.

License

LGPL 3.0