<% List list = new ArrayList(); 
    list=(List)session.getAttribute("array");
    Student student = new Student();
    //System.out.print(list.size());
    %>解开注释就报错2011-12-12 16:07:07 org.apache.catalina.core.StandardWrapperValve invoke
严重: Servlet.service() for servlet jsp threw exception
java.lang.NullPointerException
at org.apache.jsp.operate.Select_jsp._jspService(Select_jsp.java:227)
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:386)
at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:313)
at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:260)
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.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:859)
at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
at java.lang.Thread.run(Thread.java:619)
--------------------------------------------------------------------------------------------------------
session赋值的页面是这样的 session.setAttribute("array",list); 赋值给session的 经检测在赋值页面session是有值的...是什么原因导致解开上面注释就报错呢???????????   

解决方案 »

  1.   

    System.out.print(list);
    看看是不是null
      

  2.   

    果然是null  那怎么办呢?
      

  3.   


    /*
     *改成这样,写代码要加上逻辑判断
     *我估计是你把list放在session中时list就是空的
     *或者你那个session.setAttribute("array",list)逻辑没有走
     *从这几点去找的噻
     */
    List list = (List)session.getAttribute("array");
    Student student = new Student();
    if(list != null) {
        System.out.print(list.size());
    }
       
      

  4.   

    1、主动判断为空
    if(list != null) {
       ***
    }else
    {
    ***}
      
      

  5.   

    session.setAttribute("array",list); 
    这个list初始化了么?
    不会是这样吧?
    List list = null;
    session.setAttribute("array",list);