Unit testing for Spring Boot API

First of all, if you write your tests to cover "required level of tests" or requirement to "have some tests at all" having the production implementation already done, it is slightly too late. In the majority of cases having tests first, based on your requirements, contract, use case or anything it more optimal approach. Nevertheless, I don't know your situation and the thing you're trying to implement, so treat it as a suggestion and move on to the key thing you are asking about.

Your JUnit (preferably 5) and Mockito tests, which probably use MockMvc are very good unit(-like) tests to cover web communication concerns such as: HTTP request types, content type, encoding, input and output parameters JSON (de)serialization, error handling, etc. They are best to test with the service layer mocked. Thanks to that you can easily cover a lot of web cases without the need to prepare data in a database, etc.

The core logic has to also be tested. Depending what it is, it might be feasible to test it in the unit way (the easiest to write, can cover a lot of - also corner - cases). It could be supplemented with some set of integration tests to verify that it works fine also in integration (Spring Beans, DB, etc.).

If desired, you may also write some E2E test from the web call via (real) HTTP requests through controllers, services to a database/datastore (if any), but I would limit it only to the most important scenarios to use it in your CI/CD pipeline to verify that a deployment finished successfully.

Disclaimer. I've found this approach useful in multiple situations, but definitely in some other circumstances it might be good to change the balance point to better apply testing.

I think you are probably getting confused between unit and Integration tests.

If you are using Mockito you are probably referring to unit tests wherein, the scope of the Test Class should be only the current class.

Any external method calls should be mocked.So in your case the service calls should be mocked in case you are writing unit test for your controller class.

Your Test Suite should contain

• Junit for Controller-- To Test the interface contract points like HTTP Method, Request Parameters, Mandatory inputs, Valid Request Payloads.
• Junit for all other classes including Service classes- This is to test your application classes core logic
• Integration Test- Finally an integration test which can hit your controller endpoints with service classes and rest of the application code functionality.