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Kotlin Language Server

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A language server that provides smart code completion, diagnostics, hover, document symbols, definition lookup, method signature help and more for Kotlin.


Any editor conforming to LSP is supported, including VSCode and Atom.

Getting Started


Packaging status

This repository needs your help!

The original author created this project while he was considering using Kotlin in his work. He ended up deciding not to and is not really using Kotlin these days though this is a pretty fully-functional language server that just needs someone to use it every day for a while and iron out the last few pesky bugs.

There are two hard parts of implementing a language server:

  • Figuring out the dependencies
  • Incrementally re-compiling as the user types

The project uses the internal APIs of the Kotlin compiler.

Figuring out the dependencies

Dependencies are determined by the DefaultClassPathResolver.kt, which invokes Maven or Gradle to get a list of classpath JARs. Alternatively, projects can also 'manually' provide a list of dependencies through a shell script, located either at [project root]/kls-classpath or [config root]/kotlin-language-server/classpath, which outputs a list of JARs. Depending on your platform, the scripts also can be suffixed with .{sh,bat,cmd}.

  • Example of the ~/.config/kotlin-language-server/classpath on Linux:
echo /my/path/kotlin-compiler-1.4.10/lib/kotlin-stdlib.jar:/my/path/my-lib.jar
  • Example of the %HOMEPATH%\.config\kotlin-language-server\classpath.bat on Windows:
@echo off
echo C:\my\path\kotlin-compiler-1.4.10\lib\kotlin-stdlib.jar;C:\my\path\my-lib.jar

Incrementally re-compiling as the user types

I get incremental compilation at the file-level by keeping the same KotlinCoreEnvironment alive between compilations in Compiler.kt. There is a performance benchmark in OneFilePerformance.kt that verifies this works.

Getting incremental compilation at the expression level is a bit more complicated:

  • Fully compile a file and store in CompiledFile:
    • val content: String A snapshot of the source code
    • val parse: KtFile The parsed AST
    • val compile: BindingContext Additional information about the AST from typechecking
  • After the user edits the file:
    • Find the smallest section the encompasses all the user changes
    • Get the LexicalScope encompassing this region from the BindingContext that was generated by the full-compile
    • Create a fake, in-memory .kt file with just the expression we want to re-compile
      • Add space at the top of the file so the line numbers match up
    • Re-compile this tiny fake file

The incremental expression compilation logic is all in CompiledFile.kt. The Kotlin AST has a built-in repair API, which seems to be how IntelliJ works, but as far as I can tell this API does not work if the surrounding IntelliJ machinery is not present. Hence I created the "fake tiny file" incremental-compilation mechanism, which seems to be quite fast and predictable.

There is an extensive suite of behavioral tests, which are all implemented in terms of the language server protocol, so you should be able to refactor the code any way you like and the tests should still work.


Name Description
server The language server executable
shared Classpath resolution and utilities


Name Command Description python3 scripts/ Creates a tag for the current version and bumps the development version

Protocol Extensions

The Kotlin language server supports some non-standard requests through LSP. See KotlinProtocolExtensions for a description of the interface. The general syntax for these methods is kotlin/someCustomMethod.

Initialization Options

The Kotlin language server supports some custom initialization options via the initializationOptions property in the initialize request parameters. See InitializationOptions in Configuration for a list of supported properties.




Signature help

Signature Help



Go-to-definition, find all references

Find all references

Document symbols

Document symbols

Global symbols

Global symbols