List<FileItem> list = upload.parseRequest(request);list==null; why???

Servlet代码：

 1 public String doUploadPicture(HttpServletRequest request){
 2         String result = "error";
 3         try{
 4             String filepath = getServletContext().getInitParameter("photopath");//配置文件里面配置了照片路径？
 5             
 6             DiskFileItemFactory factory = new DiskFileItemFactory();// 创建文件项目工厂对象
 7             factory.setRepository(new File(TEMP_FOLDER));          // 设置临时文件夹 为TEMP_FOLDER
 8             factory.setSizeThreshold(1024 * 1024);                 // 设置缓冲区大小为 1M
 9             ServletFileUpload upload = new ServletFileUpload(factory);//用工厂实例化上传组件,ServletFileUpload用来解析文件上传请求
10             
11             List<FileItem> list = upload.parseRequest(request);// 解析结果放在List中
12             
13             FileItem item = getUploadFileItem(list);  // 获取上传的文件
14             String filename = getUploadFileName(item);    // 获取文件名
15             filename = filename.substring(filename.length() - 3, filename.length());   //取得原文件名的后缀名
16             String photo_prex = "." + filename;   
17             Date date = new Date();
18             SimpleDateFormat df = new SimpleDateFormat("yyyyMMddHHmmss");
19             String time = df.format(date);
20             filename = time + photo_prex;     //修改原来的文件名
21             // 真正写到磁盘上
22             item.write(new File(filepath, filename)); // 第三方提供的
23             //调用服务层插入一条数据
24             result = filename;
25         }catch(Exception e){
26             result = "error";
27         }finally{
28             return result;
29         }
30     }

JavaScript代码： 

 1 $("#ajaxUpload").click(function(){
 2         var title = $.trim($("#pic_title").val());
 3         if(title == ""){
 4             alert("请输入文件名！");
 5             return false;}
 6         
 7         var type = $(":input[type='file']").val().match(/^(.*)(\.)(.{1,8})$/)[3];
 8         type = type.toUpperCase();
 9         if (type == "JPG" || type == "PNG" ) {
10         } else { alert("上传类型有误,请上传jpg或png格式图片文件"); return false; }
11             
12         $.ajaxFileUpload({
13             url :'<%= server_path%>/UploadAction?action=uploadPicture', 
14             secureuri : false,
15             fileElementId : 'fileToUpload',
16             dataType : 'json',
17             success : function(dataResult, status) {
18                 if(dataResult.result == "ok" && status == "success" && dataResult.returnValue != ""){
19                     alert("success");
20                     var path = dataResult.returnValue;
21                     var url = "<%= server_path%>/UserAction?action=doAddPicture";
22                     var json = {
23                             "userid" : userid,
24                             "path" : path,
25                             "title" : title,
26                     }
27                     var dataResult  = postAjax(url, dataResult);
28                     if(dataResult.result == "ok"){
29                         alert("上传成功！");
30                         showUserPicture(path,title);
31                     }else{alert("失败！");}
32                     
33                 }
34             },
35             error : function(data,status,e){
36                 alert("上传出错！")
37             } 
38         })
39         return false;//？
40     })