<select id="xxx" fetchSize="100" resultSetType="FORWARD_ONLY">
  ...
</select>

@Override
protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain)
      throws ServletException, IOException {

  response = new ShallowEtagResponseWrapper(response);    //1.将所有的请求包装成 ShallowEtagResponseWrapper

  filterChain.doFilter(request, response);        //2. 其他 filter

  updateResponse(request, response);      //3.返回给客户端之前处理一次 response
}

/**
 * {@link HttpServletRequest} wrapper that buffers all content written to the
 * {@linkplain #getOutputStream() output stream} and {@linkplain #getWriter() writer},
 * and allows this content to be retrieved via a {@link #toByteArray() byte array}.
 */
private static class ShallowEtagResponseWrapper extends HttpServletResponseWrapper {

  private final ByteArrayOutputStream content = new ByteArrayOutputStream();

  private final ServletOutputStream outputStream = new ResponseServletOutputStream();

  private PrintWriter writer;
  ...
  public PrintWriter getWriter() throws IOException { //获取 writer, 采用自定义的 ResponsePrintWriter, 后续的写会将内容写到 content 中
      if (this.writer == null) {
          String characterEncoding = getCharacterEncoding();
          this.writer = (characterEncoding != null ? new ResponsePrintWriter(characterEncoding) :
                  new ResponsePrintWriter(WebUtils.DEFAULT_CHARACTER_ENCODING));
      }
      return this.writer;
  }
  private byte[] toByteArray() {  // 读取 content 内容
      return this.content.toByteArray();
  }

  // 自定义 ServletOutputStream, write操作将内容写入到 content 中
  private class ResponseServletOutputStream extends ServletOutputStream {
      @Override
      public void write(int b) throws IOException {
          content.write(b);   //写到 content 中
      }
      ...
  }

  // 自定义 PrintWriter, 写入到 content 中
  private class ResponsePrintWriter extends PrintWriter {
      private ResponsePrintWriter(String characterEncoding) throws UnsupportedEncodingException {
          super(new OutputStreamWriter(content, characterEncoding));
      }
      ...
  }
}

private void updateResponse(HttpServletRequest request, HttpServletResponse response) throws IOException {
  ...
  byte[] body = responseWrapper.toByteArray();        //缓冲起来的数据
  int statusCode = responseWrapper.getStatusCode();

  if (isEligibleForEtag(request, responseWrapper, statusCode, body)) {
      String responseETag = generateETagHeaderValue(body);    //基于内容生成 ETag
      response.setHeader(HEADER_ETAG, responseETag);      //设置 ETag Header

      String requestETag = request.getHeader(HEADER_IF_NONE_MATCH);   //If-None-Match 
      if (responseETag.equals(requestETag)) {                 //请求的If-None-Match和ETag相同, 则304
          response.setStatus(HttpServletResponse.SC_NOT_MODIFIED);    //304
      } else {
          copyBodyToResponse(body, response);     //将缓冲的内容写入到 response 中
      }
  } else {
      ...
      copyBodyToResponse(body, response);
  }
}

private void copyBodyToResponse(byte[] body, HttpServletResponse response) throws IOException {
  if (body.length > 0) {
      response.setContentLength(body.length);         //设置 Content-Length
      FileCopyUtils.copy(body, response.getOutputStream());       //ShallowEtagResponseWrapper数据拷贝到response中
  }
}