Introduce `ConversionSupport` in `junit-platform-commons`

[Michael1993]

DefaultArgumentConverter used by JUnit is marked as internal. I'm not sure why that is but I think it'd be great if we got some support for argument conversion, so I'm proposing partially or entirely exposing or mirroring it in the support package (or somewhere else where it makes sense).

Or if the JUnit team doesn't want to support it, maybe Pioneer can host something?

Deliverables

• All or partial functionality of DefaultArgumentConverter exposed.

[marcphilipp]

Would it suffice to introduce a StringConversionSupport class in org.junit.platform.commons.support with the following method (possibly supporting Class<T> and T as a return type)?

Object convert(String source, Class<?> targetType, ClassLoader classLoader)

[Michael1993]

That sounds superb! I'm just not entirely sure why you need ClassLoader in there?

[sbrannen]

I'm just not entirely sure why you need ClassLoader in there?

A custom ClassLoader is necessary to support use cases where classes are dynamically loaded.

A concrete example is converting from a fully-qualified class name (i.e., a String) to a Class.

We introduced ClassLoader arguments in numerous APIs in JUnit 5.10 to support custom ClassLoader arrangements. Consult the release notes for details.

---

By the way, we would likely accept null as the ClassLoader argument to imply that the default ClassLoader should be used.