jacoco-analysis

Overview

JaCoCo (the canonical JVM coverage tool) instruments Java bytecode at runtime via a Java agent, then aggregates execution data into per-class / per-method / per-line reports.

Per jacoco-counters, JaCoCo measures six counters:

This skill covers Maven (the primary deployment), with the bytecode-only key insight: "All counters work at bytecode level, making them available regardless of debug information presence" (jacoco-counters). Gradle, the XML parser, and the cross-language conversions live in the reference file.

When to use

• A JVM project (Java / Kotlin / Scala / Groovy) needs coverage and the team is on Maven or Gradle.
• A Cobertura-based pipeline is migrating to JaCoCo (typical: Cobertura is unmaintained for JDK 11+; JaCoCo handles modern bytecode).
• Cross-language aggregation needs JaCoCo XML converted to LCOV / Cobertura.

How to use

1. Confirm the JVM build is Maven / Gradle (see When to use).
2. Wire the jacoco-maven-plugin with the three primary goals - prepare-agent, report, check (below); Gradle's equivalent is in references/gradle-parsing-and-ci.md.
3. Run mvn verify; read the check verdict and the HTML at target/site/jacoco/index.html.
4. Gate on INSTRUCTION + BRANCH ratios, then parse jacoco.xml for PR comments and convert to LCOV / Cobertura for cross-language aggregation - the parser, the counter-aware gating table, the format conversions, and the full Maven verify CI job are in references/gradle-parsing-and-ci.md.

Wire the Maven plugin

Per jacoco-maven, the plugin defines several goals; the three primary ones are prepare-agent, report, and check (jacoco-maven):

<plugin>
  <groupId>org.jacoco</groupId>
  <artifactId>jacoco-maven-plugin</artifactId>
  <version>0.8.13</version>
  <executions>
    <execution>
      <id>prepare-agent</id>
      <goals>
        <goal>prepare-agent</goal>
      </goals>
    </execution>
    <execution>
      <id>report</id>
      <phase>verify</phase>
      <goals>
        <goal>report</goal>
      </goals>
    </execution>
    <execution>
      <id>check</id>
      <phase>verify</phase>
      <goals>
        <goal>check</goal>
      </goals>
      <configuration>
        <rules>
          <rule>
            <element>BUNDLE</element>
            <limits>
              <limit>
                <counter>INSTRUCTION</counter>
                <value>COVEREDRATIO</value>
                <minimum>0.80</minimum>
              </limit>
              <limit>
                <counter>BRANCH</counter>
                <value>COVEREDRATIO</value>
                <minimum>0.70</minimum>
              </limit>
            </limits>
          </rule>
        </rules>
      </configuration>
    </execution>
  </executions>
</plugin>

prepare-agent configures the JaCoCo runtime agent for the forked test JVM; report generates HTML / XML / CSV from target/jacoco.exec; check enforces thresholds.

Per jacoco-maven: "Maven 3.0 or higher and Java 1.8 or higher for the Maven runtime, Java 1.5 or higher for the test executor."

Report formats

The report goal emits three artifacts under target/site/jacoco/:

For Maven multi-module projects, the per-module reports go under each module's target/site/jacoco/. For an aggregated report, use the report-aggregate goal in a parent module.

Gating with the check goal

The check goal accepts a list of rules; each rule scopes by element (BUNDLE / PACKAGE / CLASS / SOURCEFILE / METHOD) and constrains via limits with counter / value / minimum or maximum:

<rules>
  <rule>
    <element>BUNDLE</element>
    <limits>
      <limit>
        <counter>INSTRUCTION</counter>
        <value>COVEREDRATIO</value>
        <minimum>0.80</minimum>
      </limit>
    </limits>
  </rule>
  <rule>
    <element>CLASS</element>
    <excludes>
      <exclude>*Test</exclude>
      <exclude>*IT</exclude>
    </excludes>
    <limits>
      <limit>
        <counter>METHOD</counter>
        <value>MISSEDCOUNT</value>
        <maximum>0</maximum>
      </limit>
    </limits>
  </rule>
</rules>

The two-rule pattern: a BUNDLE (whole-project) floor + a per-CLASS strict rule for production code (excluding tests).

value options (per the JaCoCo Maven plugin's check-mojo docs):

minimum floors the value; maximum caps it. Counter options are INSTRUCTION / BRANCH / LINE / METHOD / CLASS / COMPLEXITY (per jacoco-counters). Gate on INSTRUCTION for the whole-repo floor and BRANCH separately, with a bounded METHOD miss:

<limit><counter>INSTRUCTION</counter><value>COVEREDRATIO</value><minimum>0.80</minimum></limit>
<limit><counter>BRANCH</counter><value>COVEREDRATIO</value><minimum>0.70</minimum></limit>
<limit><counter>METHOD</counter><value>MISSEDCOUNT</value><maximum>5</maximum></limit>

Whole-bundle 80% instruction; 70% branch; allow up to 5 untested methods. The per-counter "when to gate on which" table is in references/gradle-parsing-and-ci.md.

Worked example

A Maven service wiring JaCoCo for the first time:

1. Add the plugin block above to pom.xml (all three executions: prepare-agent, report, check).
2. Run the build:

./mvnw -B verify

prepare-agent instruments the forked test JVM; the tests write target/jacoco.exec; report renders target/site/jacoco/index.html; check evaluates the rules.

If instruction coverage lands at 82% but branch coverage is 66%, the build fails on the BRANCH rule (minimum 0.70), naming the bundle and the missed limit. Open target/site/jacoco/index.html, drill into the red (missed) branches, add the tests that exercise them, and re-run ./mvnw -B verify until check passes.

Operating in CI

Run ./mvnw -B verify so check gates the build, then upload target/site/jacoco/ with if: always() (coverage matters most on runs that failed the gate). For a coverage SaaS or cross-language aggregation, convert jacoco.xml to LCOV in the same job and upload that. The full GitHub Actions workflow (verify, artifact upload, xml2lcov conversion, SaaS upload) is in references/gradle-parsing-and-ci.md.

Anti-patterns

Limitations

• Bytecode-level only. Per jacoco-counters, all counters work at bytecode level - useful for source-format independence but means kotlinc / scalac inlining can produce surprising line-coverage shapes.
• Per-method desc= is JVM signature. Cross-source-language reports (Kotlin generates JVM methods) need the source file as the pivot, not the method descriptor.
• prepare-agent only instruments forked test JVMs. Surefire's forkCount=0 (run-in-Maven-JVM mode) bypasses the agent; coverage data is empty. Use the default fork mode.
• No PR-context awareness. Pair with coverage-diff-reporter for the diff vs main.

References

• jacoco-counters - six counters (instructions, branches, lines, methods, classes, cyclomatic complexity), bytecode-level measurement, formula for cyclomatic complexity.
• jacoco-maven - prepare-agent / report / check goals; Maven + JVM version requirements; HTML report output path.
• references/gradle-parsing-and-ci.md - the Gradle plugin config, the jacoco.xml parser, the counter-aware gating table, JaCoCo-to-Cobertura / LCOV conversion, and the Maven verify CI job.
• cobertura-analysis - sister parser; JaCoCo XML can convert to Cobertura XML for sibling tooling.
• lcov-analysis - sister parser; JaCoCo can convert to LCOV for cross-language aggregation.
• coverage-diff-reporter, test-coverage-targeter - downstream skills consuming the parsed JaCoCo output.