The following is a simple example of a JSP page that uses the Post-Redirect-Get (PRG) pattern.
It uses scriptlets for backward compatibility and to allow the entire pattern to be written in a single JSP page.
Note: this does not mean that you should use scriptlets in your pages. It is done here for illustration purposes only!
Lines 1 through 16 simulate typical database read/write operations by reading and appending to a string of text (comments) bound to session scope.
The key to implementing the PRG pattern is to insure that a page is never displayed as a result of a request of type POST. Here, this is done by redirecting to a read-only version of the page after performing a database write (line 16).
To test, copy this page (without the line numbers) into your application and access it through the browser. Add a comment and then click the refresh button. Note that the previous write operation has not been repeated.
1 <%
2
3 String comments = (String)session.getAttribute("comments");
4 if(null == comments){
5 comments = "";
6 }
7
8
9
10 String comment = request.getParameter("comment");
11 if(null != comment){
12 comments = comments + comment + "<br />";
13 session.setAttribute("comments", comments);
14 response.sendRedirect(request.getContextPath() + "/prg.jsp");
15 return;
16 }
17 %>
18 <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
19 <html>
20 <head>
21 <title>Simple Post Redirect Get Pattern</title>
22 <meta http-equiv="Expires" content="Sat, 01 Dec 2001 00:00:00 GMT" />
23 <meta http-equiv="pragma" content="no-cache" />
24 <meta http-equiv="Cache-Control" content="no-cache" />
25 </head>
26 <body>
27 <fieldset>
28 <legend>Previous Comments</legend>
29 <%=comments%>
30 </fieldset>
31 <form method="post" action="<%=request.getContextPath()%>/prg.jsp">
32 <fieldset>
33 <legend>Add a new comment</legend>
34 <input type="text" name="comment" id="comment" />
35 <input type="submit" name="enter_button" id="Enter" />
36 </fieldset>
37 </form>
38 </body>
39 </html>
Also see
DoubleSubmitProblem
