如何从HttpServlet的doPost（）方法返回一些数据？

Question

I am writing a java class that extends HttpServlet thus overwrites doPost() method. Inside the doPost() method, some data object is assembled, but I am not sure what is a better way to return the data. For instance,

public class FormSubmission extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // http request is parsed to assemble an object, e.g., a List
        List L = parseRequest(request);

        // Question: how to return List L for use by Server?         
    }
}    

One way I can think of is to declare a static variable as

Public static List L;

and call FormSubmission.L , which basically makes List L a global variable.

Another way is to write data bytes to a file so that it can be read back into a List later.

But is there better way to handle such situation?

Use cases based on Comment:

Two forms with HTTP/POST are created in main page "index.html", Form A and B. It is required that a user submits Form A first, and then submits Form B. Submitting Form A triggers the above doPost() method, such that List L is constructed. The scenario is to how to make this List available to process Form B submission. Specifically, how to have Form B's doPost() method be able to access List L created from A during the previous stage.

Answer 1

I would consider using Session scope and put there. You can reach object from that session.

final HttpSession session = req.getSession();

Answer 2

In your FormASubmission servlet, save the List to the user's session object.

List L = parseRequest(request);

HttpSession session = request.getSession();
session.setAttribute("FormADataList", L);

Then in your FormBSumission servlet, you can retrieve this List as

HttpSession session = request.getSession();
List L = (List) session.getAttribute("FormADataList");

---

HttpSession is an interface whose underlying implementation is provided by the container. This scoped object is created only once per user/browser interacting with the application. Any user data stored here remains available for the length of the session across multiple HTTP requests from the same user.

The HttpSession 's javadoc says it

Provides a way to identify a user across more than one page request or visit to a Web site and to store information about that user.

Answer 3

you can do a RequestDispatcher

RequestDispatcher rd = request.getRequestDispatcher("name.jsp");
    rd.forward(request, response);