SpringMvc上傳文件遇到重復讀取InputStream的問題

---

文件上傳配置：

<bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver">
  <property name="defaultEncoding" value="utf-8"></property>
  <property name="maxUploadSize" value="10485760000"></property>
  <property name="maxInMemorySize" value="40960"></property>
</bean>

Controller代碼

@RequestMapping("/file")
@RestController
public class HelloController {
  @Autowired
  private UploadService uploadService;

  @RequestMapping(value="/upload", method= RequestMethod.POST)
  public void upload(@RequestParam(value = "file", required = true) MultipartFile file) {
    String md5 = DigestUtils.md5Hex(file.getInputStream()); //使用commons-codec計算md5
    uploadService.upload(file.getInputStream());
  }
}

單元測試代碼

@ContextConfiguration(locations = "classpath*:META-INF/spring/spring-*.xml")
@RunWith(SpringJUnit4ClassRunner.class)
public class UploadServiceTests extends BaseTests {
    private static final Logger logger = LoggerFactory.getLogger(UploadServiceTests.class);

    @Autowired
    private UploadService uploadService;

    @Test
    public void upload() throws Exception {
      Integer num = new Random().nextInt(9999999);
      byte[] bytes = ("HelloWorld-" + num).getBytes();
      InputStream inputStream = new ByteArrayInputStream(bytes);
      String md5 = MD5Utils.getMD5(inputStream);
      uploadService.upload(inputStream);
    }
}

遇到問題：
Controller代碼可以正常上傳，而單元測試代碼上傳只有0字節

原因：
在計算stream的md5時，已經將流的游標移動到了最后，如果不做任何處理直接使用，則無法再從流里讀到字節。而Controller里的file.getInputStream()實際每次返回的並不是同一個流，也就是計算md5和傳給upload方法的並不是同一個stream，所以Controller里的upload成功的取到了數據。單元測試的stream時手動創建的，計算md5和upload的都是同一個流，所以upload時流的游標已經在尾部了。

修改單元測試代碼為：

InputStream inputStream = new ByteArrayInputStream(bytes);
inputStream.mark(0);
String md5 = MD5Utils.getMD5(inputStream);
inputStream.reset();

關於mark和reset的理解，可以參考通過mark和reset方法重復利用InputStream