Kotlin Tooling Java To Kotlin

JetBrainsOfficialCreated by JetBrainsDistributed by JetBrains

Skill
Workflow
Software development
Apache-2.0
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From the author

Use when converting Java source files to idiomatic Kotlin, when user mentions "java to kotlin", "j2k", "convert java", "migrate java to kotlin", or when working with .java files that need to become .kt files. Handles framework-aware conversion for Spring, Lombok, Hibernate, Jackson, Micronaut, Quarkus, Dagger/Hilt, RxJava, JUnit, Guice, Retrofit, and Mockito.

— the description by JetBrains

AutoGineer summary

Written by AutoGineer from the skill’s own files, not by the author

This skill provides a disciplined, four-step methodology for migrating Java code to Kotlin. It includes framework-aware guidance for common JVM libraries such as Spring, Hibernate, Lombok, and others. The process emphasizes maintaining behavioral parity through a chain-of-thought approach, checking five specific invariants at each stage to ensure the resulting Kotlin code is idiomatic, safe, and maintainable.

What it does

Scans Java files to detect and load relevant framework-specific conversion guides.
Executes a four-step conversion process: 1:1 translation, nullability audit, collection conversion, and idiomatic transformation.
Validates code against five core invariants at every step to prevent regressions.
Provides instructions for preserving Git history during file renaming.
Offers a checklist for verifying compilation and behavioral consistency.
Handles common pitfalls like keyword conflicts, SAM ambiguity, and platform types.

When to use it

When migrating existing Java projects or individual files to Kotlin.
When you need to ensure converted code follows idiomatic Kotlin patterns.
When working with Java codebases that utilize frameworks like Spring, Hibernate, or Dagger.
When you need to maintain Git blame history while refactoring Java to Kotlin.

Advantages

Reduces manual effort by providing a systematic, repeatable conversion workflow.
Ensures framework-specific idioms are applied correctly through targeted reference guides.
Maintains code integrity by enforcing strict invariants throughout the transformation.
Minimizes common interop issues by addressing platform types and JVM annotations.

Usage examples

“Convert this Java file to idiomatic Kotlin.”: The agent will scan the file for frameworks, perform the four-step conversion, and output the resulting .kt file.
“Migrate my Java project to Kotlin.”: The agent will list all .java files, sort them by dependency order, and perform a batch conversion following the defined workflow.

Install

Download the skill as a ZIP:
https://autogineer.com/agent-skills/jetbrains/kotlin-tooling-java-to-kotlin/download

Unzip it into the cross-client skills folder of your project:

mkdir -p .agents/skills && unzip kotlin-tooling-java-to-kotlin.zip -d .agents/skills

Documentation by JetBrains

Java to Kotlin Conversion

Convert Java source files to idiomatic Kotlin using a disciplined 4-step conversion methodology with 5 invariants checked at each step. Supports framework-aware conversion that handles annotation site targets, library idioms, and API preservation.

Workflow

digraph j2k_workflow {
  rankdir=TB;
  "User specifies files" -> "Step 0: Scan & Detect";
  "Step 0: Scan & Detect" -> "Load framework guides";
  "Load framework guides" -> "Step 1: Convert";
  "Step 1: Convert" -> "Step 2: Write .kt";
  "Step 2: Write .kt" -> "Step 3: Git rename";
  "Step 3: Git rename" -> "Step 4: Verify";
  "Step 4: Verify" -> "Next file?" [label="pass"];
  "Step 4: Verify" -> "Fix issues" [label="fail"];
  "Fix issues" -> "Step 1: Convert";
  "Next file?" -> "Step 0: Scan & Detect" [label="batch: yes"];
  "Next file?" -> "Done" [label="no more files"];
}

Step 0: Scan & Detect Frameworks

Before converting, scan the Java file's import statements to detect which frameworks are in use. Load ONLY the matching framework reference files to keep context focused.

Framework Detection Table

Import prefix	Framework guide
`org.springframework.*`	SPRING.md
`lombok.*`	LOMBOK.md
`javax.persistence.`, `jakarta.persistence.`, `org.hibernate.*`	HIBERNATE.md
`com.fasterxml.jackson.*`	JACKSON.md
`io.micronaut.*`	MICRONAUT.md
`io.quarkus.`, `javax.enterprise.`, `jakarta.enterprise.*`	QUARKUS.md
`dagger.`, `dagger.hilt.`	DAGGER-HILT.md
`io.reactivex.`, `rx.`	RXJAVA.md
`org.junit.`, `org.testng.`	JUNIT.md
`com.google.inject.*`	GUICE.md
`retrofit2.`, `okhttp3.`	RETROFIT.md
`org.mockito.*`	MOCKITO.md

If javax.inject.* is detected, check for Dagger/Hilt vs Guice by looking for other imports from those frameworks. If ambiguous, load both guides.

Step 1: Convert

Apply the conversion methodology from CONVERSION-METHODOLOGY.md.

This is a 4-step chain-of-thought process:

Faithful 1:1 translation — exact semantics preserved
Nullability & mutability audit — val/var, nullable types
Collection type conversion — Java mutable → Kotlin types
Idiomatic transformations — properties, string templates, lambdas

Five invariants are checked after each step. If any invariant is violated, revert to the previous step and redo.

Apply any loaded framework-specific guidance during step 4 (idiomatic transformations).

Step 2: Write Output

Write the converted Kotlin code to a .kt file with the same name as the original Java file, in the same directory.

Step 3: Preserve Git History

To preserve git blame history, use a two-phase approach:

# Phase 1: Rename (creates rename tracking)
git mv src/main/java/com/example/Foo.java src/main/kotlin/com/example/Foo.kt
git commit -m "Rename Foo.java to Foo.kt"

# Phase 2: Replace content (tracked as modification, not new file)
# Write the converted Kotlin content to Foo.kt
git commit -m "Convert Foo from Java to Kotlin"

If the project keeps Java and Kotlin in the same source root (e.g., src/main/java/), rename in place:

git mv src/main/java/com/example/Foo.java src/main/java/com/example/Foo.kt

If the project does not use Git, simply write the .kt file and delete the .java file.

Step 4: Verify

After conversion, verify using checklist.md:

Attempt to compile the converted file
Run existing tests
Check annotation site targets
Confirm no behavioral changes

Batch Conversion

When converting multiple files (a directory or package):

List all .java files in the target scope
Sort by dependency order — convert leaf dependencies first (files that don't import other files in the conversion set), then work up to files that depend on them
Convert one file at a time — apply the full workflow (steps 0-4) for each
Track progress — report which files are done, which remain
Handle cross-references — after converting a file, update imports in other Java files if needed (e.g., if a class moved packages)

For large batches, consider converting in packages (bottom-up from leaf packages).

Common Pitfalls

See KNOWN-ISSUES.md for:

Kotlin keyword conflicts (when, in, is, object)
SAM conversion ambiguity
Platform types from Java interop
@JvmStatic / @JvmField / @JvmOverloads usage
Checked exceptions and @Throws
Wildcard generics → Kotlin variance

Files

ATTRIBUTION.mdAdded by AutoGineer1023 B
LICENSEAdded by AutoGineer10.0 KiB
SKILL.md5.6 KiB
assets/checklist.md2.5 KiB
references/CONVERSION-METHODOLOGY.md9.8 KiB
references/KNOWN-ISSUES.md9.5 KiB
references/frameworks/DAGGER-HILT.md4.3 KiB
references/frameworks/GUICE.md4.5 KiB
references/frameworks/HIBERNATE.md5.9 KiB
references/frameworks/JACKSON.md7.1 KiB
references/frameworks/JUNIT.md5.3 KiB
references/frameworks/LOMBOK.md6.3 KiB
references/frameworks/MICRONAUT.md3.3 KiB
references/frameworks/MOCKITO.md7.7 KiB
references/frameworks/QUARKUS.md3.8 KiB
references/frameworks/RETROFIT.md4.4 KiB
references/frameworks/RXJAVA.md5.6 KiB
references/frameworks/SPRING.md6.3 KiB

LICENSE

Apache License
Version 2.0, January 2004
http://www.apache.org/licenses/

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Requirements and compatibility

Works with: Claude Code, Claude apps, OpenAI Codex, GitHub Copilot, Cursor, Gemini CLI, Kiro, OpenCode, Goose, Amp, Roo Code, JetBrains Junie, Factory Droid, Devin (ex-Windsurf), Mistral Vibe, OpenHands, Letta

“Works with” means the tool reads the open Agent Skills format; a skill written for one tool may use features others lack.

Needs: Needs a command-line tool

Security

No issues found by our static scan· No issues found

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Attribution and licence

Created by

JetBrains

Publisher

JetBrains (Official)

Distributed by

JetBrains

Source

JetBrains/skills · kotlin-tooling-java-to-kotlin · commit 3f53782

Licence

Apache License 2.0 (redistribution allowed)

LICENSE (written by AutoGineer)
ATTRIBUTION.md (written by AutoGineer)

Synced

2026-11-09

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