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name: mutation-testing description: > Use PIT (Pitest) mutation testing to identify weak tests and improve test quality in Java projects. Finds tests that pass but don't actually verify behavior correctly. Use when improving test suite quality beyond line coverage metrics. compatibility: Java projects using Gradle 8.x+ with JUnit 5 or Spock metadata: version: "1.0.0" technology: java category: testing tags: - java - gradle - testing - mutation-testing - pitest - quality

Mutation Testing

Use PIT mutation testing to identify tests that pass but don't verify behavior correctly.

When to use this skill

• Improving test quality beyond line coverage metrics
• Finding weak test assertions that don't catch bugs
• Validating critical business logic has strong tests
• Setting up mutation testing in CI pipelines
• Analyzing which tests need strengthening

Skill Contents

Sections

• When to use this skill (L25-L32)
• Quick Start (L54-L90)
• Key Concepts (L91-L116)
• References (L117-L125)
• Related Rules (L126-L130)
• Related Skills (L131-L136)

Available Resources

📚 references/ - Detailed documentation

• configuration
• improving tests
• intensity levels
• troubleshooting

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Quick Start

1. Check readiness

Copybash java/scripts/check-pitest-readiness.sh

2. Apply PIT configuration

Copy java/templates/pitest.gradle to gradle/pitest.gradle and apply in build.gradle.

3. Run mutation testing

Copy# QUICK level - fast feedback
./gradlew pitest -Ppitest.mutators=DEFAULTS

# STANDARD level - recommended
./gradlew pitest -Ppitest.mutators=STRONGER

# COMPREHENSIVE level - thorough
./gradlew pitest -Ppitest.mutators=ALL

4. Analyze results

Copyopen build/reports/pitest/index.html

Focus on SURVIVED mutants - these indicate weak tests.

5. Improve tests

Add specific assertions that kill the surviving mutants.

Key Concepts

Mutation Score vs Line Coverage

• Line coverage: Does your test execute the code?
• Mutation score: Does your test actually verify the code works correctly?

High line coverage with low mutation score = tests run code but don't verify results.

Intensity Levels

Mutation Statuses

References

Related Rules

• .cursor/rules/java-testing-guidelines.mdc - Spock/JUnit patterns
• .cursor/rules/java-jacoco-coverage.mdc - Line coverage configuration

Related Skills