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 |