TestInstancePostProcessor cannot be registered via @ExtendWith on a non-static field

[sbrannen]

Overview

As discovered by @graydavid in #3433, there is an issue regarding extension registration via @ExtendWith on instance fields.

The following is a simplification of the RandomNumberDemo from the User Guide which demonstrates the problem.

// @ExtendWith(RandomNumberExtension.class)
class RandomNumberDemoTest {

	@Random
	Integer randomNumber;

	@Test
	void test() {
		assertThat(this.randomNumber).isNotNull();
	}
}

The above test fails; however, if you annotate the test class with @ExtendWith(RandomNumberExtension.class) the test then passes.

The reason is that ClassBasedTestDescriptor#instantiateAndPostProcessTestInstance() invokes ExtensionUtils.registerExtensionsFromFields() too late.

I even added the following comment when I introduced that support.

// In addition, we register extensions from instance fields here since the
// best time to do that is immediately following test class instantiation
// and post processing.

In retrospect, that is not the "best time to do that" when a field is annotated (or meta-annotated in the case of @Random) with an @ExtendWith annotation that registers an extension that implements TestInstancePostProcessor.

Related Issues

• Fix implementation of RandomNumberExtension in the User Guide #3433
• Revise implementation of RandomNumberExtension in the User Guide #3463

Deliverables

• Revise the usage of ExtensionUtils.registerExtensionsFromFields() so that a TestInstancePostProcessor can be registered via @ExtendWith on a non-static field.

[Michael1993]

Should I be able to inject a @RegisterExtension value with a field-level TestInstancePostProcessor?

Example:

class BasicTests {

  @ExtendWith(ExtensionInjector.class)
  ExtensionInjector injector;

  @RegisterExtension
  ParameterResolver extension;

  // some tests...
}

Also: can I work on this issue? Other than this one question, I think I can handle it.

[sbrannen]

Hi @Michael1993,

Should I be able to inject a @RegisterExtension value with a field-level TestInstancePostProcessor?

Ideally, yes, I believe so. At least, generally speaking a TestInstancePostProcessor should be capable of doing that.

I believe I demonstrated that once with Spring. Let me see if I can find it...

[sbrannen]

I believe I demonstrated that once with Spring. Let me see if I can find it...

https://stackoverflow.com/a/50251190/388980

[sbrannen]

Also: can I work on this issue? Other than this one question, I think I can handle it.

Thanks for volunteering!

However, this issue is assigned to me and not labeled as up-for-grabs.

In addition, it may turn out to be more challenging than you expect.

In light of that, I would like to keep this assigned to me.

FWIW, I took a glance at your spike, and I'd like to point out that we have no plans to add specific handling for a particular extension API (such as TestInstancePostProcessor). Rather, I will implement a general solution that applies to any extension type registered via @ExtendWith on both static and non-static fields, and I've already begun work on this issue.

[Michael1993]

Sorry, I don't know how I missed that. I will take a look at the up-for-grabs issues instead, then. 🙂

[sbrannen]

I pushed an "experiment" to the following feature branch.

Note, however, that this is truly a work in progress. A few existing tests fail, and we may need to take a completely different approach.

Thus, I've only pushed this to allow others (for example, @marcphilipp) to take a look at this for potential inspiration.

main...sbrannen:junit5:issues/3437-ExtendWith-on-fields-early-registration