Collecting code coverage information for Android tests

The Android-specific test suites are outlined on MDN. A more elaborate description can be found on the Wiki. This page describes how collecting code coverage information is implemented for these test suites.

Collecting and exporting code coverage information

Relevant tools and libraries

JaCoCo is the tool used to gather code coverage data. JaCoCo uses class file instrumentation to record execution coverage data. It has two operating modes:

  • online instrumentation: Class files are instrumented on-the-fly using a so called Java agent. The JaCoCo agent collects execution information and dumps it on request or when the JVM exits. This method is used for test suites which run on the JVM (generally, *UnitTest Gradle tasks).
  • offline instrumentation: At runtime the pre-instrumented classes needs be on the classpath instead of the original classes. In addition, jacocoagent.jar must be put on the classpath. This method is used for test suites with run on the Android emulator (generally, *AndroidTest Gradle tasks, including robocop).

JaCoCo is integrated with Gradle in two ways: a Gradle plugin (activated in the build.gradle file as a top-level apply plugin: 'jacoco'), and an Android-specific Gradle plugin, activated with android { buildTypes { debug { testCoverageEnabled true } } }. These two methods of activating JaCoCo have a different syntax of choosing the JaCoCo tool version, and both should be configured to use the same version. See the mobile/android/geckoview/build.gradle file for example usage.

grcov is a tool implemented in Rust that transforms code coverage reports between formats. Among others, it supports reading JaCoCo XML reports.

Generating the coverage report artifacts

All tasks that output code coverage information do so by exporting an artifact named which contains a single file named The file should contain coverage information in the lcov format. The artifact, once uploaded, is picked up and indexed by ActiveData.

The code that generates the artifact after a generally resides in the CodeCoverageMixin class, in the module testing/mozharness/mozharness/mozilla/testing/ This class is responsible for downloading grcov along with other artifacts from the build job. It is also responsible for running grcov after the tests are finished, to convert and merge the coverage reports.

Code coverage for android-test

The android-test suite is a JUnit test suite that runs locally on the host’s JVM. It can be run with mach android test. The test suite is implemented as a build task, defined at taskcluster/ci/build/android-stuff.yml.

To collect code coverage from this suite, a duplicate build task is defined, called android-test-ccov. It can be run with mach android test-ccov.

The mach subcommand is responsible for downloading and running grcov, instead of the CodeCoverageMixin class. This is because the android-test-ccov task is a build task (not a test task), so it doesn’t use mozharness to run the tests.

Code coverage for geckoview-junit

The geckoview-junit tests are on-device Android JUnit tests written for GeckoView. The tests are implemented with mochitest. The automation entry point is testing/mozharness/scripts/, which then calls testing/mochitest/ This is an out-of-tree task, so a source tree clone is unavailable.

To generate the coverage report, we need three things:

  • The classfiles before instrumentation. These are archived as a public artifact during build time, and downloaded on the test machine during testing time;
  • The file with coverage counters. This is generated while running the tests on the emulator, then downloaded with adb pull;
  • jacoco-cli, a command-line package JaCoCo component that takes the classfiles and the coverage counters as input and generates XML reports as output.

The mach android archive-geckoview-coverage-artifacts command archives the class files and exports the jacoco-cli jar file after the build is done. These files are later saved as public artifacts of the build.

To enable offline instrumentation for the test suites, the mozconfig flag --enable-java-coverage should be set. When the flag is checked both during build and test time. During test time, the flag instructs CodeCoverageMixin to download the coverage artifacts from the build task before the tests run, and generate and export the reports after testing is finished. The flag also instructs the script to insert the arguments -e coverage true to am instrument.