Working on unreleased Glean code in android-components

This is a companion to the equivalent instructions for the android-components repository.

Modern Gradle supports composite builds, which allows to substitute on-disk projects for binary publications. Composite builds transparently accomplish what is usually a frustrating loop of:

  1. change library
  2. publish library snapshot to the local Maven repository
  3. consume library snapshot in application

Note: this substitution-based approach will not work for testing updates to the Glean Gradle Plugin. For that to work, the library and the plugin need to be published to a local maven and manually imported in the consuming projects. Please refer to Using locally-published Glean in Fenix for how to do that.


Clone the Glean SDK and android-components repositories:

git clone
git clone

Cargo build targets

By default when building Android Components using gradle, the gradle-cargo plugin will compile Glean for every possible platform, which might end up in failures. You can customize which targets are built in glean/ (more information):

# For physical devices:

# For unit tests:
# rust.targets=darwin # Or linux-*, windows-* (* = x86/x64)

# For emulator only:

Substituting projects

android-components has custom build logic for dealing with composite builds, so you should be able to configure it by simply adding the path to the Glean repository in the correct file:

In android-components/


If this doesn't seem to work, or if you need to configure composite builds for a project that does not contain this custom logic, add the following to settings.gradle:

In android-components/settings.gradle:

includeBuild('../glean') {
  dependencySubstitution {
    substitute module('org.mozilla.telemetry:glean') with project(':glean')
    substitute module('org.mozilla.telemetry:glean-forUnitTests') with project(':glean')

Composite builds will ensure the Glean SDK is build as part of tasks inside android-components.


There's a big gotcha with library substitutions: the Gradle build computes lazily, and AARs don't include their transitive dependencies' JNI libraries. This means that in android-components, ./gradlew :service-glean:assembleDebug does not invoke :glean:cargoBuild, even though :service-glean depends on the substitution for :glean and even if the inputs to Cargo have changed! It's the final consumer of the :service-glean project (or publication) that will incorporate the JNI libraries.

In practice that means you should always be targeting something that produces an APK: a test or a sample module. Then you should find that the cargoBuild tasks are invoked as you expect.

Inside the android-components repository ./gradlew :samples-glean:connectedAndroidTest should work. Other tests like :service-glean:testDebugUnitTest or :support-sync-telemetry:testDebugUnitTest will currently fail, because the JNI libraries are not included.


This document is based on the equivalent documentation for application-services: Development with the Reference Browser

  1. Transitive substitutions (as shown above) work but require newer Gradle versions (4.10+).