MT-plugin – Mutation Testing Playground

Kotlin-based mutation tool that makes small code changes (rename params/locals, flip ifs, insert comments) in a target Java repo and checks whether it still builds.
Results are printed to the console and saved as JSON.
The tool can be used in two ways:

1. As a CLI tool (via Gradle run)
2. As an IntelliJ IDEA plugin (with a custom tool window)

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📂 Project Structure

MT-plugin/
├── .gradle/
├── .idea/
├── .run/
├── build/
├── fixtures/ # Example Java projects used for mutation testing
│ └── java-hello/
│ ├── .gradle/
│ ├── build/
│ └── src/main/java/demo/
│ ├── App.java # Main example file (can be simple or complex)
│ └── Target.java # Simple mutation target for testing
│ ├── build.gradle.kts
│ └── settings.gradle.kts
├── gradle/
├── src/main/kotlin/
│ │──mtspark/ # Core mutation testing implementation
│ │ ├── BuildRunner.kt # Handles build execution (Gradle commands)
│ │ ├── Diagnostics.kt # Writes logs and JSON output
│ │ ├── JavaCommentInserter # Mutation: insert comments
│ │ ├── JavaIfFlipMutator # Mutation: flip if-conditions
│ │ ├── JavaLocalVarRenamer # Mutation: rename local variables
│ │ ├── JavaParamRenamer # Mutation: rename method parameters
│ │ ├── KotlinLocalVarRenamer # Mutation for Kotlin code
│ │ ├── Main.kt # CLI entry point
│ │ ├── MetamorphicRenamer.kt # Example advanced mutation
│ │ ├── Mutation.kt # Mutation interface/definitions
│ │ ├── RenameHelpers # Shared rename helper functions
│ │ └── RepoManager # Locates and tracks files in the target repo
│ │├── org.example.mtplugin/ # IntelliJ plugin package
│ │├── runner/ # Runner for IDE integration
│ ││ └── SweAgentRunner
│ │└── ui/ # IntelliJ UI extensions
│ └── MtToolWindowFactory
├── .gitignore
├── build.gradle.kts
└── settings.gradle.kts

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🏗 Architecture

CLI Engine (mtspark/)

1. CLI Entry (Main.kt) – parses arguments (--repo, --operation, --pick-index, --limit, --json-out) and triggers a mutation run.
2. RepoManager – finds source files in the repo.
3. Mutators
   • JavaParamRenamer → renames method/constructor/lambda parameters
   • JavaLocalVarRenamer → renames locals inside blocks
   • JavaIfFlipMutator → flips if-conditions (semantic-preserving)
   • JavaCommentInserter → inserts harmless comments
4. BuildRunner – runs Gradle builds (before & after mutation).
5. Diagnostics – logs results to console and JSON (mutation details, build status, error messages).

IntelliJ Plugin (org.example.mtplugin/, runner/, ui/)

• Provides a UI tool window (MtToolWindowFactory) to launch mutations inside the IDE.
• SweAgentRunner integrates with the build runner and mutators.
• resources/META-INF contains plugin descriptors for IntelliJ.
• Defined in resources/META-INF/plugin.xml as an IntelliJ plugin descriptor.

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⚙️ Setup

Requirements:

• Java 17+
• Gradle (or just use the wrapper ./gradlew)

git clone https://github.com/leanerr/MT-plugin.git cd MT-plugin ./gradlew clean build

🚀 Running Mutations

Example 1: Rename a parameter

./gradlew run --args="--repo fixtures/java-hello
--language java
--operation rename-param
--pick-index 2
--limit 1
--json-out /tmp/mtspark.json"

Example 2: Rename a local variable

./gradlew run --args="--repo fixtures/java-hello
--language java
--operation rename-local
--limit 1
--json-out /tmp/mtspark.json"

Example 3: Flip an if-condition

./gradlew run --args="--repo fixtures/java-hello
--language java
--operation flip-if
--limit 1
--json-out /tmp/mtspark.json"

Example 4: Insert a comment

./gradlew run --args="--repo fixtures/java-hello
--language java
--operation insert-comment
--limit 1
--json-out /tmp/mtspark.json"

Continue even if build fails

./gradlew run --args="--repo fixtures/java-hello
--language java
--operation rename-param
--pick-index 2
--limit 1
--json-out /tmp/mtspark.json" || true

1) Build the plugin

./gradlew clean build

2) Run a simple mutation

Example: rename parameter at index 2 in App.java:

./gradlew run --args="--repo /IdeaProjects/MT-plugin/fixtures/java-hello \ --language java \ --operation rename-param \ --pick-index 2 \ --limit 1 \ --json-out /tmp/mtspark.json"

3) Continue even if build fails

./gradlew run --args="--repo /IdeaProjects/MT-plugin/fixtures/java-hello \ --language java \ --operation rename-param \ --pick-index 2 \ --limit 1 \ --json-out /tmp/mtspark.json"||true

🧩 IntelliJ Plugin Usage

MT-plugin is also packaged as an IntelliJ IDEA plugin. The IntelliJ plugin is a wrapper/host: it gives you a UI tool window in IDEA, and when you click “Run Mutation” it calls the same mutators and SweAgentRunner.

▶️ Run in IntelliJ

1. Open this project in IntelliJ IDEA (Community or Ultimate).

2. Use the Gradle task to launch a sandbox IDE with the plugin installed:

   ./gradlew runIde

✨ Features in the IDE • A new MT Tool Window appears (provided by MtToolWindowFactory). • From there, you can select a project and run mutations: • rename-param • rename-local • flip-if • insert-comment • Results are displayed inside the IDE, using SweAgentRunner + Diagnostics.

🛠 Edit / Extend the Plugin • Plugin code lives in: • org.example.mtplugin/ • runner/ • ui/ • Update resources/META-INF/plugin.xml if you add new actions or tool windows. • Build a distributable plugin zip with:

./gradlew buildPlugin

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## 📄 Example JSON Output (Build-breaking mutation)```json{"repo": "/IdeaProjects/MT-plugin/fixtures/java-hello","language": "java","operation": "rename-param","target_file": "src/main/java/demo/App.java","mutation": {"file": "src/main/java/demo/App.java","old": "x","new": "x_mt","changed": true },"baseline_build": {"success": true,"exitCode": 0 },"mutated_build": {"success": false,"exitCode": 1,"stderr_head": "App.java:51: error: cannot find symbol variable x_mt ..." }}

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