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The send is being called by my SUT and the test is on the return of the function calling send in the SUT.

I am not sure if I understand you correctly and I don’t know Vert.x but can’t you just mock the object which has this `send` method and return a mock from it, which will return you a message you want

If it is not the case can you share a snippet of the test code?

If you look at the stackoverflow, that's what I'm trying to do -- but it's not working.

But what you mean by ‘not working’, any errors?

No, my test just freezes and doesn't continue