Writing tests for your code provides you with a number of development benefits, from proving that your code meets set requirements to ensuring that future code changes don’t introduce bugs or break existing functionality. It’s also important to know how much of your code is covered by your tests and what areas in your code aren’t yet covered.

Jacoco is a framework that creates code coverage reports that show you how much of your code has or has not been touched in your tests. This post is an excellent guide to setting up your Android Gradle project to use Jacoco.

To generate the Jacoco test coverage report, execute the following in the commandline:

./gradlew cleanTest test jacocoTestReport

If you’re using Dagger in your Android Gradle project, you might encounter the following error:

FAILURE: Build failed with an exception.

* What went wrong:
Execution failed for task ':app:jacocoTestReport'.
> Error while creating report

To dig deeper into this error, execute the same command with the stacktrace option:

./gradlew cleanTest test jacocoTestReport --stacktrace

In the stack trace, you’ll see an entry similar to this:

Caused by: java.io.FileNotFoundException: /app/build/intermediates/classes/debug/com/yourproject/AndroidModule$ModuleAdapter$ProvideAccountManagerProvidesAdapter.class (No such file or directory)
	at org.apache.tools.ant.types.resources.FileResource.getInputStream(FileResource.java:217)
	at org.jacoco.ant.ReportTask.createBundle(ReportTask.java:568)
	at org.jacoco.ant.ReportTask.createReport(ReportTask.java:547)
	at org.jacoco.ant.ReportTask.execute(ReportTask.java:494)
	... 67 more

Jacoco is expecting to find generated files with a single “$” in their name, but Dagger generates files with “$$” in their name. However, we don’t need to check test coverage in these generated classes, so we can tell Jacoco to ignore these files by adding the following to the “excludes” array (in addition to any existing entries) in the fileTree passed to “classDirectories”, like so:

classDirectories = fileTree(
 dir: '../app/build/intermediates/classes/debug',
 excludes: [
 '**/*InjectAdapter*.*',
 '**/*StaticInjection*.*',
 '**/*ModuleAdapter*.*'
 ]
 )