SpringMvc上传文件遇到重复读取InputStream的问题

---

文件上传配置：

<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代码

@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计算md5
    uploadService.upload(file.getInputStream());
  }
}

单元测试代码

@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);
    }
}

遇到问题：
Controller代码可以正常上传，而单元测试代码上传只有0字节

原因：
在计算stream的md5时，已经将流的游标移动到了最后，如果不做任何处理直接使用，则无法再从流里读到字节。而Controller里的file.getInputStream()实际每次返回的并不是同一个流，也就是计算md5和传给upload方法的并不是同一个stream，所以Controller里的upload成功的取到了数据。单元测试的stream时手动创建的，计算md5和upload的都是同一个流，所以upload时流的游标已经在尾部了。

修改单元测试代码为：

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

关于mark和reset的理解，可以参考通过mark和reset方法重复利用InputStream