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Thread: http status 500

  1. 12-14-2010, 03:36 PM #1
    swathi dharmaraj
    swathi dharmaraj is offline Member
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    Default http status 500

    i am working on a web application called "fast learn", an online study center and ive encountered the following error for the code below.. plz help
    HTTP Status 500 -

    type Exception report

    message

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

    exception

    java.lang.ClassCastException: course cannot be cast to java.util.List
    ListCourses.doGet(ListCourses.java:51)
    javax.servlet.http.HttpServlet.service(HttpServlet .java:617)
    javax.servlet.http.HttpServlet.service(HttpServlet .java:717)
    org.netbeans.modules.web.monitor.server.MonitorFil ter.doFilter(MonitorFilter.java:393)
    //code
    import java.io.IOException;
    import java.io.PrintWriter;
    import javax.servlet.ServletException;
    import javax.servlet.http.HttpServlet;
    import javax.servlet.http.HttpServletRequest;
    import javax.servlet.http.HttpServletResponse;
    import java.util.*;


    public class ListCourses extends HttpServlet {

    private List listcourse=null;

    protected void doGet(HttpServletRequest request, HttpServletResponse response)
    throws ServletException, IOException {
    listcourse=new LinkedList();
    listcourse.add(new course("c#"));
    listcourse.add(new course("asp.net"));
    String pageTitle="list of courses";
    response.setContentType("text/html;charset=UTF-8");
    PrintWriter out = response.getWriter();
    out.println("<html>");
    out.println("<head>");
    out.println("<title>"+pageTitle+"</title>");
    out.println("</head>");
    out.println("<body bgcolor='white'>");
    out.println("<!--Page Heading-->");
    out.println("<table border='1' cellpadding='5' cellspacing='0' width='400'>");
    out.println("<tr bgcolor='#CCCCFF' align='center' valign='center' height='20'>");
    out.println("<td> <h3>"+pageTitle+"</h3> </td>");
    out.println("</tr>");
    out.println("</table>");
    out.println("<p>");
    out.println("list of courses are");
    out.println("</p>");
    out.println("<ul>");
    Iterator items=listcourse.iterator();
    while(items.hasNext())
    {
    List list=(List)items.next();
    out.println("<li>"+course.getTitle()+"</li>");
    }
    out.println("</ul>");
    out.println("</body>");
    out.println("</html>");
    }
    }
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  2. 12-14-2010, 03:42 PM #2
    gcalvin
    gcalvin is offline Senior Member
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    Default

    Java Code: 
            Iterator items=listcourse.iterator();
        while(items.hasNext())
        {
            List list=(List)items.next();
    Here items.next() will be an Object of type course, and you are trying to assign it to list, which is type List. Can't do that. I guess you probably meant to do something like:
    Java Code: 
                list.add(items.next());
    -Gary-
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  3. 12-14-2010, 04:13 PM #3
    swathi dharmaraj
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    Default

    thnx a lot :)
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  4. 12-14-2010, 04:41 PM #4
    Tolls
    Tolls is offline Moderator
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    Default

    You also have a Bug In Waiting in that Servlet.

    Servlets should not have state. That is, in general, a Servlet should not have any attributes. Yours does, in the form of your listcourse attribute.

    You might ask why, but it's because a single instance of a Servlet will be handling multiple requests at one time, so need to be thread safe.

    So, say a request comes in and starts processing and gets to a point just after adding stuff to the list, but before displaying. Then another request comes in to be processed by this Servlet and gets as far as the new LinkedList() bit. This wipes the stuff in listcourses out. Now the original thread (request) takes over again and displays...well...nothing, because it's now an empty list.

    So don't put attributes onto your Servlets.
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  5. 12-14-2010, 04:46 PM #5
    swathi dharmaraj
    swathi dharmaraj is offline Member
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    Default

    thnx a lot :)
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