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
• When asked to "improve test quality with mutation testing"

Skill Contents

Sections

• When to use this skill
• Quick Start
• Key Concepts
• References
• Related Rules
• Related Skills

Available Resources

📚 references/ - Detailed documentation

• configuration
• improving tests
• intensity levels
• troubleshooting

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Quick Start

1. Check readiness

bash 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

# 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

open 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

Level	Mutators	Use Case	Time
QUICK	DEFAULTS (~11)	Initial analysis, CI	1x
STANDARD	STRONGER (~14)	Regular improvement	1.5x
COMPREHENSIVE	ALL (~30+)	Critical logic	3-5x

Mutation Statuses

Status	Meaning	Action
Killed	Test detected mutation	Good
Survived	Test missed mutation	Improve test
No Coverage	No test covers code	Write test
Timed Out	Infinite loop	Usually OK

References

Reference	Description
references/intensity-levels.md	Detailed mutator descriptions
references/improving-tests.md	Patterns for killing mutants
references/configuration.md	PIT configuration options
references/troubleshooting.md	Common issues and solutions

Related Rules

• java-testing-guidelines - Spock/JUnit patterns
• java-jacoco-coverage - Line coverage configuration

Related Skills

Skill	Purpose
java-testing	Test configuration
java-coverage	JaCoCo coverage