PIT mutation tuning

Why mutation testing

Coverage proves the code ran. Mutation proves the test would have caught a real defect. A mutant is a small change (negate a condition, return null, change > to <=); a "killed" mutant means a test failed; a "survived" mutant means your test suite is blind.

Default configuration

<plugin>
    <groupId>org.pitest</groupId>
    <artifactId>pitest-maven</artifactId>
    <version>1.17.0</version>
    <configuration>
        <targetClasses>
            <param>com.example.shop.*</param>
        </targetClasses>
        <targetTests>
            <param>com.example.shop.*</param>
        </targetTests>
        <mutators>
            <mutator>STRONGER</mutator>
        </mutators>
        <outputFormats>
            <param>HTML</param>
            <param>XML</param>
        </outputFormats>
        <timestampedReports>false</timestampedReports>
        <features>
            <feature>+GIT(from[HEAD~1])</feature>     <!-- only changed files -->
        </features>
        <mutationThreshold>80</mutationThreshold>
        <coverageThreshold>90</coverageThreshold>
    </configuration>
    <dependencies>
        <dependency>
            <groupId>org.pitest</groupId>
            <artifactId>pitest-junit5-plugin</artifactId>
            <version>1.2.1</version>
        </dependency>
    </dependencies>
</plugin>

Scope: incremental, not full

Full PIT runs are slow. Always scope to changed code:

• Locally / in /build refactor phase: +GIT(from[HEAD~1]).
• In CI for PRs: +GIT(from[origin/main]).
• Nightly: full run, results posted to dashboard but not blocking.

What to do with surviving mutants

For every survived mutant in changed packages:

1. Look at the mutated line and the mutation type (e.g. "negated conditional").
2. Write a test that would have failed under the mutation.
3. Re-run; the mutant should now be killed.
4. Log the new test in 06-test-plan.md.

If a mutant is genuinely equivalent (impossible to kill — e.g. a defensive if that can never be false), suppress it via @CoverageIgnore on the specific method, with a comment explaining why. Track in 08-code-review.md waivers.

Common mutation types worth caring about

Mutator	Example	What it tells you
NegateConditionals	if (x > 0) → if (x <= 0)	Boundary tests missing.
ReturnVals	return x; → return null;	Caller doesn't validate non-null.
MathMutator	a + b → a - b	Arithmetic untested with non-zero values.
VoidMethodCalls	removes a call	Side effect not asserted.

Reading mutations.xml

<mutation status="SURVIVED" detected="false">
  <sourceFile>PriceCalculator.java</sourceFile>
  <mutatedClass>com.example.shop.checkout.PriceCalculator</mutatedClass>
  <mutatedMethod>apply</mutatedMethod>
  <mutator>org.pitest.mutationtest.engine.gregor.mutators.NegateConditionalsMutator</mutator>
  <description>negated conditional</description>
  <lineNumber>42</lineNumber>
</mutation>

spring-validator parses this and lists every SURVIVED mutant in changed files in 07-validation-report.md.

Anti-patterns

• Lowering mutationThreshold to make the build green.
• Excluding entire packages because they're "hard to test".
• Marking equivalent mutants without a one-line justification.
• Running PIT against the whole codebase on every build.