Add JUnit 5 OutputCapture extension

[philwebb]

We'll need to provide JUnit 5 versions of our @Rule classes (such as OutputCapture).

[d1231]

Hi,

Is someone working on it?

Can I work on it?

I thought of starting with OutputCapture, using Junit5 Extension that implements BeforeTestExecutionCallback, AfterTestExecutionCallback, ParameterResolver.

[philwebb]

@d1231 I think @mbhave has started to think about JUnit5 support. Also @sbrannen has a proof of concept for this here.

[sbrannen]

Indeed... and I've heard that some people actually use that proof of concept in their projects.

For example, Pivotal's education team has included it in the most recent versions of the Core Spring training course as a replacement for the OutputCapture rule support.

[sbrannen]

FYI: this test class demonstrates example usage.

[sbrannen]

If Boot provides a Jupiter extension for OutputCapture, I would recommend the following (unless the Boot team feels that migration from the rule-based OutputCapture should be as simple as possible).

1. Base expectations on AssertJ instead of Hamcrest.
2. Track SYSOUT and SYSERR separately so that developers can assert what was written to which stream.