getAttribute() returns null for drop down list in

问题:

I am trying to pass value from database to drop down menu using getAttribute(). However, it returns null.

This is my jsp (updateLecturer.jsp) file:

<form action="updateLecturer" class="sky-form">
<header>Update Lecturer Information</header>
<center>
<fieldset>

<section>
<label class="select">
<select name="selectLecturer" id="lecturerFullname">
<option value="0" selected disabled>Lecturers Name</option>        
**<option name="lecturerFullname"><%=((LecturerBean)request.getAttribute("LecturerFullname"))%></option>** 
</select>

</label>
</section>

</center>
<footer>
<center><button type="submit" class="button">Update</button></center>
</footer>
</form>

This is my servlet UpdateLecturerServlet.java) :

public void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, java.io.IOException {

                    String lecturerFullname = request.getParameter("LecturerFullame"); 
                    LecturerBean lecturer = new LecturerBean(); 
                    lecturer.setLecturerFullname(lecturerFullname); 

                    request.setAttribute("LecturerFullname",lecturerFullname); 

                    RequestDispatcher view = getServletContext().getRequestDispatcher("/updateLecturer.jsp"); 
                    view.forward(request,response);


    }

This is my UpdateLecturerDAO :

static Connection currentCon = null;
static ResultSet rs = null;

public static LecturerBean selectlecturer(LecturerBean Lbean) {

    // preparing some objects for connection
    System.out.println("JIJIJI");
    Statement stmt = null;
    String lecturerFullname = Lbean.getLecturerFullname();



System.out.println("j444444");


String searchQuery = "select lecturerfullname from lecturer";

System.out.println("Your lecturer is " + lecturerFullname);
System.out.println("Query: " + searchQuery);

try {
    // connect to DB
    currentCon = JavaConnectionDB.getConnection();
    stmt = currentCon.createStatement();
    rs = stmt.executeQuery(searchQuery);
   // boolean more = rs.next();

    while(rs.next())
    {
    LecturerBean lecturer = new LecturerBean();
                lecturer.setLecturerFullname(rs.getString("LecturerFullname"));
                lecturer.add(lecturer);
    }


}

catch (Exception ex) {
    System.out.println("Select failed: An Exception has occurred! " + ex);
}

PLEASEEEE HELP :( thank you very much

回答1:

If you set some attribute in request like this

request.setAttribute("key",obj);

You can get it display in jsp by a scriplet like this

<%=request.getAttribute("key")%>

In your case please check the following points

1. Check whether the following code is giving you a null.

   String lecturerFullname = request.getParameter("LecturerFullame"); If this gives you null then check whether your passing parameter LecturerFullame in url.

2. In the jsp please cast the following to correct object

   <option name="lecturerFullname"><%=((String)request.getAttribute("LecturerFullname"))%></option>

And let me know.

回答2:

I've never worked with any of this type of code before, so I'm not sure how the attribute system works, but I noticed an anomaly in your code:

Where you are setting: request.setAttribute("LecturerFullname",lecturerFullname);

Where you are getting: lecturer.setLecturerFullname(request.getAttribute("lecturerFullname",lecturer));

Notice it yet? Where you are setting it, you put a capital "L" and I believe it's case sensitive. Doesn't hurt to try.

回答3:

In UpdateLecturerServlet.java.

 //calling selectlecturer() to retrieve list full names and store it in session
 request.setAttribute("LecturerFullname",selectlecturer()); 

 RequestDispatcher view = getServletContext().getRequestDispatcher("/updateLecturer.jsp"); 
 view.forward(request,response);

Now selectlecturer() method.

public static List<LecturerBean> selectlecturer() {

//DB query to retrieve all lecturer fullnames

List<LecturerBean> lecturers = new ArrayList<LecturerBean>();

while(rs.next())
{
    LecturerBean lecturer = new LecturerBean();
    lecturer.setLecturerFullname(rs.getString("LecturerFullname"));
    lecturers.add(lecturer);
}

return lecturers;//return the list lecturer fullnames
}

In updateLecturer.jsp.

<%
    java.util.List<LecturerBean> lists = (java.util.List<LecturerBean>)request.getAttribute("LecturerFullname"));
    for(LecturerBean bean:lists)
    {
%>
<option name="lecturerFullname"><%=bean.getLecturerFullname()%></option>
<%
    }
%>

But you should avoid Java code in JSP. Just for the reference here I'm giving this code.