Java Controller单元测试JSON响应与实际API不符问题排查

问题概述 #

测试depositUserBalance端点控制器时遇到两个问题：

• 测试生成的请求体为{"balance":3000.0}，但根据UserAmountDTO的配置，请求体应仅包含amount字段（amount标记为WRITE_ONLY）。
• 测试断言时响应内容为空字符串，报错org.opentest4j.AssertionFailedError: expected: [{"balance":3000.0}] but was: []，但Postman调用API能正常返回带balance的JSON。

相关代码 #

控制器代码

@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);
}

DTO代码

@Getter
@Setter
@AllArgsConstructor
public class UserAmountDTO {
    
    @JsonProperty(access = Access.WRITE_ONLY)
    public double amount;

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

}

原测试代码

@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);
}

问题原因及修复 #

问题1：请求体包含无效的balance字段 #

原因：直接序列化完整的UserAmountDTO实例时，ObjectMapper默认会序列化所有public字段，忽略了@JsonProperty(access = Access.READ_ONLY)的序列化规则（除非配置了特定的序列化过滤）。

修复方式：
选择以下任意一种方式构造正确的请求体：

1. 手动编写符合要求的JSON字符串：

   String requestBody = "{\"amount\":2000.0}";
   

2. 配置ObjectMapper仅序列化WRITE_ONLY字段：

   ObjectMapper requestMapper = new ObjectMapper();
   SimpleFilterProvider filterProvider = new SimpleFilterProvider();
   filterProvider.addFilter("writeOnlyFilter", SimpleBeanPropertyFilter.filterOutAllExcept("amount"));
   requestMapper.setFilterProvider(filterProvider);
   String requestBody = requestMapper.writeValueAsString(new UserAmountDTO(2000.0, 0));
   

问题2：响应内容为空 #

原因：mockuserService.updateUserBalance时使用了具体的userAmountDTO实例作为参数匹配条件，但Spring MVC处理请求时会自动创建新的UserAmountDTO实例，导致mock方法未被触发，返回null，最终控制器返回null，响应内容为空。

修复方式：
使用Mockito参数匹配器替代具体实例，确保任何UserAmountDTO实例调用都能触发mock返回：

// 导入所需类：org.mockito.ArgumentMatchers.eq、org.mockito.ArgumentMatchers.any
when(userService.updateUserBalance(eq(1), any(UserAmountDTO.class))).thenReturn(userAmountDTO);

修复后的完整测试代码 #

import static org.mockito.ArgumentMatchers.any;
import static org.mockito.ArgumentMatchers.eq;

@Test
public void Test_DepositUserBalance() throws Exception {
    // Arrange
    UserAmountDTO mockResponseDto = new UserAmountDTO(0, 3000.0);
    when(request.getAttribute("userId")).thenReturn(1);
    // 用参数匹配器确保mock触发
    when(userService.updateUserBalance(eq(1), any(UserAmountDTO.class))).thenReturn(mockResponseDto);
    // 构造正确的请求体
    String requestBody = "{\"amount\":2000.0}";

    // 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 = "{\"balance\":3000.0}";
    assertEquals(HttpStatus.OK.value(), status);
    assertEquals(expectedJson, content);
}

内容的提问来源于stack exchange，提问作者Jessica