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"I want to be able to mock the REST callout without performing any actual i/o."

Why? Because it's hard for you to mock out actual I/O, which could be solved by a tool that makes it easy to mock out actual I/O.

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We're not in the business of testing I/O. We outsource whether or not I/O is correct that to the people writing the JVM.

Testing it as part of your test cases violates the "Test one thing and one thing exactly" principle. The goal is to reduce the surface area of the test as far as possible to distill what the author of the code wrote is correct.

That's an arbitrary division of "one thing" though. You've decided the "one thing" includes the HTTP request and response, but it doesn't involve the socket call for that request and response. I'm not saying the test has to involve a network, or a network card, or anything like that, but perhaps a more accurate reflection of the "unit" to be tested is something that reads and writes to a channel or even a socket (and again, not necessarily a TCP/IP socket).

In reality, you're actually testing a bunch of method invocations that won't happen when the code is being used, and to do it, you're also breaking a lot of the contracts that one would otherwise expect to be maintained... maybe that's not the optimal way to do things.

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