java dispatcher.forward抛出null异常

Question

I got the following error

HTTP Status 500 -

type Exception report

message

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

exception

org.apache.jasper.JasperException: java.lang.NullPointerException
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:548)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:471)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:389)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:333)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    ahp.CompareCriterionSlider.doGet(CompareCriterionSlider.java:35)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
root cause

java.lang.NullPointerException
    org.apache.jsp.CompareCriterionSlider_jsp._jspService(CompareCriterionSlider_jsp.java:91)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:433)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:389)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:333)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)
    ahp.CompareCriterionSlider.doGet(CompareCriterionSlider.java:35)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:621)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:722)

For the line: dispatcher.forward(request, response);

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        HttpSession session = request.getSession(true);
        session.setAttribute("criterions", CriterionRecommander.values() );
        RequestDispatcher dispatcher = getServletContext().getRequestDispatcher("/CompareCriterionSlider.jsp");
        dispatcher.forward(request, response);
    }

I have CompareCriterionSlider.jsp in the dir WebContent/

I tried to set break point to debug it, but eclipse(Version: Indigo Service Release 2) gives me: Source not found.

Whoever down voted me care to give a reason?

Answer 1

There is a null pointer exception thrown from your JSP and you need to find the exact line of code in your JSP that is causing it.
One way to do it is to simply open the compiled java file of the jsp ie CompareCriterionSlider_jsp.java:91 and then map it to the line in the JSP.
Once you do that , given it is a null pointer it should be straightforward figuring the error.