Java - Incorrect output for response JSON in unit test

I am testing my depositUserBalance endpoint controller and this is just a simple test but I am not sure why it is not getting the correct response JSON as compared to the one returned in my REST API.

// My Controller
@PutMapping("/api/deposit")
public ResponseEntity<UserAmountDTO> depositUserBalance(HttpServletRequest request,
                @RequestBody UserAmountDTO userAmountDTO) {
        Integer userId = (Integer) request.getAttribute("userId");
        userAmountDTO = userService.updateUserBalance(userId, userAmountDTO);
        return new ResponseEntity<>(userAmountDTO, HttpStatus.OK);
}

// My UserAmountDTO Data Transfer Object
@Getter
@Setter
@AllArgsConstructor
public class UserAmountDTO {
    
    @JsonProperty(access = Access.WRITE_ONLY)
    public double amount;

    @JsonProperty(access = Access.READ_ONLY)
    public double balance;

}

@Test
public void Test_DepositUserBalance() throws Exception {
    // Arrange
    UserAmountDTO userAmountDTO = new UserAmountDTO(2000.0, 3000.0);
    HttpServletRequest request = mock(HttpServletRequest.class);
    when(request.getAttribute("userId")).thenReturn(1);
    when(userService.updateUserBalance(1, userAmountDTO)).thenReturn(userAmountDTO);
    String requestBody = objectMapper.writeValueAsString(userAmountDTO);
    System.out.println("requestBody: " + requestBody);

    // Act
    MvcResult mvcResult = mockMvc.perform(put(DEPOSIT_PATH)
            .contentType(MediaType.APPLICATION_JSON)
            .content(requestBody).with(requestBuilder -> {
                requestBuilder.setAttribute("userId", 1);
                return requestBuilder;
            }))
            .andExpect(status().isOk())
            .andReturn();

    // Assert
    int status = mvcResult.getResponse().getStatus();
    String content = mvcResult.getResponse().getContentAsString();
    String expectedJson = String.format("{\"balance\":%.1f}", 3000.0);
    assertEquals(HttpStatus.OK.value(), status);
    assertEquals(expectedJson, content);
}

In my unit test, the requestBody was returned as balance here requestBody: {"balance":3000.0} when it should be amount because I set the @JsonProperty(acccess = Access.WRITE_ONLY) to amount.

The error from the test was org.opentest4j.AssertionFailedError: expected: [{"balance":3000.0}] but was: []. Why is it an empty string? When I tried it in the REST API on POSTMAN, it actually returned me this JSON balance amount. Coupled with the fact that I added @JsonProperty(acccess = Access.READ_ONLY) to balance.

UPDATE ON UNIT TESTING POST REQUEST ON RESTCONTROLLER

I managed to resolve the mistake and in summary it is because I was passing in an object to the mocked service a different memory address. To put it simply, when(userService.updateUserBalance(1, userAmountDTO)).thenReturn(userAmountDTO);, this is an appropriate way to mock the userService.updateUserBalance method.

However, inside the @Test, the userAmountDTO memory address could be something like @21835tj3. In my controller, userAmountDTO = userService.updateUserBalance(userId, userAmountDTO);, the memory address of userAmountDTO being passed into the userService.updateUserBalance method could be different like @21943jf3 which was why it does not match the mocked service arguments in the unit test. Objects with the same fields and values doesn't mean they share the same reference, thus the mocked method did not return the expected value.

To fix this:

Use the ArgumentMatchers.any() Mockito to match any object of the UserAmountDTO class. So, change the mocked method in the unit test to when(userService.updateUserBalance(eq(1), ArgumentMatchers.any(UserAmountDTO))).thenReturn(userAmountDTO);

This usually happens with POST requests as we pass in a deserialized JSON object. Also thanks to @JorgeCampos for supporting with the debug process.