function postImg(){
 if ($.trim($("#imgFile").val()) == "") { 
   alert("请选择图片！"); 
   return; 
  } 
 console.log($("#imgFile")[0].files[0].size);//小于100*1024，下面的请求就可以成功
 var option = {
  url : '/cloudnote/user/insertUserPhoto.do',
  type : 'POST',
//  dataType : 'json',
  headers : {"ClientCallMode" : "ajax"}, //添加请求头部
  success : function(XMLHttpRequest, textStatus, errorThrown){
   console.log(XMLHttpRequest);
   console.log(textStatus);
   console.log(errorThrown);
   console.log("前端输出上传成功");
   $("#imgClose").click();
  },
  error: function(XMLHttpRequest, textStatus, errorThrown) {
   console.log(XMLHttpRequest);
   console.log(textStatus);
   console.log(errorThrown);
   console.log("前端输出上传失败"); 
  }
 };
 $("#imgForm").ajaxSubmit(option);
 return false; 
}

<form id="imgForm" > 
　　<div class="modal-content">
 　　<div class="modal-header">
   <button type="button" class="close" data-dismiss="modal" aria-hidden="true">×</button>
   <h4 class="modal-title" id="myModalLabel">修改头像</h4>
  </div>
  <div class="modal-body">
  　　<input type="file" id="imgFile" name="imgFile"/> 
   <input id="imgId" name="userId" value="${user.id }" style="display:none" />
  </div>
  <div class="modal-footer">
  　　<button type="button" class="btn btn-default" data-dismiss="modal" id="imgClose">关闭</button>
   <button type="button" class="btn btn-primary" onclick="postImg();" id="imgSubmit">上传</button>
  </div>
 </div>
</form>

//更新用户头像
 @RequestMapping(value="/insertUserPhoto.do",method = RequestMethod.POST)
 public void insertUserPhoto(
   HttpServletRequest req, HttpServletResponse res){
  System.out.println("----- 插入图片 -------");
  try{
   String id = req.getParameter("userId"); 
   System.out.println(id);
   MultipartHttpServletRequest multipartRequest = (MultipartHttpServletRequest) req;
   MultipartFile file = multipartRequest.getFile("imgFile");
   byte[] photo = file.getBytes();
   boolean result = serv.insertUserPhoto(id, photo); 
   res.setContentType("text/html;charset=utf8"); 
   res.getWriter().write("result:" + result);   
  }catch(Exception e){
   e.printStackTrace();
  }
  System.out.println("----- 插入图片end -------");
 }
 /**
  * 读取用户头像
  * @param req
  * @param res
  */
 @RequestMapping(value="/readPhoto.do", method=RequestMethod.GET)
 public void readPhoto(HttpServletRequest req, HttpServletResponse res){
  System.out.println("------readPohto-----");
  String id = Utils.getSessionUserId(req);
  try {
   User user = serv.selectUserPhoto(id);
   res.setContentType("image/jpeg");
   res.setCharacterEncoding("utf-8"); 
   OutputStream outputStream = res.getOutputStream(); 
   InputStream in = new ByteArrayInputStream(user.getPhoto()); 
   int len = 0; 
   byte[] buf = new byte[1024]; 
   while((len = in.read(buf,0,1024)) != -1){ 
    outputStream.write(buf, 0, len);
   } 
   outputStream.close(); 
  } catch (IOException e) { 
   e.printStackTrace(); 
  } 
  System.out.println("-----readPohto end-----");
  return;
 }

//查找用户图片（头像）
 public User selectUserPhoto(String id) throws ImageException {
  User user = userDao.findUserById(id);
  if(user == null){
   throw new UserNameException("用户名不存在!");
  }
  Map<String, Object> data = userDao.selectUserPhoto(id);
  System.out.println(data);
  user.setPhoto((byte[]) data.get("photo"));
  return user;
 }
 //更新用户图片（头像）
 public boolean insertUserPhoto(String userId, byte[] photo) throws ImageException, UserNameException {
  if(userId == null || userId.trim().isEmpty()){
   throw new UserNameException("用户id不存在");
  }
  User user = userDao.findUserById(userId);
  if(user == null){
   throw new UserNameException("用户不存在");
  }
  user.setPhoto(photo);
  int n = userDao.updateUserPhoto(user);
  System.out.println("插入图片：" + n);
  return n==1?true:false; 
 }

<!-- 更新图片 -->
 <update id="updateUserPhoto" parameterType="cn.tedu.note.entity.User"> 
  UPDATE user set id = #{id}, photo = #{photo,jdbcType=BLOB} <!-- 这里试了，如果不加jdbcType=BLOB 会出错，虽然不是很理解，但也照做了 --> 
  WHERE id = #{id}
 </update> 
 <!-- 获取图片 -->
 <select id="selectUserPhoto" parameterType="String" resultType="Map"> 
   SELECT id as id, photo as photo from user 
   WHERE id=#{id} 
 </select>

 <!-- 文件上传表单的视图解析器 --> 
 <bean id="multipartResolver" class="org.springframework.web.multipart.commons.CommonsMultipartResolver"> 
  <property name="maxUploadSize"><value>100000</value></property> 
  <property name="defaultEncoding"><value>UTF-8</value></property> 
 </bean>