Understanding JaCoCo and Maven Integration | Generated by AI

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Got it—here’s how the JaCoCo pieces in your exact POM work together, step-by-step, and why.

The two coverage streams this POM creates

Unit-test coverage (Surefire path)

You bind JaCoCo’s prepare-agent without any special config:
```
<execution>
  <id>prepare-agent</id>
  <goals><goal>prepare-agent</goal></goals>
</execution>
```
This injects -javaagent:...org.jacoco.agent-<ver>-runtime.jar into the Surefire test JVM via the argLine property. • Default destfile is ${project.build.directory}/jacoco.exec. • Default append is true (JaCoCo agent appends when the file already exists). • Effect: when you run unit tests (if any) during test, coverage goes into target/jacoco.exec.

Integration-test coverage (Jetty path)

You define a separate file for IT coverage:

<properties>
  <jacoco.it.exec>${project.build.directory}/jacoco-it.exec</jacoco.it.exec>
</properties>

You start Jetty with its own JaCoCo agent pointing to that file:

<plugin>
  <artifactId>jetty-maven-plugin</artifactId>
  ...
  <execution>
    <id>start-jetty</id>
    <phase>pre-integration-test</phase>
    <goals><goal>start</goal></goals>
    <configuration>
      <daemon>true</daemon>
      <jvmArgs>
        -javaagent:${settings.localRepository}/org/jacoco/org.jacoco.agent/${jacoco.version}/org.jacoco.agent-${jacoco.version}-runtime.jar=destfile=${jacoco.it.exec}
      </jvmArgs>
      ...
    </configuration>
  </execution>
</plugin>

• Jetty runs as a separate JVM; its agent writes to target/jacoco-it.exec. • Because append isn’t specified, JaCoCo’s default append=true applies—so multiple Jetty launches append to the same file unless you clean or set append=false.

Lifecycle flow (what happens on `mvn verify`)

compile
- Spotless formats (spotless-maven-plugin) and Checkstyle runs (maven-checkstyle-plugin).
- Your WAR is prepared (maven-war-plugin).
test (Surefire)
- If you have unit tests, they run under the prepare-agent injected argLine → coverage goes to target/jacoco.exec.
pre-integration-test
- Jetty is started in daemon mode:
```
<daemon>true</daemon>
```
  Maven immediately regains control; Jetty keeps running with the JaCoCo agent attached, writing to jacoco-it.exec.

integration-test

Your Python tests hit the running app:

<plugin>
  <artifactId>exec-maven-plugin</artifactId>
  <execution>
    <id>python-integration-tests</id>
    <phase>integration-test</phase>
    <goals><goal>exec</goal></goals>
    <configuration>
      <executable>python3</executable>
      <workingDirectory>${project.parent.basedir}</workingDirectory>
      <arguments>
        <argument>-m</argument><argument>unittest</argument>
        <argument>discover</argument><argument>tests/</argument>
        <argument>-v</argument>
      </arguments>
    </configuration>
  </execution>
</plugin>

These tests exercise Jetty, so coverage accumulates in target/jacoco-it.exec via the Jetty JVM’s agent.

post-integration-test

Jetty is cleanly stopped:

<execution>
  <id>stop-jetty</id>
  <phase>post-integration-test</phase>
  <goals><goal>stop</goal></goals>
  ...
</execution>

verify

You generate a separate IT report using the IT data file and a custom output directory:

<execution>
  <id>report-it</id>
  <phase>verify</phase>
  <goals><goal>report</goal></goals>
  <configuration>
    <dataFile>${jacoco.it.exec}</dataFile>
    <outputDirectory>${project.reporting.outputDirectory}/jacoco-it
    </outputDirectory>
  </configuration>
</execution>

Result: HTML goes to target/site/jacoco-it/index.html summarizing only integration coverage (from Jetty).

Note: If you also want a unit-test report, you could add another jacoco:report execution that points at the default target/jacoco.exec and outputs to target/site/jacoco—keeping unit and IT reports separate.

About “create new or replace?” for the `.exec` files

JaCoCo agent default is append=true.
- For unit tests, the file is target/jacoco.exec. When you run mvn verify repeatedly without clean, data may append across runs.
- For Jetty IT, your jvmArgs don’t override append, so target/jacoco-it.exec also appends by default.

Best practice (to avoid mixing old data):

Run mvn clean verify, or add append=false in the Jetty agent line:

-javaagent:...org.jacoco.agent-...-runtime.jar=destfile=${jacoco.it.exec},append=false

If you ever want to merge multiple .exec files on purpose, use jacoco:merge before jacoco:report.

What each relevant block in your original XML is doing (by reference)

IT file location

<properties>
  <jacoco.it.exec>${project.build.directory}/jacoco-it.exec</jacoco.it.exec>
  <it.report.skip>false</it.report.skip>
</properties>

Defines where integration coverage is stored. it.report.skip is declared but not yet wired into the report execution (see tip below).

Unit test agent (Surefire path)

<plugin>
  <artifactId>jacoco-maven-plugin</artifactId>
  <executions>
    <execution>
      <id>prepare-agent</id>
      <goals><goal>prepare-agent</goal></goals>
    </execution>
    ...
  </executions>
</plugin>

Jetty with JaCoCo for IT

<plugin>
  <artifactId>jetty-maven-plugin</artifactId>
  ...
  <jvmArgs>
    -javaagent:${settings.localRepository}/org/jacoco/org.jacoco.agent/${jacoco.version}/org.jacoco.agent-${jacoco.version}-runtime.jar=destfile=${jacoco.it.exec}
  </jvmArgs>
  ...
</plugin>

Python integration tests while Jetty runs

<plugin>
  <artifactId>exec-maven-plugin</artifactId>
  <execution>
    <id>python-integration-tests</id>
    <phase>integration-test</phase>
    <goals><goal>exec</goal></goals>
    ...
  </execution>
</plugin>

Generate the IT coverage report at verify

<execution>
  <id>report-it</id>
  <phase>verify</phase>
  <goals><goal>report</goal></goals>
  <configuration>
    <dataFile>${jacoco.it.exec}</dataFile>
    <outputDirectory>${project.reporting.outputDirectory}/jacoco-it</outputDirectory>
  </configuration>
</execution>

Practical tips / small improvements

Avoid stale IT data Add append=false to the Jetty agent (shown above), or always run mvn clean verify.

Wire your it.report.skip switch If you want to skip generating the IT report via a property:

<execution>
  <id>report-it</id>
  <phase>verify</phase>
  <goals><goal>report</goal></goals>
  <configuration>
    <skip>${it.report.skip}</skip>
    <dataFile>${jacoco.it.exec}</dataFile>
    <outputDirectory>${project.reporting.outputDirectory}/jacoco-it</outputDirectory>
  </configuration>
</execution>

Keep unit vs IT reports separate You’ve already separated the IT report to site/jacoco-it. If you also need the unit report, add another jacoco:report (no dataFile override) writing to site/jacoco.
Multi-module note This module-local setup reports coverage for PlaygroundWeb only. If you later want a combined view, use jacoco:merge (to combine .exec files) and/or report-aggregate from a parent pom with packaging=pom (JaCoCo 0.8.8+).

TL;DR

Unit tests → prepare-agent injects JaCoCo into Surefire JVM → target/jacoco.exec.
Integration tests → Jetty starts with its own JaCoCo agent → target/jacoco-it.exec.
Report → At verify, jacoco:report renders only the IT coverage to target/site/jacoco-it.
File behavior → JaCoCo appends by default; prefer mvn clean verify or append=false to avoid mixing runs.

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