MockHttpServletResponse returning empty body for Pageable endpoint

I have the following test code where I'm testing a Pageable endpoint that list all entries for student.

    @Autowired
private MockMvc mockMvc;

@MockBean
private StudentRepository studentRepository;

private PageableHandlerMethodArgumentResolver pageableArgumentResolver = new PageableHandlerMethodArgumentResolver();

@BeforeEach
public void init() {
    mockMvc = MockMvcBuilders.standaloneSetup(new StudentEndpoint(studentRepository))
            .setCustomArgumentResolvers(pageableArgumentResolver)
            .build();
}

@Test
@WithMockUser(username = "xx", password = "xx", roles = "USER")
public void whenListStudentUsingCorrectStudentUsernameAndPassword_thenReturnStatusCode200 () throws Exception {
    List<Student> students = asList(new Student(1L, "Legolas", "legolas@lotr.com"),
            new Student(2L, "Aragorn", "aragorn@lotr.com"));

    when(studentRepository.findAll()).thenReturn(students);


    mockMvc.perform(get("http://localhost:8080/v1/protected/students/"))
            .andExpect(status().isOk())
            .andDo(print());

    verify(studentRepository, times(1)).findAll();
}

The problem here is that the verify(studentRepository, times(1)).findAll(); doesn't work because MockHttpServletResponse is returning a null Body.

Thats my endpoint:

    @GetMapping(path = "protected/students")
public ResponseEntity<?> listAll (Pageable pageable) {
    return new ResponseEntity<>(studentDAO.findAll(pageable), HttpStatus.OK);
}

And my log:

   MockHttpServletResponse:
           Status = 200
    Error message = null
          Headers = []    
     Content type = null
             Body = 
    Forwarded URL = null  Redirected URL = null
          Cookies = []

Argument(s) are different! Wanted:
br.com.devdojo.repository.StudentRepository#0 bean.findAll(
    
);
-> at br.com.devdojo.TestingTestTech.whenListStudentUsingCorrectStudentUsernameAndPassword_thenReturnStatusCode200(TestingTestTech.java:68)

Actual invocations have different arguments:

br.com.devdojo.repository.StudentRepository#0 bean.findAll(

    Page request [number: 0, size 20, sort: UNSORTED]
);    

Could someone please help with the right way to test pageable response? Thanks.

Finally, I found how to fix it.

You just need to pass a Pageable object as parameter to your findAll method that returns a Pageable JSON.

Thats my new working code:

        Page<Student> pagedStudents = new PageImpl(students);

    when(studentRepository.findAll(isA(Pageable.class))).thenReturn(pagedStudents);


    mockMvc.perform(get("http://localhost:8080/v1/protected/students/"))
            .andExpect(status().isOk())
            .andDo(print());

    verify(studentRepository).findAll(isA(Pageable.class));

And the MockHttpServletResponse:

    MockHttpServletResponse:
           Status = 200
    Error message = null
          Headers = [Content-Type:"application/json"]
     Content type = application/json
             Body = {"content":[{"id":1,"name":"Legolas","email":"legolas@lotr.com"},{"id":2,"name":"Aragorn","email":"aragorn@lotr.com"}],"pageable":"INSTANCE","totalElements":2,"totalPages":1,"last":true,"size":2,"number":0,"sort":{"sorted":false,"unsorted":true,"empty":true},"first":true,"numberOfElements":2,"empty":false}
    Forwarded URL = null
   Redirected URL = null
          Cookies = []