Testing Java Callbacks with Mockito

If you're working with Java code that works with callbacks, you may be having some difficulty with mocking the response.

I had this problem recently when working with the Wiremock project, in which there was a class implementing the RequestHandler interface:

public interface RequestHandler {
	void handle(Request request, HttpResponder httpResponder);
}

This method call expects the HttpResponder to be a callback that can return the Response to the caller:

public interface HttpResponder {
    void respond(Request request, Response response);
}

I had control over the RequestHandler, but didn't want to inject in a factory or something that may add unnecessary complexity, but fortunately Mockito's doAnswer came to the rescue:

Request request = mock(Request.class);
Response response = mock(Response.class);
// setup

doAnswer((i) -> {
    HttpResponder responder = i.getArgument(1, HttpResponder.class);
    responder.respond(request, response);
    return null;
}).when(handler()).handle(any(), any());

This allows us to retrieve the real implementation of a HttpResponder that's passed into the method, then invoke it with the arguments we want to inject in.