requestDispatcher Interface Vs sendRedirect

Question

Hi,

Can I know the difference between the RequestDispatcher interface and sendRedirect().

Thanks Ravi

Answer 1

The RequestDispatcher interface allows you to do a server side forward/include whereas sendRedirect() does a client side redirect. In a client side redirect, the server will send back an HTTP status code of 302 (temporary redirect) which causes the web browser to issue a brand new HTTP GET request for the content at the redirected location. In contrast, when using the RequestDispatcher interface, the include/forward to the new resource is handled entirely on the server side.

Answer 2

Either of these methods may be "better", i.e. more suitable, depending on what you want to do.

A server-side redirect is faster insofar as you get the data from a different page without making a round trip to the browser. But the URL seen in the browser is still the original address, so you're creating a little inconsistency there.

A client-side redirect is more versatile insofar as it can send you to a completely different server, or change the protocol (e.g. from HTTP to HTTPS), or both. And the browser is aware of the new URL. But it takes an extra back-and-forth between server and client.

Answer 3

In short: a forward reuses the current request and a redirect creates a new request, hereby losing the initial request, including all of its attributes and parameters.

A forward is extremely useful in the MVC paradigm and/or when you want to hide JSP's from direct access. You can put JSP's in /WEB-INF folder and use a Servlet which controls, preprocesses and postprocesses the requests. The JSP's in /WEB-INF folder are not directly accessible by URL, but the Servlet can access them using RequestDispatcher.

You can for example have a JSP file in /WEB-INF/login.jsp and a LoginServlet which is mapped on an url-pattern of /login. When you invoke http://example.com/context/login, then the servlet's doGet() will be invoked. You can do any preprocessing stuff in there and finally forward the request like:

request.getRequestDispatcher("/WEB-INF/login.jsp").forward(request, response);

When you submit a form, you normally want to use POST:

<form action="login" method="post">

This way the servlet's doPost() will be invoked and you can do any postprocessing stuff in there (e.g. validation, business logic, login the user, etc). If there are any errors, then you normally want to forward the request back to the same page and display the errors there next to the input fields and so on. You can use the RequestDispatcher for this.

If a POST is been successful, you normally want to redirect the request, so that the request won't be resubmitted when the user refreshes the request (e.g. pressing F5 or nagivating back in history).

User user = userDAO.find(username, password);
if (user != null) {
    request.getSession().setAttribute("user", user); // Login user.
    response.sendRedirect("home"); // Redirects to http://example.com/context/home after succesful login.
} else {
    request.setAttribute("error", "Unknown login, please try again."); // Set error.
    request.getRequestDispatcher("/WEB-INF/login.jsp").forward(request, response); // Forward to same page so that you can display error.
}

A redirect namely instructs the client to fire a new GET request. Refreshing the request would then only refresh the redirected request and not the initial request. This will avoid "double submits" and confusion and bad user experience. This is also called the POST-Redirect-GET pattern.