Coverage Engines

Delta Coverage supports two coverage engines. The plugin doesn't collect coverage itself — it uses data from these engines.

JaCoCo (Default)

The standard coverage engine for JVM projects. Works well with Java and most Kotlin code.

Kotlin DSLGroovy DSLCLI

configure<io.github.surpsg.deltacoverage.gradle.DeltaCoverageConfiguration> {
    coverage {
        engine = CoverageEngine.JACOCO
    }
}

deltaCoverageReport {
    coverage {
        engine = 'JACOCO'
    }
}

java -jar delta-coverage-cli.jar \
  --engine JACOCO \
  --diff-file changes.diff \
  --coverage-binary build/jacoco/test.exec \
  --classes build/classes/java/main \
  --sources src/main/java \
  --console

• Produces .exec binary files
• Widely supported and documented
• Default choice for most projects

IntelliJ Coverage

Better accuracy for Kotlin projects, especially with inline functions and coroutines.

Kotlin DSLGroovy DSLCLI

configure<io.github.surpsg.deltacoverage.gradle.DeltaCoverageConfiguration> {
    coverage {
        engine = CoverageEngine.INTELLIJ
    }
}

deltaCoverageReport {
    coverage {
        engine = 'INTELLIJ'
    }
}

java -jar delta-coverage-cli.jar \
  --engine INTELLIJ \
  --diff-file changes.diff \
  --coverage-binary build/coverage/test.ic \
  --classes build/classes/kotlin/main \
  --sources src/main/kotlin \
  --console

• Produces .ic binary files
• Uses CoverJet plugin under the hood
• Recommended for Kotlin-heavy projects

When to use IntelliJ engine

If you see incorrect coverage for Kotlin inline functions, suspend functions, or coroutines, switch to IntelliJ engine.

Auto-Apply Behavior

By default, the Gradle plugin auto-applies the coverage engine plugin:

coverage {
    autoApplyPlugin = true  // default
}

This means:

• JaCoCo engine: Applies jacoco plugin to all projects
• IntelliJ engine: Applies cover-jet plugin to all projects

To disable auto-apply (if you configure coverage manually):

coverage {
    autoApplyPlugin = false
}

CLI

Auto-apply is a Gradle plugin feature. The CLI requires you to provide pre-generated coverage binary files via --coverage-binary.

Comparison

Feature	JaCoCo	IntelliJ
Java accuracy	Excellent	Excellent
Kotlin accuracy	Good	Excellent
Inline functions	Partial	Full
Coroutines	Partial	Full
Binary format	.exec	.ic
Ecosystem support	Wide	Growing

Mixing Engines

You cannot mix engines in a single build. Choose one engine for your entire project.

Troubleshooting

No coverage data found

1. Verify tests ran before deltaCoverage
2. Check binary files exist (build/jacoco/*.exec or build/coverage/*.ic)
3. Run with -PexplainOnly to see detected files

Wrong coverage for Kotlin

Switch to IntelliJ engine:

Kotlin DSLGroovy DSLCLI

coverage {
    engine = CoverageEngine.INTELLIJ
}

deltaCoverageReport {
    coverage {
        engine = 'INTELLIJ'
    }
}

java -jar delta-coverage-cli.jar --engine INTELLIJ ...