Cannot understand mocks - spring-boot

Question

I'm trying to write tests for my Spring Boot application that has some end points, it's a REST application.
I have the "usual" simple web application with a controller, a service and a repository. CRUD operations.
In my update endpoint I call the service layer to perform the update, like this:
#PutMapping
public Post updatePost(#RequestBody Post post) {
return postService.updatePost(post);
}
The updatePost method on the PostService class makes some checks about the object before updating in it, and if the checks all pass, then the update operation is perforrmed, like this:
public Post updatePost(Post post) {
if (post == null || post.getId() == null) {
throw new PostGenericException();
}
Post postToUpdate = postRepo.findById(post.getId()).orElseThrow(PostGenericException::new);
bool isOk = true;
// some other checks..
if (!isOk) {
throw new PostGenericException();
}
// update operation
postToUpdate.setMessage(post.getMessage());
....
return postRepo.save(postToUpdate);
}
From what I've seen online in the test class I have to do something like this:
#WebMvcTest(PostController.class)
public class PostControllerTest {
#Autowired
private MockMvc mockMvc;
#Autowired
private ObjectMapper mapper;
#MockBean
private PostService postService;
#Test
public void updatePost() throws Exception {
Post post = new Post(...);
Mockito.when(postService.updatePost(post)).thenReturn(post);
mockMvc.perform(MockMvcRequestBuilders.put("/posts")
.contentType(MediaType.APPLICATION_JSON)
.content(this.mapper.writeValueAsString(post)))
.andExpect(status().isOk())
.andExpect(jsonPath("$", notNullValue()));
}
}
So here in the test method I'm completely replacing the logic of the updatePost method of the service layer with a fake one.
How is this useful?
The only good reason I can think of is that here I'm trying to test the endpoint by itself, meaning that I simply want to check if I can reach that endpoint, but I don't really care about the implementation at all, i.e how the update operation is performed. I'm expecting that if I make a PUT request to that endpoint I get a result, if the test fails I know that the controller doesn't handler that endpoint anymore.
Is this all about it or am I missing something?
If I remember correctly, Kent Beck also said that you don't want to test the implementation but only the public APIs, it doesn't make much sense to test the implementation anyway, you could have a lot of tests and at some point have more test code than production code.

Answer 1

Using mocks may be against your testing philosophy, but looking at the practicalities:
Web layer is a nice chunk to be tested separately. There is a good amount of things that you can check:
routing
request and response deserialization
validation
error handling
authentication
This also allows business logic tests to skip these concerns.
Additional benefits:
they are easy to set up and run on a single machine (or even single process)
they are reasonably fast