Spring MVC Controller

正在嘗試用junit5和mockito測試我的web層（springboot，springmvc）。http方法的所有其他測試（get、put等）工作正常，但更新。遵循代碼。

Controller:

@PutMapping(value = "{id}")
public ResponseEntity<?> putOne(@PathVariable Integer id, @Valid @RequestBody Customer customerToUpdate) {
    Customer updated = customerService.update(id, customerToUpdate);
    return ResponseEntity.ok(updated);
}    

Service:

public Customer update(Integer customerId, Customer customerToUpdate) {
    Customer customerFound = customerRepository.findById(customerId).orElseThrow(() -> {
        throw new CustomerControllerAdvice.MyNotFoundException(customerId.toString());
    });

    customerToUpdate.setId(customerFound.getId());

    return customerRepository.save(customerToUpdate);
}

最后，測試：

static final Customer oneCustomer = Customer.of(3,"john", LocalDate.of(1982, 11, 8));
    
@Test
void putOneTest() throws  Exception {
    when(customerService.update(oneCustomer.getId(), oneCustomer)).thenReturn(oneCustomer);

    mockMvc.perform(put(CUSTOMER_URL + oneCustomer.getId())
                    .contentType(MediaType.APPLICATION_JSON)
                    .content(objectMapper.writeValueAsString(oneCustomer)))
            .andDo(print())
            .andExpect(jsonPath("$.name").value(oneCustomer.getName()))
                .andExpect(jsonPath("$.birthDate").value(oneCustomer.getBirthDate().toString()))
                .andExpect(status().isOk());
}

Result:

java.lang.AssertionError: No value at JSON path "$.name"

CustomerService中的update(...）方法只返回null。我不明白怎么回事。請給出建議。