Question

servlet转发时候出现空指针异常

Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
at org.apache.jsp.forquestion_jsp._jspService(forquestion_jsp.java:342)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:374)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:646)
at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:436)
at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:374)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:302)
at com.gh.zhaoping.servlet.ForQuestion.doGet(ForQuestion.java:37)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at com.wabacusdemo.util.SetCharacterEncodingFilter.doFilter(SetCharacterEncodingFilter.java:64)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke
private String user_qestion = "/forquestion.jsp";
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
// TODO Auto-generated method stub
CommonServiceImpl.setQuestioninfo(request);
// System.out.println(request.getAttribute("lxrname"));
ServletContext context = getServletContext();
RequestDispatcher dispatcher = context.getRequestDispatcher(user_qestion);
// System.out.println("--------------------");
dispatcher.forward(request, response);
}
这是我的servlset 现在出的问题是 假如我有 a,b,c三个页面 直接由b->servlet->c 能成功，但是由a 跳到 b 再走b->servlet->c 就会报这个异常 不理解怎么回事

Answer 1

问题出在你的service方法里，是不是直接访问servlet？可能有些变量需要通过其他页面提交过来的，结果你直接访问，就出现空指针了

Answer 2

从a到b你传了哪些值，把他打印出来，再在servlet中把你所传进来的参数打印下，传出去的参数也打印下，就能知道那个对象空了

Answer 3

设个断点，看一下那个变量没有数据，用degug模式运行

Answer 4

如果猜的没错 你应该在页面取东西了 ，问题在于RequestDispatcher dispatcher = context.getRequestDispatcher(user_qestion);
你这行代码是Dispathcher 只对第二个页面有效 所以参数是传不到重定向页面的换成forward 试试

Answer 5

　　servlet的空指针是由于jsp页面应用了null对象导致，里面的service方法抛出的异常。
　　遇到这种情况要及从错误里面捕捉异常抛出的原因，跟踪代码调试修复。

　　处理方式 1) 检查到对象为空时，设置对象值为空字符串或一个默认值;
　　处理方式 2) 检测到对象为空时，根本不执行某操作，直接跳转到其他处理中。
　　处理方式 3) 检查到对象为空时，提示用户操作有错误。
　

Answer 6

你这样没有人可以回答的出，贴程序吧