Best way to run fast JUnit tests in Android project in Android Studio

Question

I have some plain old Java classes and simple JUnit tests in my Android Studio project. They currently live inside my Android Module. If I run them as Android tests they pass fine, but they're really slow as it requires launching or connecting to the emulator.

If I try to run one as a JUnit test, I hit the !!! JUnit version 3.8 or later expected JUnit version problem. This answer explains how to workaround that issue. Unfortunately, I then hit the java.lang.NoClassDefFoundError: junit/textui/ResultPrinter, which I can't figure out how to fix.

I then tried to move my JUnit tests into a separate Java module that depends on the Android Module. Everything compiles fine. I added a new configuration to launch the JUnit tests (:MyJUnitTests:test), but the tests fail with package com.myproject.util does not exist. It looks like maybe the classpath isn't including the classes from the dependent Android Module.

Does anybody know how to fix this classpath issue? I've looked at lots of related answers and none of them seem to work for me. Or is it a just a bad idea to try and keep my JUnit tests in their own Module?

I can get the JUnit tests to run fast and pass if I move my plain Java classes and their JUnit tests into a completely separate Module (e.g. here), and reverse the dependency so the Android Module depends on the Java Module. Is that the preferred solution?

Answer 1

The basic problem is that Android Framework classes don't work well outside the context of Android OS, so any code that has dependencies on Framework classes doesn't run in a regular JUnit environment, as you've found.

Your solution of trying to move the JUnit tests into a separate module won't work because you can't have a plain Java module depend on an Android module. Android Gradle modules don't act like Java modules, because Android builds are much more complex, and because the end result of an Android module build will be an APK or an AAR, which other module types won't understand.

If you can move your plain Java classes and unit tests into a plain Java module and have the Android modules depend on that, it will be the best approach that will get the most support from official features in the IDE. It will also have an architectural benefit in that it will keep those classes free from Android abstractions, making it more portable, and enforcing a separation of business logic from UI or storage or other things more device-specific.

If that's difficult for you to do, then a lot of developers in your shoes go with Robolectric, which is a test harness allowing code depending on many parts of the Android Framework to run in a JUnit environment without an emulator or device. It's not officially supported by Google, but Google is aware that it's widely used and tries hard to not wantonly break it.