为什么我收到空指针异常？

Question

So I am trying to implement a user profile page, I've implemeted it using a bean, servlet and a corresponding jsp page.

When I try to visit /profile/username (the account definitely exists in the database), I get the following error displayed in my browser:

exception

org.apache.jasper.JasperException: An exception occurred processing JSP page /profile.jsp at line 87

84:         <div class="container">
85:             <div class="row">
86:                 <div class="col-lg-12 text-center user-profile">
87:                     <h1>Welcome <%=profile.getUsername()%>!</h1>
88:                 </div>
89:                 <div class="col-sm-4 text-center">
90:                     <div class="user-post" style="background-color: red">


Stacktrace:
    org.apache.jasper.servlet.JspServletWrapper.handleJspException(JspServletWrapper.java:574)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:476)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)
    [PACKAGE].servlets.Profile.doGet(Profile.java:47)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)

root cause

java.lang.NullPointerException
    org.apache.jsp.profile_jsp._jspService(profile_jsp.java:218)
    org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:438)
    org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:396)
    org.apache.jasper.servlet.JspServlet.service(JspServlet.java:340)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)
    [PACKAGE].servlets.Profile.doGet(Profile.java:47)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
    javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    org.netbeans.modules.web.monitor.server.MonitorFilter.doFilter(MonitorFilter.java:393)

When I go to where the error is (line 47 of the servlet Profile.java) it's just:

rd.forward(request, response);

I'm using the code exactly as I do in other servlets but it just doesn't work.

Am I missing something really obvious? Thanks in advance.

Answer 1

You commented (on a deleted answer):

Sorry I should've added the full jsp page, I declare profile above as follows:

 ProfileStore profile = (ProfileStore) session.getAttribute("Profile");

.... but you set the attribute in the servlet code like this:

request.setAttribute("Profile", profile);

You should use the same "scoping" when setting and getting the attribute. If you fetch an attribute that is not defined in the scope you are trying to fetch it from, then you will get a null .