在线等 十万火急 新手求助 JSP ServletContext 报空指针错误 ！！！！！！！！

小龙王2010 2010-11-24 05:14:14
错误信息：

type Exception report

message

description The server encountered an internal error () that prevented it from fulfilling this request.

exception

java.lang.NullPointerException
com.dvd.view.ListLibraryServlet.doGet(ListLibraryServlet.java:87)
javax.servlet.http.HttpServlet.service(HttpServlet.java:690)
javax.servlet.http.HttpServlet.service(HttpServlet.java:803)


note The full stack trace of the root cause is available in the Apache Tomcat/6.0.14 logs.


--------------------------------------------------------------------------------

源代码：
ListLibraryServlet.java

package com.dvd.view;



import java.io.IOException;

import java.io.PrintWriter;

import java.util.List;

import java.util.Iterator;

//import java.util.ArrayList;

 



import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import com.dvd.model.DVDItem;

import javax.servlet.ServletContext;



public class ListLibraryServlet extends HttpServlet {



	/**

	 * Constructor of the object.

	 */

	public ListLibraryServlet() {

		super();

	}



	/**

	 * Destruction of the servlet. <br>

	 */

	public void destroy() {

		super.destroy(); // Just puts "destroy" string in log

		// Put your code here

	}



	/**

	 * The doGet method of the servlet. <br>

	 *

	 * This method is called when a form has its tag value method equals to get.

	 * 

	 * @param request the request send by the client to the server

	 * @param response the response send by the server to the client

	 * @throws ServletException if an error occurred

	 * @throws IOException if an error occurred

	 */

	@SuppressWarnings("unchecked")

	public void doGet(HttpServletRequest request, HttpServletResponse response)

			throws ServletException, IOException {

		//List dvds = new ArrayList();

		//Create the set of dvds

		//dvds.add(new DVDItem("Star Wars Episode I","1999","Sci-Fi"));

		//dvds.add(new DVDItem("Star Wars","1977","Sci-Fi"));

		//Set page title

		String pageTitle ="ListLibraryServlet";

		

		//Retrieve the list of dvdItems from the context-scope

		ServletContext context = getServletContext();

		List dvdList = (List)context.getAttribute("dvdList");

		//Specify the content type is HTML

		response.setContentType("text/html");

		PrintWriter out = response.getWriter();

		//Generate the HTML response

		out

				.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");

		out.println("<HTML>");

		out.println("  <HEAD><TITLE>"+pageTitle+"</TITLE></HEAD>");

		out.println("  <BODY bgcolor='white'>");

		 

		//Generate page heading

		out.print("<!--Page Heading-->");

		out.print("<table border='0' cellpadding='3'cellspacing='1'width='400'>");

		out.println("<tr bgcolor='#ccccff'");

		out.println("<td align='center'><font size='5'>ListLibraryServlet</font></td>");

		out.println("</tr>");

		out.println("</table>");

		

		//Generate main body

		out.println("<p>");

		int Size = 0;

		//if(dvdList.size() != 0){ Size =dvdList.size();}

		out.println("You currently have <font color='red' size='3'><b>"+Size+"</b></font> DVDs in your collection:");

		out.println("</p>");

		out.println("<table cellpadding='3' cellspacing='1'width='400'>");

		out.println("<tr bgcolor='#ccccff'>");

		out.println("<td align='left'>Title</td>");

		out.println("<td align='center'>Year</td>");

		out.println("<td align='center'>Genre</td>");

		out.println("</tr>");

		Iterator items = dvdList.iterator();

		while(items.hasNext()){

			DVDItem item = (DVDItem)items.next();

			out.println("<tr>");

			out.println("<td>"+item.getTitle()+"</td>");

			out.println("<td align='center'>"+item.getYear()+"</td>");

			out.println("<td align='center'>"+item.getGenre()+"</td>");

			out.println("</tr>");

		}

		out.println("</table>");

		out.println("  </BODY>");

		out.println("</HTML>");

		out.flush();

		out.close();

	}



	/**

	 * The doPost method of theservlet. <br>

	 *

	 * This method is called when a form has its tag value method equals to post.

	 * 

	 * @param request the request send by the client to the server

	 * @param response the response send by the server to the client

	 * @throws ServletException if an error occurred

	 * @throws IOException if an error occurred

	 */

	public void doPost(HttpServletRequest request, HttpServletResponse response)

			throws ServletException, IOException {



		response.setContentType("text/html");

		PrintWriter out = response.getWriter();

		out

				.println("<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.01 Transitional//EN\">");

		out.println("<HTML>");

		out.println("  <HEAD><TITLE>A Servlet</TITLE></HEAD>");

		out.println("  <BODY>");

		out.print("    This is ");

		out.print(this.getClass());

		out.println(", using the POST method");

		out.println("  </BODY>");

		out.println("</HTML>");

		out.flush();

		out.close();

	}



	/**

	 * Initialization of the servlet. <br>

	 *

	 * @throws ServletException if an error occurs

	 */

	public void init() throws ServletException {

		// Put your code here

	}



}


InitializeLibrary.java



package com.dvd.web;



import java.io.InputStream;

import java.io.InputStreamReader;

import java.io.BufferedReader;



import javax.servlet.ServletContextListener;

import javax.servlet.ServletContextEvent;

import javax.servlet.ServletContext;



import com.dvd.model.DVDItem;

import java.util.List;

import java.util.LinkedList;



public class InitializeLibrary implements ServletContextListener {

	@SuppressWarnings("unchecked")

	 public void contextInitialized(ServletContextEvent event){ 

		ServletContext context = event.getServletContext();

		List dvdList = new LinkedList();

		String dvdsFile = context.getInitParameter("dvd_file");

		InputStream is = null;

		BufferedReader reader = null;

		

		try{

			is = context.getResourceAsStream(dvdsFile);

			if(is != null){

			reader = new BufferedReader(new InputStreamReader(is));

			}

			String record ;

			

			//Read every record (one per line)

			

			while((record = reader.readLine().toString()) != null){

				String [] fields = record.split("\t");

				

				//Extract the data fields for the record

				String title = fields[0];

				String year =fields[1];

				String genre = fields[2];

				

				//Add the new DVD Item to the list

				DVDItem item = new DVDItem(title,year,genre);

				dvdList.add(item);

			}

			context.setAttribute("dvdList", dvdList);

			context.log("The DVD list has been loaded.");

		}catch(Exception e){

			context.log("Exception occured while proccessing the dvdsFile.",e);

		}finally{

			if(is != null){

				try{is.close();}catch(Exception e){}

			}

			if(reader !=null){

				try {reader.close();}catch(Exception e){}

			}

		}

	 }

	 public void contextDestroyed(ServletContextEvent event){  }

	

}
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wangan27 2010-11-25
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ServletContext context = getServletContext();
改成
ServletContext context=request.getServletContext();
或者
ServletContext context=request.getSession().getServletContext();

你上面的context肯定是取不到的 绝对是null
只看了第一个页面 估计是这个原因报空指针异常
小龙王2010 2010-11-25
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谢谢 问题已解决 报错的原因是 保存数据的DVDFiles.txt文件里面的初始数据没有用tab键隔开 所以读不出来
谢谢各位哈
montao 2010-11-25
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com.dvd.view.ListLibraryServlet.doGet(ListLibraryServlet.java:87)

发这个文件的代码出来 ListLibraryServlet.java 在87行后面加一个标注 我们好看一些！
小龙王2010 2010-11-24
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每次运行tomcat Server的时候 都报这个空指针 郁闷：

错误提示：
严重: Exception occured while proccessing the dvdsFile.
java.lang.NullPointerException
at com.dvd.web.InitializeLibrary.contextInitialized(InitializeLibrary.java:36)
at org.apache.catalina.core.StandardContext.listenerStart(StandardContext.java:3843)
at org.apache.catalina.core.StandardContext.start(StandardContext.java:4342)
.......................................
2010-11-24 23:44:37 org.apache.catalina.core.ApplicationContext log
信息: ContextListener: contextInitialized()
2010-11-24 23:44:37 org.apache.catalina.core.ApplicationContext log
信息: SessionListener: contextInitialized()
2010-11-24 23:44:38 org.apache.coyote.http11.Http11Protocol start
信息: Starting Coyote HTTP/1.1 on http-8080
2010-11-24 23:44:38 org.apache.jk.common.ChannelSocket init
信息: JK: ajp13 listening on /0.0.0.0:8009
2010-11-24 23:44:38 org.apache.jk.server.JkMain start
信息: Jk running ID=0 time=0/31 config=null
2010-11-24 23:44:38 org.apache.catalina.startup.Catalina start
信息: Server startup in 2916 ms


代码：

package com.dvd.web;

import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.BufferedReader;
import javax.servlet.ServletContextListener;
import javax.servlet.ServletContextEvent;
import javax.servlet.ServletContext;
import com.dvd.model.DVDItem;
import java.util.List;
import java.util.LinkedList;

public class InitializeLibrary implements ServletContextListener {
@SuppressWarnings("unchecked")

public void contextInitialized(ServletContextEvent event){
ServletContext context = event.getServletContext();
List dvdList = new LinkedList();
String dvdsFile = context.getInitParameter("dvd_file");
InputStream is = null;
BufferedReader reader = null;

try{
is = context.getResourceAsStream(dvdsFile);
if(is != null){
reader = new BufferedReader(new InputStreamReader(is));
}
String record ;

//Read every record (one per line)
record = reader.readLine().toString();
while((record = (String)reader.readLine().toString()) != null){ // 这一行就是每次提示报空指针错误的行 真是想不明白啊
String [] fields = record.split("\t");

//Extract the data fields for the record
String title = fields[0];
String year =fields[1];
String genre = fields[2];

//Add the new DVD Item to the list
DVDItem item = new DVDItem(title,year,genre);
dvdList.add(item);
}
context.setAttribute("dvdList", dvdList);
context.log("The DVD list has been loaded.");
}catch(Exception e){
context.log("Exception occured while proccessing the dvdsFile.",e);

}finally{
if(is != null){
try{is.close();}catch(Exception e){}
}
if(reader !=null){
try {reader.close();}catch(Exception e){}
}
}
}
public void contextDestroyed(ServletContextEvent event){ }

}
小龙王2010 2010-11-24
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恩， 可以这么说吧 生成静态HTML

shengfq 2010-11-24
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请问你这个代码就是通过servlet生成静态html页面吗
学习了，帮顶
小龙王2010 2010-11-24
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没人回答么？
小龙王2010 2010-11-24
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还有相关源代码 请各位大侠帮忙解答 谢谢
AddDVDServlet.java


package com.dvd.controller;



import java.io.IOException;





import javax.servlet.RequestDispatcher;

import javax.servlet.ServletException;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.ServletContext;



import com.dvd.model.DVDItem;

import java.util.List;

import java.util.ArrayList;



public class AddDVDServlet extends HttpServlet {



	/**

	 * Constructor of the object.

	 */

	public AddDVDServlet() {

		super();

	}



	/**

	 * Destruction of the servlet. <br>

	 */

	public void destroy() {

		super.destroy(); // Just puts "destroy" string in log

		// Put your code here

	}



	/**

	 * The doPost method of the servlet. <br>

	 *

	 * This method is called when a form has its tag value method equals to post.

	 * 

	 * @param request the request send by the client to the server

	 * @param response the response send by the server to the client

	 * @throws ServletException if an error occurred

	 * @throws IOException if an error occurred

	 */

	@SuppressWarnings("unchecked")

	public void doPost(HttpServletRequest request, HttpServletResponse response)

			throws ServletException, IOException {

		List errorMsgs = new ArrayList();

		request.setAttribute("errorMsgs", errorMsgs);

		

		try{

			//Retrieve form parameters

			String titleStr = request.getParameter("title").trim();

			String yearStr = request.getParameter("year").trim();

			String genreStr = request.getParameter("genre").trim();

			String newgenreStr = request.getParameter("newgenre").trim();

			

			if(titleStr.length() == 0){

				errorMsgs.add("Please enter the title of DVD Item");

			}

			int year = -1;

			try{

				year = Integer.parseInt(yearStr);

			}catch(NumberFormatException nfe){

				errorMsgs.add("The 'year' field must be a positive integer");

			}

			//Verify form parameters

			if((year != -1)&&((year <2000)||(year>2100))){

				errorMsgs.add("The 'year' field must within 2000 to 2100");

			}

			if(genreStr.equals("unknown")&&newgenreStr.equals("Add new genre")){

				errorMsgs.add("Please select a genre Or define the new Genre");

			}

			if(!(genreStr.equals("unknown"))&&!(newgenreStr.equals("Add new genre"))){

				errorMsgs.add("Please just select one genre:genre Or newgenre");

			}

			//Send the user back to the AddDVD form, if there were errors

			if(! errorMsgs.isEmpty()){

				 RequestDispatcher view

                 = request.getRequestDispatcher("add_dvd.view");

         view.forward(request, response);

			}

			//Perform business logic

			if(genreStr.equals("unknown")){

				genreStr = newgenreStr;

			}

			//Store the new DVD Application in the request-scope

			DVDItem dvdItem = new DVDItem(titleStr,yearStr,genreStr);

			request.setAttribute("dvdItem", dvdItem);

			

			//Store the new DVD in the dvdList  context-scope attribute

			ServletContext context = getServletContext();

			List dvdList =(List)context.getAttribute("dvdList");

			dvdList.add(dvdItem);

			

			//send the success view

			RequestDispatcher view

			=request.getRequestDispatcher("success.view");

			view.forward(request, response);

		}catch(Exception e){

			errorMsgs.add(e.getMessage());

			

		}

	}



	/**

	 * Initialization of the servlet. <br>

	 *

	 * @throws ServletException if an error occurs

	 */

	public void init() throws ServletException {

		// Put your code here

	}



}
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