Legacy Projects¶

Incrementally improve code coverage in legacy codebases without requiring full project refactoring.

The Problem¶

Legacy projects often have low test coverage (10-30%). Requiring 90% coverage immediately is impractical and demoralizing.

The Solution¶

Delta Coverage enforces standards only on new and modified code. Legacy code remains untouched until you change it.

Strategy¶

Set a reasonable threshold for new code (80-90%)
Don't touch old code — coverage improves naturally as you modify it
Track progress with full coverage reports

Configuration¶

configure<io.github.surpsg.deltacoverage.gradle.DeltaCoverageConfiguration> {
    diffSource.git.compareWith("refs/remotes/origin/main")

    reports {
        html.set(true)
        fullCoverageReport.set(true)  // Track overall progress
    }

    reportViews {
        val test by getting {
            violationRules.failIfCoverageLessThan(0.8)  // Start at 80%
        }
    }
}

Phased Approach¶

Phase 1: Establish Baseline (Week 1)¶

Start with a lower threshold:

violationRules.failIfCoverageLessThan(0.6)

Phase 2: Increase Gradually (Month 1-3)¶

Raise the threshold as the team adapts:

violationRules.failIfCoverageLessThan(0.7)

Phase 3: Target State (Month 3+)¶

Reach your target:

violationRules.failIfCoverageLessThan(0.9)

Handling Large Legacy Files¶

When modifying a large legacy file, you might trigger coverage requirements for the entire file. Options:

Option 1: entityCountThreshold¶

Ignore violations for small changes:

violationRules {
    rule(CoverageEntity.LINE) {
        minCoverageRatio.set(0.8)
        entityCountThreshold.set(20)  // Only enforce if 20+ lines changed
    }
}

Option 2: Exclude Specific Files¶

Temporarily exclude problematic files:

excludeClasses.value(
    listOf("**/LegacyMonolith.class")
)

Option 3: Lower View-Specific Threshold¶

Create a separate view for legacy modules:

reportViews {
    view("legacyModule") {
        includeClasses.value(listOf("**/legacy/**/*.class"))
        violationRules.failIfCoverageLessThan(0.5)
    }

    val test by getting {
        excludeClasses.value(listOf("**/legacy/**/*.class"))
        violationRules.failIfCoverageLessThan(0.9)
    }
}

Tracking Progress¶

Enable full coverage reports to see overall project coverage:

reports {
    fullCoverageReport.set(true)
}

This generates reports in: - delta-coverage/ — your changes only - full-coverage-report/ — entire project

Track the full coverage percentage over time to see improvement.

Tips¶

Don't Boil the Ocean¶

Focus on covering new code well. Legacy code coverage will improve naturally.

Celebrate Wins¶

When legacy files get updated with tests, acknowledge the improvement.

Exclude Generated Code¶

Generated code shouldn't affect your metrics:

excludeClasses.value(
    listOf("**/generated/**/*.*")
)

Use CI Reports¶

Post coverage comments on PRs to make progress visible:

- uses: gw-kit/delta-coverage-action@v1
  with:
    report-path: build/reports/coverage-reports/delta-coverage/test/report.md