A Controller (in this context, an implementation of HttpServlet) may perform either a forward or a redirect operation at the end of processing a request. It’s important to understand the difference between these two cases, in particular with respect to browser reloads of web pages.

Forward

a forward is performed internally by the servlet
the browser is completely unaware that it has taken place, so its original URL remains intact
any browser reload of the resulting page will simple repeat the original request, with the original URL

Redirect

a redirect is a two step process, where the web application instructs the browser to fetch a second URL, which differs from the original
a browser reload of the second URL will not repeat the original request, but will rather fetch the second URL
redirect is marginally slower than a forward, since it requires two browser requests, not one
objects placed in the original request scope are not available to the second request

In general, a forward should be used if the operation can be safely repeated upon a browser reload of the resulting web page; otherwise, redirect must be used. Typically, if the operation performs an edit on the datastore, then a redirect, not a forward, is required. This is simply to avoid the possibility of inadvertently duplicating an edit to the database.

More explicitly (in terms of common SQL operations) :

for SELECT operations, use a forward
for INSERT, UPDATE, or DELETE operations, use a redirect

In HTML, a

tag can either GET or POST its data. In this context, a GET corresponds to a SELECT-then-forward, and a POST corresponds to an edit-then-redirect.

It’s strongly recommended that forms for the input of search criteria should use GET, while forms for editing database records should use POST.

The most common symptom of not using forward/redirect properly is a warning message in a browser, asking the user if they really wish to POST their form data a second time.

Example

This example is after the style of the WEB4J Controller class. The important methods of the Servlet API are:

ServletRequest.getRequestDispatcher(String)
RequestDispatcher.forward(request, response)
HttpServletResponse.sendRedirect(String)

   import java.io.IOException;
import javax.servlet.*;
import javax.servlet.http.*;
import hirondelle.web4j.action.Action;
import hirondelle.web4j.action.ResponsePage;
import hirondelle.web4j.request.RequestParser;
import hirondelle.web4j.model.BadRequestException;

public class RedirectForward extends HttpServlet {
  
  //..many items elided

  @Override public final void doGet(
      HttpServletRequest aRequest, HttpServletResponse aResponse
   ) throws ServletException, IOException {
    processRequest(aRequest, aResponse);
  }

  @Override public final void doPost(
      HttpServletRequest aRequest, HttpServletResponse aResponse
   ) throws ServletException, IOException {
    processRequest(aRequest, aResponse);
  }

  /**
  * Handle all HTTP GET and POST requests.
  */
  protected void processRequest(
    HttpServletRequest aRequest, HttpServletResponse aResponse
  ) throws ServletException, IOException {
    RequestParser requestParser = RequestParser.getInstance(aRequest, aResponse);
    try {
      Action action = requestParser.getWebAction();
      ResponsePage responsePage = action.execute();
      if ( responsePage.getIsRedirect() ) {
        redirect(responsePage, aResponse);
      }
      else {
        forward(responsePage, aRequest, aResponse);
      }
    }
    catch (BadRequestException ex){
      //..elided
      //use Response.sendError()
    }
    catch (Throwable ex) {
      //..elided
      //use Response.sendError()
    }
  }
  
  // PRIVATE //
  
  private void redirect(
    ResponsePage aDestinationPage, HttpServletResponse aResponse
  ) throws IOException {
    String urlWithSessionID = aResponse.encodeRedirectURL(aDestinationPage.toString());
    aResponse.sendRedirect( urlWithSessionID );
  }

  private void forward(
    ResponsePage aResponsePage, HttpServletRequest aRequest, HttpServletResponse aResponse
  ) throws ServletException, IOException {
    RequestDispatcher dispatcher = aRequest.getRequestDispatcher(aResponsePage.toString());
    dispatcher.forward(aRequest, aResponse);
  }
}