Hi all. I’m trying to mock RestTemplate in a Sprin...

# spring

[Larry Garfield]

Hi all. I’m trying to mock RestTemplate in a Spring app for testing purposes. The only docs I’ve found for that is this page, which is Java-targeted: https://www.baeldung.com/spring-mock-rest-template

However, the expect() call there doesn’t seem to translate to Kotlin very easily. I… have no idea what requestTo() turns into, and IDE autocomplete is not being helpful. Any suggestions on what the Kotlin version of that would be?

[Jacob]

That article isn't really about mocking resttemplate. It's about mocking the server. If anything you're testing the template to ensure that it's generating the correct requests

[Jacob]

And it's just java. No reason I can think of for it not translating to kotlin

[Jacob]

https://docs.spring.io/spring-framework/docs/6.1.11/kdoc-api/spring-test/index.html I see kotlin extension functions for other spring mocks but not for the rest template tests. Which makes sense since resttemplate itself is practically deprecated.

But the extension functions are just for convenience. You should be able to test using basic java interop

[thanksforallthefish]

did you check the Github repository linked at the bottom of the article? I find it quite annoying when people omit imports in their sample code, but in this case you can see the whole source code, eg: https://github.com/eugenp/tutorials/blob/master/spring-web-modules/spring-resttemp[…]baeldung/mock/EmployeeServiceMockRestServiceServerUnitTest.java

that said, we recently migrated to org.springframework.web.client.RestClient since RestTemplate is kept around in maintenance mode only because it's used a lot, but it has a lot of limitations (like inability to stream), but we had a bunch of test using MockRestServiceServer bound to a RestTemplate before and did not have any problem

[Larry Garfield]

@thanksforallthefish Oh! Yes, the lack of imports in examples is super annoying, but that looks like it’s showing some of the pieces I’m missing. Let me see if those help.

I know RestTemplate is deprecated; we only just migrated off of Spring Boot 2. 😬 I have a ticket in our queue to upgrade to WebClient, but haven’t gotten there yet, so I’m stuck with this for now.

As for translating Java to Kotlin, I’m sure it can be, I’m just not very good at it yet. (Only using Kotlin a few months, and no JVM before that, and when examples don’t show the imports they’re using I have to just kinda guess why “requestTo” is undefined, and such.)

[Jacob]

FYI spring never implemented any similar testing tool for webclient. They recommended just using a mock server library

[Larry Garfield]

How does that even work? You stick a fake server behind the real rest client somehow? That seems… complicated.

(I’m coming from a PHP background; most HTTP clients in PHP have a very simple interface they support, making mocking/faking it super easy. All this server wrapper stuff is novel to me.)

[Larry Garfield]

OK, the imports got me further. It’s recognizing requestTo() now. But requestTo() returns a RequestMatcher instance, and the code is then trying to call andExpect().andRespond() on it. Neither of which methods appear to be defined.

[thanksforallthefish]

FYI spring never implemented any similar testing tool for webclient. They recommended just using a mock server library

but they extended MockRestServiceServer to support the even newer RestClient, so I guess they are still making up their minds, probably based on community feedback. A mock server library, like Wiremock or okhttp3.mockwebserver would also be an option and it would transparent for future changes (should you remove the RestTemplate at some point you would not need to change anything in the test). I find them more cumbersome to use though, but it's probably just a matter of habit

[thanksforallthefish]

to your problem, it's probably some as simple as wrong parenthesis position, andExpect is not a method on the RequestMatcher , eg:

import org.springframework.test.web.client.match.MockRestRequestMatchers.method
import org.springframework.test.web.client.match.MockRestRequestMatchers.requestTo
import org.springframework.test.web.client.response.MockRestResponseCreators.withSuccess
import org.springframework.http.HttpMethod

server.expect(
  requestTo("...")
)
.andExpect(
  method(HttpMethod.POST)
)
.andRespond(withSuccess())

[Jacob]

How does that even work? You stick a fake server behind the real rest client somehow? That seems… complicated.

You add a list of canned responses to the server. The server saves every request it gets. You look at the list of requests and assert that they look right.