空指针错误。。。。。。。。。。。。。。。。。。。。

Question

我在JSP内写了    <%  {List list=(List)request.getAttribute("list"); %>
          <% Iterator i=list.iterator(); 
           while (i.hasNext()){ 
              SongBean song = (SongBean)i.next(); 
            %> 
    <tr> <td> <%=song.getId()%> </td> 
           </tr> 
        <%}%>  
写了这段后报错，页面内表格不见了，执行后又出现了 报的错误如下，告诉下怎么解决：：：谢谢java.lang.NullPointerException
at org.apache.jsp.Main_jsp._jspService(org.apache.jsp.Main_jsp:547)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:97)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:322)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:291)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:241)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.ApplicationDispatcher.invoke(ApplicationDispatcher.java:672)
at org.apache.catalina.core.ApplicationDispatcher.processRequest(ApplicationDispatcher.java:463)
at org.apache.catalina.core.ApplicationDispatcher.doForward(ApplicationDispatcher.java:398)
at org.apache.catalina.core.ApplicationDispatcher.forward(ApplicationDispatcher.java:301)
at org.apache.struts.action.RequestProcessor.doForward(RequestProcessor.java:1063)
at org.apache.struts.action.RequestProcessor.processForwardConfig(RequestProcessor.java:386)
at org.apache.struts.action.RequestProcessor.process(RequestProcessor.java:229)
at org.apache.struts.action.ActionServlet.process(ActionServlet.java:1194)
at org.apache.struts.action.ActionServlet.doGet(ActionServlet.java:414)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:802)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:252)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:173)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:213)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:178)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:126)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:105)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:107)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:148)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:856)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.processConnection(Http11Protocol.java:744)
at org.apache.tomcat.util.net.PoolTcpEndpoint.processSocket(PoolTcpEndpoint.java:527)
at org.apache.tomcat.util.net.LeaderFollowerWorkerThread.runIt(LeaderFollowerWorkerThread.java:80)
at org.apache.tomcat.util.threads.ThreadPool$ControlRunnable.run(ThreadPool.java:684)
at java.lang.Thread.run(Thread.java:595)
2009-4-28 18:09:01 org.apache.catalina.core.StandardWrapperValve invoke

Answer 1

去查查servlet传过来list之前，list有没有没有赋值的变量

Answer 2

song.getId() 拿到的是空值.....也就是说你没有传值过去.
你也可以在后台打印出来...一句
System.out.println(song.getId());就行了.

Answer 3

[com.bean.SongBean@16f5b17, com.bean.SongBean@1c55e69, com.bean.SongBean@b950d1, com.bean.SongBean@68dc2d, com.bean.SongBean@4f0f97, com.bean.SongBean@889c4e, com.bean.SongBean@cdf872, com.bean.SongBean@7227a8, com.bean.SongBean@899e6a, com.bean.SongBean@ff665a, com.bean.SongBean@198c113, com.bean.SongBean@1abcd5e, com.bean.SongBean@37eaab, com.bean.SongBean@10f80a9, com.bean.SongBean@c803e6, com.bean.SongBean@1780f30, com.bean.SongBean@12d297a, com.bean.SongBean@1fe3238, com.bean.SongBean@19331eb, com.bean.SongBean@b27bb5, com.bean.SongBean@1d7ce63, com.bean.SongBean@12ecba8, com.bean.SongBean@13c8540, com.bean.SongBean@504f3c, com.bean.SongBean@820a0c, com.bean.SongBean@161d282, com.bean.SongBean@14362d9]
list内的东西

Answer 4

怎么解决呢，上面是打印出来的list  照理说有值

Answer 5

看一下你相应的Action中有没有
request.setAttribute("list",yourList);
return mapping.findForward("你现在的页面");
同时在你的SongBean中有相应的getid（）方法

Answer 6

<%{
    List list=(List)request.getAttribute("list"); 
    if(list == null){
        out.print("list is null");
        return;
    }else{
    Iterator i=list.iterator(); 
    while (i.hasNext()){ 
        SongBean song = (SongBean)i.next(); 
        if(song == null){
            out.print("song is null");
            return;
        }else{
%> 
    <tr> 
        <td> <%=song.getId()%> </td> 
    </tr> 
<%}}}%>
加上一些判空值语句，就算是空指针也应该也可显示页面了，
但我始终认为在页面中用JSP显示方式不好，还是用JSTL吧
<c:if test="${!empty list}">
     <c:forEach items="${list}" var="songBean">
        <c:if test="${!empty songBean}">
            <tr><td>${songBean.id}</td>
        </c:if>
    </c:forEach>
</c:if>

Answer 7

先测试list是否有值在里面，如果没有值，下面就会引发空指针的异常了。

Answer 8

看一下有没有把SongBean所在的包导进JSP页面
再看一下你相应的Action中有没有 
request.setAttribute("list",yourList); 
然后看一下list是否有值了

Answer 9

 <% Iterator i=list.iterator(); 
          while (I!=NULL && i.hasNext()){               SongBean song = (SongBean)i.next(); 
            %> 
    <tr> <td> <%=song.getId()%> </td> 
          </tr> 
        <%}%>  

Answer 10

request.getAttribute("list")会不会是空值

Answer 11

估计多半是list集合没传过来。。你仔细查下

Answer 12

你可以打印下list.size()，看看值是多少，若是0则list里没有值啊。
再看看你是不是用request.setAttribute();保存的，有的时候这个也会出问题的。