How to configure multi-module Maven + Sonar + JaCoCo to give merged coverage report?

I was in the same situation as you, the half answers scattered throughout the Internet were quite annoying, since it seemed that many people had the same issue, but no one could be bothered to fully explain how they solved it.

The Sonar docs refer to a GitHub project with examples that are helpful. What I did to solve this was to apply the integration tests logic to regular unit tests (although proper unit tests should be submodule specific, this isn’t always the case).

In the parent pom.xml, add these properties:

<properties>
    <!-- Sonar -->
    <sonar.java.coveragePlugin>jacoco</sonar.java.coveragePlugin>
    <sonar.dynamicAnalysis>reuseReports</sonar.dynamicAnalysis>
    <sonar.jacoco.reportPath>${project.basedir}/../target/jacoco.exec</sonar.jacoco.reportPath>
    <sonar.language>java</sonar.language>
</properties>

This will make Sonar pick up unit testing reports for all submodules in the same place (a target folder in the parent project). It also tells Sonar to reuse reports ran manually instead of rolling its own. We just need to make jacoco-maven-plugin run for all submodules by placing this in the parent pom, inside build/plugins:

<plugin>
    <groupId>org.jacoco</groupId>
    <artifactId>jacoco-maven-plugin</artifactId>
    <version>0.6.0.201210061924</version>
    <configuration>
        <destFile>${sonar.jacoco.reportPath}</destFile>
        <append>true</append>
    </configuration>
    <executions>
        <execution>
            <id>agent</id>
            <goals>
                <goal>prepare-agent</goal>
            </goals>
        </execution>
    </executions>
</plugin>

destFile places the report file in the place where Sonar will look for it and append makes it append to the file rather than overwriting it. This will combine all JaCoCo reports for all submodules in the same file.

Sonar will look at that file for each submodule, since that’s what we pointed him at above, giving us combined unit testing results for multi module files in Sonar.