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+Title: Generating test coverage for Sonar with maven, surefire and jacoco
+Date: 2020-05-11
+Lang: en
+
+At work we're using [Sonar](https://www.sonarqube.org) to keep our code quality under control. One integral part of code quality is test coverage and Sonar offers coverage metrics in the UI. However, the [Sonar docs on code coverage](https://docs.sonarqube.org/latest/analysis/coverage/) are a bit sparse (at best) and don't tell you the exact steps to run for getting coverage with a Maven build.
+
+With a bit of googling I was eventually able to find the correct steps to get coverage. The key to success is the [Jacoco Maven plugin](https://www.eclemma.org/jacoco/trunk/doc/maven.html). In a first Maven execution we let jacoco record coverage statistics while running the unit tests:
+
+    :::bash
+    mvn clean org.jacoco:jacoco-maven-plugin:prepare-agent install
+
+In the build log you see the jacoco-maven-plugin in action (we'll get back to this in a while):
+
+    [INFO] --- jacoco-maven-plugin:0.8.5:prepare-agent (default-cli) @ sonar-code-coverage ---
+    [INFO] argLine set to -javaagent:/.m2/repository/org/jacoco/org.jacoco.agent/0.8.5/org.jacoco.agent-0.8.5-runtime.jar=destfile=/sonar-code-coverage/target/jacoco.exec
+
+Then in a second Maven execution we'll pick up those coverage statistics and convert them to XML so that Sonar can include them in its analysis:
+
+    :::bash
+    mvn org.jacoco:jacoco-maven-plugin:report org.sonarsource.scanner.maven:sonar-maven-plugin:sonar
+
+**Hooray! Coverage info in Sonar.**
+
+The trouble starts when you need to pass custom arguments through surefire down to the JVM that will run the tests. This is done through the `<argLine>` configuration parameter of maven-surefire-plugin:
+
+    :::xml
+    <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-surefire-plugin</artifactId>
+        <configuration>
+            <argLine>-Xmx128m</argLine>
+        </configuration>
+    </plugin>
+
+Looking back at the output of jacoco-maven-plugin above you'll notice that we're setting exactly the same `<argLine>` parameter as jacoco-maven-plugin does. If you don't take extra steps only one configuration can win - and that's the configuration you put into your pom. You have to run `mvn -X` to actually see this, though:
+
+    [DEBUG] Forking command line: /bin/sh -c cd /sonar-code-coverage && /opt/openjdk-bin-8.252_p09/jre/bin/java -Xmx128m -jar /sonar-code-coverage/target/surefire/surefirebooter9099169912642065145.jar /sonar-code-coverage/target/surefire 2020-05-12T07-39-04_225-jvmRun1 surefire3709014631518520641tmp surefire_01706858247537265191tmp
+
+Note how the `-javaagent:...` argLine is not included in the command line arguments of the JVM that's forked to run the tests.
+
+A way to fix this is through creative use of Maven build properties. Configure the jacoco-maven-plugin to set a different property than `argLine`:
+
+    :::xml
+    <plugin>
+        <groupId>org.jacoco</groupId>
+        <artifactId>jacoco-maven-plugin</artifactId>
+        <configuration>
+            <propertyName>surefire.argLine.sonar</propertyName>
+        </configuration>
+    </plugin>
+
+Next, configure maven-surefire-plugin to include that property into its `<argLine>`:
+
+    :::xml
+    <plugin>
+        <groupId>org.apache.maven.plugins</groupId>
+        <artifactId>maven-surefire-plugin</artifactId>
+        <configuration>
+            <argLine>-Xmx128m @{surefire.argLine.sonar}</argLine>
+        </configuration>
+    </plugin>
+
+Note that you have to use [late evaluation](http://maven.apache.org/surefire/maven-surefire-plugin/test-mojo.html#argLine) of the property because it's not present when the Maven reactor starts but only after jacoco-maven-plugin has run.
+
+Just one last hack is required to make the build work even if you don't run jacoco coverage as part of your build. Specify an empty property `surefire.argLine.sonar` in your project's properties so that it always exists and can be expanded.
+
+For further reference I've put [a demo project up on github](https://github.com/dirk-olmes/sonar-code-coverage).