How to prevent duplicate entries while refreshing?

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I have an index.jsp page where I have a form where the user enters some data that gets stored in the database using a Controller servlet.

I want to disp

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谎友^

2021-01-02 23:32

I would add an invisible ID to the page. If the data is new to the database (ID = unknown), insert and create an ID and update the page with the ID. That way you know if it is a ID != unknown, and you don't have to do an insert. And if the data hasn't changed, you don't even have to do an update...

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暗喜

2021-01-02 23:38

I thought of using POST-REDIRECT-GET pattern but the problem is when I redirect I don't get those data to be displayed using JSTL.

Just send a request parameter along identifying the information you'd like to display in the new GET request.

// ...
Long id = dataService.save(data);
// ...
response.sendRedirect(request.getContextPath() + "/index?editId=" + id);

and then in the servlet which is mapped on an URL pattern of /index

@Override
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    Long editId = Long.valueOf(request.getParameter("editId")); // Handle nullcheck yourself.
    Data editData = dataService.find(editId);
    request.setAttribute("editData", editData); // For the edit form.
    List<Data> allData = dataService.list();
    request.setAttribute("allData", allData); // For the table/list of all data.
    request.getRequestDispatcher("/index.jsp").forward(request, response);
}

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我在风中等你

2021-01-02 23:44

Here is the simplest solution

<%@page import="java.util.*"%>
<%@page contentType="text/html" pageEncoding="UTF-8"%>
<%@ taglib uri="http://java.sun.com/jsp/jstl/core" prefix="c" %>

<%!
//This List is just for demonstration purposes
List<String> names = new ArrayList<String>();
%>
<%
if(request.getParameter("name")!=null ){
    names.add(request.getParameter("name"));
    session.setAttribute("nameList", names);
    //Here you put your database insert code

    //Whenever the code execution reaches this line , 
    //then it means that you have a new page submission
    //and not a refresh or f5 case

    response.sendRedirect("index.jsp");
}
%>

<!DOCTYPE html>
<html>
    <head>
    </head>
    <body>
        <form action="index.jsp" method="post">
            <input type="text" id="name" name="name"/>
        </form>
        <p>
            <table>
                <c:forEach items="${nameList}" var="element">    
                    <tr>
                        <td>Name:  <c:out value="${element}"/> </td>
                    </tr>
                </c:forEach>
            </table>
        </p>
    </body>
</html>

The trick lies with the response.sendRedirect("index.jsp");. This makes null all request parameters. If an f5 or refresh is hit, then the if is never executed. If a normal submit is the case, then the if is executed and you call the response.sendRedirect("index.jsp");.

In summary, all you really need to do is:

1) Check if(request.getParameter("name")!=null )

2) If above is true then do database inserts

3) If above is true then response.sendRedirect("index.jsp");

UPDATE

if(request.getParameter("name")!=null ){
    DbUtility.addNameToDb(request.getParameter("name"));
    ArrayList<String> currentList = DbUtility.getAllNamesFromDb();
    session.setAttribute("nameList", currentList);
    response.sendRedirect("index.jsp");
}

You just have to implement these 2 methods. The addNameToDb(String) will make the insert in your database. And the getAllNamesFromDb() will return an ArrayList<String> Object that will represent on the entries from your database. (And you no longer need the names list introduced before in my first answer)

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北荒

2021-01-02 23:44

I think PRGP (Post Redirect Get Pattern) is the way to go for this. If you're using Spring Web Flow it has a FlashScope where you can put the data that you want to be retained after the Post-get-redirection. You can retain more than just an edit Id using this approach.

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