[New Feature] Allow injection in Extensions

[ghost]

As I understand currently this extension allows injection only inside test classes. It would be good to have ability to inject fields inside junit5 custom Extension classes

[JeffFaer]

What's your use case? I think this is already supported if you use @RegisterExtension:

@IncludeModule(MyModule.class)
class MyTest {
  @RegisterExtension @Inject MyExtension myExtension;
}

Will add a test shortly to confirm

[ghost]

Tried your suggestion and it seems it doesn't work. Does it work for you?

My usecase: I need to be able to inject config objects etc. into my custom extensions. Sinse there're several ways to add extension to test class would you like to consider providing guice injector for extensions directly? Perhaps it can be done via base class like InjectableExtension. What do you think about it?

@IncludeModule(MyModule.class)
public MyExtension extends InjectableExtension implements BeforeAllCallback {

    @Inject private InjectedObject obj;

}

[JeffFaer]

This did work for me. See https://github.com/JeffreyFalgout/junit5-extensions/blob/master/guice-extension/src/test/java/name/falgout/jeffrey/testing/junit/guice/InjectExtensionsTest.java

Adding @IncludeModule directly to an extension should be possible, but I'm not convinced it's a good idea.

1. It seems a little surprising to special type extensions for this type of support. We could support this on all types, but I definitely wouldn't want @IncludeModule on random injectable classes in my code base
2. It tightly couples your extension to a single module, which limits its reusability (and goes counter to Guice's philosophy)

[ghost]

Try implementing BeforeAllCallback. beforeAll() method is not executed for me.

[JeffFaer]

Ah, that's tricky.

Instance fields cannot implement BeforeAllCallback: https://junit.org/junit5/docs/current/user-guide/#extensions-registration-programmatic-instance-fields:

Thus, if such an instance extension implements class-level or instance-level extension APIs such as BeforeAllCallback, AfterAllCallback, or TestInstancePostProcessor, those APIs will not be honored

And static @RegisterExtension fields are registered before the GuiceExtension gets a chance to inject anything.

Perhaps we need a method to create an instance with GuiceExtension manually. Then we could do something like

  @RegisterExtension private static final MyExtension myExtension = GuiceExtension.getInstance(MyExtension.class);

[ghost]

Sounds good to me