Programming language: Kotlin
Tags: Editors    
Latest version: v0.5.2

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Kotlin Language Server

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

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

Getting Started

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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.

Dependencies are determined by the [findClassPath](server/src/main/kotlin/org/javacs/kt/classpath/findClassPath.kt) function, which invokes Maven or Gradle and tells it to output a list of dependencies. Currently, both Maven and Gradle projects are supported.

I get incremental compilation at the file-level by keeping the same KotlinCoreEnvironment alive between compilations in [Compiler.kt](server/src/main/kotlin/org/javacs/kt/Compiler.kt). There is a performance benchmark in [OneFilePerformance.kt](server/src/test/kotlin/org/javacs/kt/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](server/src/main/kotlin/org/javacs/kt/CompiledFile.kt):
    • 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](server/src/main/kotlin/org/javacs/kt/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](server/src/test/kotlin/org/javacs/kt), 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
update_kt_version.py python3 scripts/update_kt_version.py Finds and updates the Kotlin compiler/plugin version for this project
bump_version.py python3 scripts/bump_version.py Increments the project version and creates a new tag

Protocol Extensions

The Kotlin language server supports some non-standard requests through LSP. See [KotlinProtocolExtensions](server/src/main/kotlin/org/javacs/kt/KotlinProtocolExtensions.kt) for a description of the interface. The general syntax for these methods is kotlin/someCustomMethod.




Signature help

[Signature Help](images/SignatureHelp.png)



Go-to-definition, find all references

[Find all references](images/FindAllReferences.png)

Document symbols

[Document symbols](images/DocumentSymbols.png)

Global symbols

[Global symbols](images/GlobalSymbols.png)