Mateusz Pachulski

· budding · gradle, testing, modularisation

testFixtures share fakes between modules without shipping them

The java-test-fixtures plugin gives a module a third source set for fakes and builders. Other modules' tests can use it, and it stays out of the production jar.

The gap

I kept seeing testFixtures(project(...)) in our modules' test dependencies and couldn't say what it actually published, who could use it, or why the fakes didn't just live in main.

The pieces

  • The java-test-fixtures plugin adds a src/testFixtures/ source set to a module.
  • Code in there can see the module's own main code.
  • Another module's tests use it with testImplementation(testFixtures(project(":pricing"))).
  • Libraries the fixtures themselves need go in testFixturesApi(...) or testFixturesImplementation(...). That's the same split as in A green build doesn't prove implementation() is right.

The click

Test fixtures are a small extra library, published next to the real one and used only by tests. The fake for an interface sits in the same module as the interface, so when the interface changes, the fake breaks there, once, and not in every module's test folder.

How I checked it

In the same Gradle 8.14.3 project as the api() card, pricing has FakePricing in src/testFixtures/:

  • checkout's tests compile against FakePricing.
  • checkout's main code referencing FakePricing fails with error: cannot find symbol.
  • pricing.jar contains only Pricing.class. FakePricing.class is in a separate pricing-test-fixtures.jar.

This was on plain JVM modules. Android library modules switch test fixtures on through the Android Gradle plugin, which I haven't checked here.

The project and a script that reruns it are in this site's repository.

Check yourself

Can production code in checkout use FakePricing?

No. Test fixtures are only on the consumer's test classpath, so main code fails with cannot find symbol.

Does the fake end up in the production jar?

No. It goes into a separate -test-fixtures jar.

Where does a library that only the fakes need belong?

In the providing module, under testFixturesImplementation(...), or testFixturesApi(...) if consumers' tests need to see it too.

Sources

Linked from