严重: Servlet.service() for servlet () threw exception java.lang.NullPoint

[cyqcyq2008]

servlet运行错误：严重: Servlet.service() for servlet show_pathway threw exception java.lang.NullPointerException
把代码贴出来：我想根据前台传过来的参数“org_name”，通过连接数据库pathway，把“map_location” 属性里存储的图片路径取出来并把图片显示在浏览页。“map_location”存储的内容是："/WEB-INF/KEGG PATHWAY Glycolysis - Gluconeogenesis - Reference pathway (KO)_files/ko00010_1.017949.png", 这列的数据类型是varchar。错误在哪里？

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.PageContext;

import java.io.*;
import java.sql.*;

public class show_pathway extends HttpServlet {
	
  public void service(HttpServletRequest request,
  HttpServletResponse response) throws ServletException,IOException
  {	 
	    //response.setContentType("text/html;charset=UTF-8");
	    //request.setCharacterEncoding("UTF-8");   
	 
	    String org_name = request.getParameter("org_name");
	  	String mapno = request.getParameter("mapno");
		String mapscale = request.getParameter("mapscale");
		String show_description = request.getParameter("show_description");
		String location = null;
		 
        try{
			   Class.forName("com.mysql.jdbc.Driver");
			   String url = "jdbc:mysql://127.0.0.1:3306/pathway";
			   String user = "root";
			   String password = "";
			   Connection conn = DriverManager.getConnection(url, user, password);

			   // statement用来执行SQL语句
		       Statement statement = conn.createStatement();
		       // 要执行的SQL语句
		       String sql = "select map_location from show_map where org_name = "+org_name+" ";
		       // 结果集
		       ResultSet rs = statement.executeQuery(sql);
		       while(rs.next())
		       {
		    	   location = rs.getString("map_location");
		       }
		       conn.close();
        }catch(ClassNotFoundException e){
        	e.printStackTrace();
         }catch(SQLException e){
        	 e.printStackTrace();
         }
         		
	     //response.setContentType("image/png");
                
         InputStream in = this.getServletContext().getResourceAsStream(location);
         OutputStream outstr = response.getOutputStream();
		 byte[] buffer = new byte[1024];
		 int length = -1;
		 while( (length = in.read(buffer))!= -1)
		 {
		     outstr.write(buffer, 0, length);
		 }
		 
		 in.close();
		 outstr.close();	
		 
	}
  
 } 
    

[cyqcyq2008]

引用 楼主 cyqcyq2008 的回复:

servlet运行错误：严重: Servlet.service() for servlet show_pathway threw exception java.lang.NullPointerException 把代码贴出来：我想根据前台传过来的参数“org_name”，通过连接数据库pathway，把“map_location” 属性里存储的图片路径取出来并把图片显示在浏览页。“map_loc……

补一句：如果我的查询元素变成另一列属性“mapno”,即第32行变成：String sql = "select map_location from show_map where mapno = "+mapno+" "; 就会正常显示图像。为何？

[花谢尊前不敢香]

int length = 0;