jacoco

JaCoCo - Quick Reference

When to Use This Skill

• Configure code coverage in Java/Maven projects
• Set coverage thresholds
• Integrate coverage in CI/CD

When NOT to Use This Skill

• JavaScript/TypeScript coverage - Use Vitest skill for frontend coverage
• SonarQube configuration - Use sonarqube skill for quality gates
• Test writing - Use Spring Boot Test or JUnit skills
• Gradle projects - JaCoCo works but syntax differs from Maven

Deep Knowledge: Use mcp__documentation__fetch_docs with technology: jacoco for comprehensive documentation.

Essential Patterns

Maven Configuration

<plugin>
    <groupId>org.jacoco</groupId>
    <artifactId>jacoco-maven-plugin</artifactId>
    <version>0.8.12</version>
    <executions>
        <execution>
            <id>prepare-agent</id>
            <goals><goal>prepare-agent</goal></goals>
        </execution>
        <execution>
            <id>report</id>
            <phase>test</phase>
            <goals><goal>report</goal></goals>
        </execution>
        <execution>
            <id>check</id>
            <goals><goal>check</goal></goals>
            <configuration>
                <rules>
                    <rule>
                        <element>BUNDLE</element>
                        <limits>
                            <limit>
                                <counter>LINE</counter>
                                <value>COVEREDRATIO</value>
                                <minimum>0.80</minimum>
                            </limit>
                        </limits>
                    </rule>
                </rules>
            </configuration>
        </execution>
    </executions>
</plugin>

Exclusions

<excludes>
    <exclude>com.company.config.*</exclude>
    <exclude>com.company.dto.*</exclude>
    <exclude>com.company.entity.*</exclude>
    <exclude>com.company.mapper.*Impl</exclude>
</excludes>

Commands

mvn clean verify           # Test + coverage check
mvn jacoco:report          # Generate report
open target/site/jacoco/index.html  # View report

Counter Types

Counter	Description
LINE	Lines of code
BRANCH	Branch if/switch
METHOD	Methods

Anti-Patterns

Anti-Pattern	Why It's Bad	Correct Approach
Aiming for 100% coverage	Diminishing returns, test bloat	Target 80% LINE, 70% BRANCH
Including DTOs/entities in coverage	Inflates numbers, no logic to test	Exclude with <excludes>
Only checking LINE coverage	Misses untested branches	Check both LINE and BRANCH
No coverage threshold in CI	Can't enforce quality	Add <check> goal with minimums
Excluding too much code	False sense of security	Only exclude generated/config code
Not versioning jacoco.exec	Can't track coverage trends	Archive reports in CI

Quick Troubleshooting

Issue	Likely Cause	Solution
Coverage report shows 0%	Tests not running with agent	Ensure prepare-agent goal runs before tests
Build fails on coverage check	Coverage below threshold	Add tests or adjust minimum in <limit>
Report excludes nothing	Wrong exclude pattern	Use fully qualified names (com.company.dto.*)
Report missing classes	Classes not loaded during tests	Add integration tests to cover them
Coverage lower in CI than local	Different test execution	Ensure CI runs all test phases
SonarQube shows no coverage	Wrong report path	Check sonar.coverage.jacoco.xmlReportPaths

Further Reading

For advanced configurations: mcp__documentation__fetch_docs

• Technology: jacoco (not yet in MCP - use official docs)
• JaCoCo Docs