Substituting the test class

[bric3]

In the deprecated org.junit.platform.runner.JUnitPlatform, one could easily write the following

@RunWith(CustomRunner.class)
public MyTest {
  // ...
}

with

public class CustomRunner extends JUnitPlatform {
  public CustomRunner(final Class<?> clazz)
      throws NoSuchFieldException, SecurityException, IllegalArgumentException,  IllegalAccessException {
    super(shadowTestClass(clazz));
  }
  // ...
  private static Class shadowTestClass(Class testClass) {
    return modified test class, poissibly with custom classloader;
  }
}

I'm not quite sure it's possible to do the same using current JUnit feature set. From I found, in particular in ClassSelectorResolver the class is loaded then immediately passed to the ClassTestDescriptor which is then propagated along the way.

I've also seen this selector org.junit.platform.engine.discovery.DiscoverySelectors#selectClass(java.lang.ClassLoader, java.lang.String) but I'm not sure how that fits in the JUnit 5 extensions at this point.

[sormuras]

This sounds like a job for a custom test instance factory: https://docs.junit.org/current/user-guide/#extensions-test-instance-factories

Common use cases include acquiring the test instance from a dependency injection framework or invoking a static factory method to create the test class instance.

[bric3]

I'll look into this avenue. Thanks a lot.

I wonder if the ctx.getRequiredTestClass() in extensions method like public void beforeAll(ExtensionContext ctx) can see the modified class though.

Indeed after exploring selectors it appears a bit too early in the process, it even appears in gradle itself.

[bric3]

Hey didn't had a chance to go back to you, thanks for the suggestion it's very useful ! However after some discussions with teammates, it's probably better to not go that path if there are other ways.
Thank you !

[sbrannen]

To support your use case, you may be able to register a custom LauncherInterceptor. See the example in the User Guide for details.

Related Issues

• Introduce extension API for providing a custom ClassLoader (e.g., for Powermock) #201
• Revise ClassLoader lookups across the platform #806
• Introduce extension API for customizing the ClassLoader in Jupiter #3028

[sbrannen]

As a side note, if you're interested in support for a custom ClassLoader, you really should search the open issues for discussions related to "ClassLoader" before starting a discussion.

[bric3]

Thanks for the feedback I was initially interested to load a "customized" test class. That's why I didn't found anything that might apply with the other classloader discussions.

However, after some sleep, we may want to look at something more regular and drop the existing approach that patch the test class.