1. package com.example;
   
2. 
   
3. public class Dog {
   
4.         private String breed;
   
5.         public Dog(String breed) {
   
6.         this.breed = breed;
   
7.         }
   
8.         public String getBreed() {
   
9.         return breed;
   
10.         }
    
11. }
    

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1. package com.example;
   
2. 
   
3. import java.io.IOException;
   
4. import java.io.PrintWriter;
   
5. 
   
6. import javax.servlet.ServletException;
   
7. import javax.servlet.http.HttpServlet;
   
8. import javax.servlet.http.HttpServletRequest;
   
9. import javax.servlet.http.HttpServletResponse;
   
10. 
    
11. public class ListenerTester extends HttpServlet {
    
12. 
    
13.        
    
14.         public void doGet(HttpServletRequest request, HttpServletResponse response)
    
15.                         throws ServletException, IOException {
    
16. 
    
17.                 response.setContentType("text/html");
    
18.                 PrintWriter out = response.getWriter();
    
19.                 out.println("test context attributes set by listener<br>");
    
20.                 out.println("<br>");
    
21.                 Dog dog = (Dog) getServletContext().getAttribute("dog");
    
22.                 out.println("Dog’s breed is: " + dog.getBreed());
    
23.         }
    
24. 
    
25. }
    

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1. package com.example;
   
2. 
   
3. import javax.servlet.ServletContext;
   
4. import javax.servlet.ServletContextEvent;
   
5. import javax.servlet.ServletContextListener;
   
6. 
   
7. public class MyServletContextListener implements ServletContextListener {
   
8. 
   
9.         @Override
   
10.         public void contextDestroyed(ServletContextEvent event) {
    
11.                 ServletContext sc = event.getServletContext();
    
12.                 String dogBreed = sc.getInitParameter("breed");
    
13.                 Dog d = new Dog(dogBreed);
    
14.                 sc.setAttribute("dog", d);
    
15.         }
    
16. 
    
17.         @Override
    
18.         public void contextInitialized(ServletContextEvent arg0) {
    
19.                 // TODO Auto-generated method stub
    
20. 
    
21.         }
    
22. 
    
23. }
    

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1. <?xml version="1.0" encoding="UTF-8"?>
   
2. <web-app version="3.0"
   
3.         xmlns="http://java.sun.com/xml/ns/javaee"
   
4.         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   
5.         xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
   
6.         http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
   
7.   <display-name></display-name>
   
8.   <servlet>
   
9.     <servlet-name>ListenerTester</servlet-name>
   
10.     <servlet-class>com.example.ListenerTester</servlet-class>
    
11.   </servlet>
    
12. 
    
13.   <servlet-mapping>
    
14.     <servlet-name>ListenerTester</servlet-name>
    
15.     <url-pattern>/ListenTest.do</url-pattern>
    
16.   </servlet-mapping>       
    
17.   <welcome-file-list>
    
18.     <welcome-file>index.jsp</welcome-file>
    
19.   </welcome-file-list>
    
20.   
    
21.   <context-param>
    
22.         <param-name>breed</param-name>
    
23.         <param-value>Great Dane</param-value>
    
24.   </context-param>
    
25.   <listener>
    
26.         <listener-class>
    
27.           com.example.MyServletContextListener
    
28.         </listener-class>
    
29.   </listener>
    
30.   
    
31. </web-app>
    

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这个webapp是测试listener的，但总是出现空指针异常，自己找了很久，没找出错误来，望各位鼎力相助。



http://localhost:8080/ListenerTest/ListenTest.do
HTTP Status 500 -

type Exception report

message

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

exception

java.lang.NullPointerException
        com.example.ListenerTester.doGet(ListenerTester.java:22)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
        javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
note The full stack trace of the root cause is available in the Apache Tomcat/6.0.33 logs.

Apache Tomcat/6.0.33

com.example.ListenerTester.doGet(ListenerTester.java:22) 错误
Dog dog = (Dog) getServletContext().getAttribute("dog")
说明 dog == null

如果dog ==null 再去调用该空对象的方法是报NullPointException


你代码写错地方了，初始化代码写在 contextInitialized  而contextDestroyed是在web应用退出的时候调用的。
下面是改写的代码。楼主可以测试下。

1. package com.example;
   
2. 
   
3. 
   
4. import javax.servlet.ServletContext;
   
5. 
   
6. import javax.servlet.ServletContextEvent;
   
7. 
   
8. import javax.servlet.ServletContextListener;
   
9. 
   
10. 
    
11. public class MyServletContextListener implements ServletContextListener {
    
12. 
    
13. 
    
14.         @Override
    
15. 
    
16.         public void contextDestroyed(ServletContextEvent event) {
    
17. 
    
18.                  
    
19.         }
    
20. 
    
21. 
    
22.         @Override
    
23. 
    
24.         public void contextInitialized(ServletContextEvent event) {
    
25. 
    
26.                 ServletContext sc = event.getServletContext();
    
27. 
    
28.                 String dogBreed = sc.getInitParameter("breed");
    
29. 
    
30.                 Dog d = new Dog(dogBreed);
    
31. 
    
32.                 sc.setAttribute("dog", d);
    
33. 
    
34.         }
    
35. 
    
36. 
    
37. }

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哈，非常感谢{:soso_e100:}，程序通过调试，是我粗心大意了，浪费3个小时