JaCoCo Coverage File and Append Mode | Generated by AI
That’s a subtle but important detail in how JaCoCo works.
Behavior of jacoco:prepare-agent
- When Maven runs the
prepare-agentgoal, it doesn’t actually generate the coverage file immediately. - Instead, it configures the JVM (usually via a
-javaagent:jacocoagent.jarargument) so that coverage data will be collected while your tests or app run. - The location of the
.execfile is controlled by the propertyjacoco.destFile(default istarget/jacoco.exec, but often customized, e.g.target/jacoco-it.exec).
What happens if the file already exists?
- By default, JaCoCo will overwrite the file when the JVM starts with the agent.
- This means every run of
prepare-agent→ test execution will produce a fresh.execfile. - It doesn’t append automatically — old data is lost unless you configure it otherwise.
How to append instead
If you want multiple runs to accumulate into the same exec file, you must explicitly enable append mode:
<plugin>
<groupId>org.jacoco</groupId>
<artifactId>jacoco-maven-plugin</artifactId>
<version>0.8.12</version>
<configuration>
<destFile>${project.build.directory}/jacoco-it.exec</destFile>
<append>true</append> <!-- ✅ append coverage data -->
</configuration>
</plugin>
With <append>true>, if the file exists, the new coverage data will be merged into it.
So in short
- Default: Every run replaces the old
.exec. - With
append=true: Coverage from multiple JVM runs (or multiple modules/tests) gets merged into the same file.
Do you want me to also explain how this interacts with multi-module Maven builds (where each module may try to write its own JaCoCo exec), since that’s often why people hit overwrite vs append issues?