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”.

Your test is probably using a dependency that we haven't needed yet. You probably need to find the dependency you need, add it to the flutter/android/robolectric_bundle CIPD package, and then re-run gclient sync. See “Updating a CIPD dependency” below.

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

You could be using a brand new dependency. If so, you'll need to add it to the CIPD package for the robolectric tests. See “Updating a CIPD dependency” below.

Then you'll also need to add the jar to the robolectric_tests build target. Add //third_party/robolectric/lib/<dependency.jar> to robolectric_tests._jar_dependencies in /shell/platform/android/BUILD.gn.

There‘s also a chance that you’re using a dependency that we‘re relying on at runtime, but not compile time. If so you’ll just need to update _jar_dependencies in BUILD.gn.

Updating a CIPD dependency

See the Chromium instructions on “Updating a CIPD dependency” for how to upload a package update to CIPD. Download and extract the latest package from CIPD and then copy shell/platform/android/test/cipd.yaml into the extracted directory to use as the base for the pre-existing package. Add new dependencies to lib/.

Once you‘ve uploaded the new version, also make sure to tag it with the updated timestamp and robolectric version (most likely still 3.8, unless you’ve migrated all the packages to 4+).

$ cipd set-tag flutter/android/robolectric_bundle --version=<new_version_hash> -tag=last_updated:<timestamp>

Example of a last-updated timestamp: 2019-07-29T15:27:42-0700

You can generate the same date format with date +%Y-%m-%dT%T%z.

$ cipd set-tag flutter/android/robolectric_bundle --version=<new_version_hash> -tag=robolectric_version:<robolectric_version>

You can run cipd describe flutter/android/robolectric_bundle --version=<new_version_hash> to verify. You should see:

Package:       flutter/android/robolectric_bundle
Instance ID:   <new_version_hash>
...
Tags:
 last_updated:<timestamp>
 robolectric_version:<robolectric_version>

Then update the DEPS file (located at /src/flutter/DEPS) to use the new version by pointing to your new last_updated_at tag.

  'src/third_party/robolectric': {
     'packages': [
       {
        'package': 'flutter/android/robolectric_bundle',
        'version': 'last_updated:<timestamp>'
       }
     ],
     'condition': 'download_android_deps',
     'dep_type': 'cipd',
   },

You can now re-run gclient sync to fetch the latest package version.