Load dependency in webmvctest

I have a rest controller which I am trying to Unit Test:

It has a few dependencies autowired

@RestController
@RequestMapping("/test")
public class TestController {

    private final Dep1 dep1;
    private final Dep2 dep2;
    private final Dep3 dep3;

    public TestController(final Dep1 dep1,
                          final Dep2 dep2,
                          final Dep3 dep3) {
        this.dep1 = dep1;
        this.dep2 = dep2;
        this.dep3 = dep3;
    }
   
}

I recently added dep3

@Service
public class Dep3 {

    private final IValidator validator;

    public Dep3(final IValidator validator) {
        this.validator= validator;
    }

    public void validate(final Request req) {
        validator.validate(req);
    }

}

Dep3 has its own Autowired Dependency IValidator

Here is my test class:

@WebMvcTest(TestController.class)
public class TestControllerTest {

    @MockBean
    private Dep1 dep1;

    @MockBean
    private Dep2 dep2;

    @MockBean
    private Dep3 dep3;

    @Autowired
    private MockMvc mockMvc;

    @Test
    public void someTest() throws Exception {

    }

    @TestConfiguration
    static class InnerConfiguration {

        @Bean
        IValidator validator() {
            return new SomeValidator();
        }
    }

}

For my test, I need the code to actually run the dep3.validate(..) with the implementation SomeValidator(). I am unsure how to achieve this. Perhaps I am missing an annotation?

Don't mock Dep3. Instead configure Dep3 bean in @TestConfiguration:

@TestConfiguration
static class InnerConfiguration {

        @Bean
        Dep3 dep3() {
            return new Dep3(validator());
        }

        @Bean
        IValidator validator() {
            return new SomeValidator();
        }
    }

Spring will inject fully configured Dep3 into TestController

You have to configure your Mock correctly.
Something like this:

Mockito.when(dep3.validate(Mockito.any(Request.class))).thenReturn(new SomeValidator());

You can do this in your test method or in the setup method before the actual call.