How to Correctly Configure JaCoCo in Maven for Java Code Coverage Reporting

How can I properly configure JaCoCo with Maven to generate a code coverage report for my Java project?

<plugin>
    <groupId>org.jacoco</groupId>
    <artifactId>jacoco-maven-plugin</artifactId>
    <version>0.8.7</version>
    <configuration>
        <outputDirectory>${project.build.directory}/jacoco-report</outputDirectory>
        <reportFormat>html</reportFormat>
    </configuration>
    <executions>
        <execution>
            <id>prepare-agent</id>
            <goals>
                <goal>prepare-agent</goal>
            </goals>
        </execution>
        <execution>
            <id>generate-report</id>
            <phase>prepare-package</phase>
            <goals>
                <goal>report</goal>
            </goals>
        </execution>
    </executions>
</plugin>

JaCoCo is a widely used code coverage library for Java applications. When using it with Maven, ensuring the correct configuration in the `pom.xml` is crucial for generating the desired coverage reports. If you're facing issues with reports not being generated, this guide provides an overview of necessary steps and common pitfalls.

<plugin>
    <groupId>org.jacoco</groupId>
    <artifactId>jacoco-maven-plugin</artifactId>
    <version>0.8.7</version>
    <executions>
        <execution>
            <id>prepare-agent</id>
            <goals>
                <goal>prepare-agent</goal>
            </goals>
        </execution>
        <execution>
            <id>generate-report</id>
            <phase>prepare-package</phase>
            <goals>
                <goal>report</goal>
            </goals>
        </execution>
    </executions>
</plugin>

Causes

• Outdated JaCoCo version: Using older versions like 0.5.10 can lead to compatibility issues with JDK 1.8.
• Incorrect execution phase: If the report generation goal is not bound to the appropriate build phase, it may not execute properly.
• Missing dependencies: Ensure that all necessary dependencies for testing are included in the pom.xml.

Solutions

• Update JaCoCo to the latest version (currently 0.8.7) to ensure compatibility and access to the latest features.
• Set the execution phase of the report goal to 'prepare-package' to ensure it runs after tests complete but before packaging.
• Ensure your Java tests are executing successfully to generate coverage data needed for reports. Add sufficient test cases that cover your code.

Mistake: Not running tests before generating the report. If the tests haven't run, there's no data to report on.

Solution: Always run `mvn test` before `mvn jacoco:report`.

Mistake: Using outdated JaCoCo version that isn't compatible with the Java version used.

Solution: Upgrade to a recent version of JaCoCo, such as 0.8.7, that supports Java 1.8.

Mistake: Failing to include the JaCoCo configuration in the project's pom.xml.

Solution: Ensure the JaCoCo plugin is correctly added and configured in your pom.xml.