为什么SpringMVC中请求的body不支持多次读取

{
  "timestamp": "2019-12-27T02:48:50.544+0000",
  "status": 400,
  "error": "Bad Request",
  "message": "Required request body is missing: ...省略非关键信息...",
  "path": "/"
}

@Slf4j
@WebFilter
public class InputStreamFilter implements Filter {

  @Override
  public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws ServletException, IOException {
    ServletInputStream inputStream=servletRequest.getInputStream();
    String charSetStr = servletRequest.getCharacterEncoding();
    if (charSetStr == null) {
      charSetStr = "UTF-8";
    }
    Charset charSet = Charset.forName(charSetStr);
    log.info("请求的body为:\n{}",StreamUtils.copyToString(inputStream,charSet));
    filterChain.doFilter(servletRequest,servletResponse);
  }

  @Override
 protected <T> Object readWithMessageConverters(NativeWebRequest webRequest, MethodParameter parameter,
  Type paramType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
    ...省略非关键代码...
    // 关键代码
 Object arg = readWithMessageConverters(inputMessage, parameter, paramType);
 if (arg == null && checkRequired(parameter)) {
  throw new HttpMessageNotReadableException("Required request body is missing: " +
   parameter.getExecutable().toGenericString(), inputMessage);
 }
 return arg;
 }

  @Nullable
 protected <T> Object readWithMessageConverters(HttpInputMessage inputMessage, MethodParameter parameter,
  Type targetType) throws IOException, HttpMediaTypeNotSupportedException, HttpMessageNotReadableException {
    ...省略非关键代码...
 EmptyBodyCheckingHttpInputMessage message;
 try {
   // 此处调用构造方法时进行了流的读取
  message = new EmptyBodyCheckingHttpInputMessage(inputMessage);
      for (HttpMessageConverter<?> converter : this.messageConverters) {
  Class<HttpMessageConverter<?>> converterType = (Class<HttpMessageConverter<?>>) converter.getClass();
  GenericHttpMessageConverter<?> genericConverter =
   (converter instanceof GenericHttpMessageConverter ? (GenericHttpMessageConverter<?>) converter : null);
  if (genericConverter != null ? genericConverter.canRead(targetType, contextClass, contentType) :
   (targetClass != null && converter.canRead(targetClass, contentType))) {
   if (message.hasBody()) {
   ...省略非关键代码...
   }
   else {
     // 此处是处理流读取返回null的情况
   body = getAdvice().handleEmptyBody(null, message, parameter, targetType, converterType);
   }
   break;
  }
  }
 }
 catch (IOException ex) {
  throw new HttpMessageNotReadableException("I/O error while reading input message", ex, inputMessage);
 }

 ...省略非关键代码...

 MediaType selectedContentType = contentType;
 Object theBody = body;
 LogFormatUtils.traceDebug(logger, traceOn -> {
  String formatted = LogFormatUtils.formatValue(theBody, !traceOn);
  return "Read \"" + selectedContentType + "\" to [" + formatted + "]";
 });

 return body;
 }
 

    // 从请求中获取到的InputStream对象
    @Nullable
 private final InputStream body;

 public EmptyBodyCheckingHttpInputMessage(HttpInputMessage inputMessage) throws IOException {
  this.headers = inputMessage.getHeaders();
  InputStream inputStream = inputMessage.getBody();
  // 判断InputStream是否支持mark
  if (inputStream.markSupported()) {
    // 标记流的起始位置
  inputStream.mark(1);
  // 读取流
  this.body = (inputStream.read() != -1 ? inputStream : null);
  // 重置流，下次读取会从起始位置重新开始读取
  inputStream.reset();
  }
  else {
    // 回退输入流，支持重复读取的InputStream
  PushbackInputStream pushbackInputStream = new PushbackInputStream(inputStream);
  // 读取流
  int b = pushbackInputStream.read();
  if (b == -1) {
    // 返回-1表示流无数据存在
   this.body = null;
  }
  else {
    // 流存在数据，直接赋值
   this.body = pushbackInputStream;
   // 回退流到起始位置，等价于reset()方法
   pushbackInputStream.unread(b);
  }
  }
 }