Unit testing Java code

All Java code in the engine should now be able to be tested with Robolectric 3.8 and JUnit 4. The test suite has been added after the bulk of the Java code was first written, so most of these classes do not have existing tests. Ideally code after this point should be tested, either with unit tests here or with integration tests in other repos.

Adding a new test

1. Create a file under test/ matching the path and name of the class under test. For example, shell/platform/android/io/flutter/util/Preconditions.java -> shell/platform/android/**test**/io/flutter/util/Preconditions**Test**.java.
2. Add your file to the sources of the robolectric_tests build target in /shell/platform/android/BUILD.gn. This compiles the test class into the test jar.
3. Add your class to the @SuiteClasses annotation in FlutterTestSuite.java. This makes sure the test is actually executed at run time.
4. Write your test.
5. Build and run with testing/run_tests.py [--type=java] [--java-filter=<test_class_name>].

Q&A

Why are we using Robolectric 3.8 when Robolectric 4+ is current?

Robolectric 4+ uses the AndroidX libraries, and the engine sources use the deprecated android.support ones. See flutter/flutter#23586. If this is an issue we could use Jetifier on flutter.jar first and then run the tests, but it would add an extra point of failure.

My new test won‘t run. There’s a “ClassNotFoundException”.

See Updating Embedding Dependencies.

My new test won‘t compile. It can’t find one of my imports.

See Updating Embedding Dependencies.