JaCoCo in Maven Multi-Module Projects

Can you blow my whistle baby, whistle baby, Let me know Girl I'm gonna show you how to do it And we start real slow You just put your lips together. (Whistle - Flo Rida)

Code coverage is an important measure used during our development that describes the degree to which source code is tested.

In this post I am going to explain how to run code coverage using Maven and JaCoCo plugin in multi-module projects.

JaCoCo is a code coverage library for Java, which has been created by the EclEmma team. It has a plugin for Eclipse, and can be run with Ant and Maven too.

Now we will focus only in Maven approach.

In a project with only one module is as easy as registering a build plugin:

And now running mvn package, in site/jacoco directory, a coverage report will be present in different formats.

But with multimodule projects a new problem arises. How to merge metrics of all subprojects into only one file, so we can have a quick overview of all subprojects? For now Maven JaCoCo Plugin does not support it.

There are many alternatives and I am going to cite the most common:

• Sonar. It has the disadvantage that you need to install Sonar (maybe you are already using, but maybe not).
• Jenkins. Plugin for JaCoCo is still under development. Moreover you need to run a build job to inspect your coverage. This is good in terms of continuous integration but could be a problem if you are trying to "catch" some piece of code that has not covered with already implemented tests.
• Arquillian JaCoCo Extension. Arquillian is a container test framework that has an extension which during test execution can capture the coverage. Also a good option if you are using Arquillian. The disadvantage is that maybe your project does not require a container.
• Ant. You can use Ant task with Maven. JaCoCo Ant task can merge results from multiple JaCoCo files result. Note that is the most generic solution, and this is the chosen approach that we are going to use.

First thing to do is add JaCoCo plugin to parent pom so all projects could generate coverage report. Of course if there are modules which does not require coverage, plugin definition should be changed from parent pom to specific projects.

Next step is creating a specific submodule for appending all results of JaCoCo plugin by using Ant task. I suggest  using something like project-name-coverage.

Then let's open generated pom.xml and we are going to insert required plugins to join all coverage information. To append them, as we have already written we are going to use a JaCoCo Ant task which has the ability to open all JaCoCo output files and append all their content into one. So first thing to do is download the jar which contains the JaCoCo Ant task. To automatize download process, we are going to use maven dependency plugin:

During process-test-resources phase Jacoco Ant artifact will be downloaded and copied to target directory, so can be registered into pom without worrying about jar location.

We also need a way to handle Ant tasks from Maven. And this is as simple as using maven antrun plugin, which you can specify any ant command in its configuration section. See next simple example:

Notice that into target tag we can specify any Ant task. And now we are ready to start configuring JaCoCo Ant task. JaCoCo report plugin requires you set the location of build directory, class directory, source directory or generated-source directory. For this purpose we are going set them as properties.

And now the Ant task part which will go into target tag of antrun plugin.

First we need to define report task.

See that org.jacoco.ant.jar file is downloaded by dependency plugin, you don't need to worry about copying it manually.

Then we are going to call report task as defined in taskdef section.

Within executiondata element, we specify locations where JaCoCo execution data files are stored. By default is target directory, and for each project we need to add one entry for each submodule.

Next element is structure. This element defines the report structure, and can be defined with hierarchy of group elements. Each group  should contain class files and source files of all projects that belongs to that group. In our example only one group is used.

And finally we are setting output format using html, xml and csv tags.

Complete Code:

And now simply run mvn clean verify and in my-project-coverage/target/coverage-report, a report with code coverage of all projects will be presented.

Hope you find this post useful.

We Keep Learning,
Alex.

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