servlet将空值打印到jsp

Question

I have a web form in a jsp that is supposed to get values from a servlet, but it is printing out null values. I am including the code for the jsp and for the servlet below. Can anyone show me how to fix the code below so that it prints out the values from the request object instead of printing null?

Here is the code for my.jsp:

<jsp:useBean id="errors" scope="request" type="java.util.Map" class="java.util.HashMap" />
<form method="post">
    <table>
        <tr>
            <td width=302>
            </td>
            <td width=250>
                <table>
                    <tr>
                        <td width=150 align="right">tha: </td>
                        <td><input type="text" name="tha" value="c3t" size="15" />
                            <%if (errors.containsKey("tha")) {out.println("<span class=\"error\">" + errors.get("tha") + "</span>");}  
                            else{out.println(request.getParameter("tha"));}%>  
                        </td>
                    </tr>
                    <tr>
                        <td width=150 align="right">min: </td>
                        <td><input type="text" name="min" value="0" size="15" />
                            <% if (errors.containsKey("min")) {out.println("<span class=\"error\">" + errors.get("min") + "</span>");}  
                            else{out.println(request.getParameter("min"));}%>  
                        </td>
                    </tr>
                    <tr>
                        <td width=150 align="right">max: </td>
                        <td><input type="text" name="max" value="2*pi" size="15" />
                        <% if (errors.containsKey("max")) {out.println("<span class=\"error\">" + errors.get("max") + "</span>");}
                        else{out.println(request.getParameter("max"));}%>
                        </td>
                    </tr>
                    <tr>
                        <td></td>
                        <td align="right">
                        <input type="submit" name="submit-button" value="Click To Plot" />
                        </td>
                    </tr>
                </table>
            </td>
        </tr>
    </table>
</form>  

And here is the code for the servlet:

public class PlotPolarServlet extends HttpServlet{
    private RequestDispatcher jsp;

    public void init(ServletConfig config) throws ServletException {
       ServletContext context = config.getServletContext();
       jsp = context.getRequestDispatcher("/WEB-INF/jsp/my.jsp");
    } 

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) 
        throws ServletException, IOException {
        jsp.forward(req, resp);
    }

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) 
        throws ServletException, IOException{
        Map<String, String> errors = validate(req);
        if (!errors.isEmpty()){  
            jsp.forward(req, resp);
            return;
        }
        resp.sendRedirect("my");
    }

    public static Map<String, String> validate(HttpServletRequest req){
        HashMap<String, String> errors = new HashMap<String, String>();
        req.setAttribute("errors", errors);
        String tha = req.getParameter("tha");
        if (tha == null || tha.trim().length() == 0){
            errors.put("tha", "tha required.");
        }
        String min = req.getParameter("min");
        if (min == null || min.trim().length() == 0){
            errors.put("min", "min required.");
        }
        String max = req.getParameter("max");
        if (max == null || max.trim().length() == 0){
            errors.put("max", "max required.");
        }
        return errors;
    }
}  

Answer 1

I originally misread your question. The issue is when parameters are input correctly, not when there is an error. The else part of this code

<% if (errors.containsKey("min")) {out.println("<span class=\"error\">" + errors.get("min") + "</span>");}  
else{out.println(request.getParameter("min"));}%>  

can't print anything but null because, in this request, there are no parameters with those keys.

A HttpServletResponse#sendRedirect(String) returns a 302 HTTP status code which would make your browser send a new HTTP request to the location described by the String parameter. The request parameters would not be in the new request (the request context/scope is cleared once the request is handled). You would need to put them in the session attributes.

/*
   for every parameter, put it in the session attributes
*/
req.getSession(true).setAttribute("myParam", req.getParameter("myParam"));
resp.sendRedirect("my");

This is known as flash scope and flash attributes and it can be implemented as explained here with a servlet Filter . You basically store attributes you want to reuse between 2 requests, then delete them.

As for Edit:

Unless you've copy-pasted the code in your edit incorrectly

<td><input type="text" name="yX" value="cx" size="15" />
    <%if (errors.containsKey("yX")) {   // errors doesn't contain yX as a Key, it contains imageParam1
         out.println("<span class=\"error\">" + errors.get("yX") + "</span>");
    }else{out.println(request.getParameter("yX"));}%> // will print the value of the request parameter with key yX
</td>

you're POST ing a parameter called yX , but looking for imageParam . Your doPost() method will create an errors request attribute and then forward (instead of sendRedirect ).

errors.put("imageParam1", "imageParam1 required.");
...
if (!errors.isEmpty()){
    jsp.forward(req, resp);
    return;
}

not yX . Therefore, the else gets evaluated and the parameter exists since it's the same request.

MORE EXPLANATION

In your my example:

1. If you didn't enter a required field, the doPost() method gets called, the errors map is populated and your code does jsp.forward(req, resp); which uses the same request and the parameters are available when the jsp is rendered.
2. If you entered all the required fields, the doPost() is called and resp.sendRedirect("my"); is executed. This causes your browser to send a new GET HTTP request with new parameters/attributes. This causes the doGet() to be called to process the new request, which forwards to your jsp. The original request parameters aren't included in this request so there is nothing to show, thus null when else{out.println(request.getParameter("min")); gets rendered.

In your myother example, because of the difference between the <input> parameter name in the jsp and the parameter name you're looking for in the servlet, you're getting completely irrelevant results which I've explained above. Disregard this servlet. It is not doing what you thought it was doing correctly.