取BLOB图片输出到页面异常 抛SQLException必须登陆到服务器


Blob fileData = result.getZjwj();
             InputStream pi = null;
              int blobsize = 0;
                 try{
                     pi = fileData.getBinaryStream();
                     blobsize = (int)fileData.length();
                 } catch(SQLException ex) { 
                  ex.printStackTrace();
                 }
                 if(blobsize > 0){
                  response.reset();
                     response.setContentType("image/jpeg");
                     OutputStream sos = response.getOutputStream();
                     byte blobbytes[] = new byte[blobsize];
                     for(int bytesRead = 0; (bytesRead = pi.read(blobbytes)) != -1;)
                         sos.write(blobbytes, 0, bytesRead);                     pi.close();
                     sos.flush();
                 }else{
                  RequestDispatcher dis = request.getRequestDispatcher("../images/nophoto1.jpg");
                     dis.forward(request, response);
                 }在pi = fileData.getBinaryStream();时每三次操作就会出现一次SQLException必须登陆到服务器
而且有时该异常报的非常勤快，但不是每次都发生请问这可能是由于什么原因引起的？