RequestDispatcher.forward() vs HttpServletResponse.sendRedirect()

Question

In the web development world, the term "redirect" is the act of sending the client an empty HTTP response with just a Location header containing the new URL to which the client has to send a brand new GET request. So basically:

• Client sends a HTTP request to some.jsp.
• Server sends a HTTP response back with Location: other.jsp header
• Client sends a HTTP request to other.jsp (this get reflected in browser address bar!)
• Server sends a HTTP response back with content of other.jsp.

You can track it with the web browser's builtin/addon developer toolset. Press F12 in Chrome/IE9/Firebug and check the "Network" section to see it.

Exactly the above is achieved by sendRedirect("other.jsp"). The RequestDispatcher#forward() doesn't send a redirect. Instead, it uses the content of the target page as HTTP response.

• Client sends a HTTP request to some.jsp.
• Server sends a HTTP response back with content of other.jsp.

However, as the original HTTP request was to some.jsp, the URL in browser address bar remains unchanged. Also, any request attributes set in the controller behind some.jsp will be available in other.jsp. This does not happen during a redirect because you're basically forcing the client to create a new HTTP request on other.jsp, hereby throwing away the original request on some.jsp including all of its attribtues.

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The RequestDispatcher 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 the /WEB-INF folder and use a Servlet which controls, preprocesses and postprocesses the requests. The JSPs in the /WEB-INF folder are not directly accessible by URL, but the Servlet can access them using RequestDispatcher#forward().

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 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 navigating 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 thus instructs the client to fire a new GET request on the given URL. 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 experiences. This is also called the POST-Redirect-GET pattern.

See also:

• How do servlets work? Instantiation, sessions, shared variables and multithreading
• doGet and doPost in Servlets
• How perform validation and display error message in same form in JSP?
• HttpServletResponse sendRedirect permanent

requestDispatcher - forward() method

1. When we use the forward method, the request is transferred to another resource within the same server for further processing.

2. In the case of forward, the web container handles all processing internally and the client or browser is not involved.

3. When forward is called on the requestDispatcherobject, we pass the request and response objects, so our old request object is present on the new resource which is going to process our request.

4. Visually, we are not able to see the forwarded address, it is transparent.

5. Using the forward() method is faster than sendRedirect.

6. When we redirect using forward, and we want to use the same data in a new resource, we can use request.setAttribute() as we have a request object available.

SendRedirect

1. In case of sendRedirect, the request is transferred to another resource, to a different domain, or to a different server for further processing.

2. When you use sendRedirect, the container transfers the request to the client or browser, so the URL given inside the sendRedirect method is visible as a new request to the client.

3. In case of sendRedirect call, the old request and response objects are lost because it’s treated as new request by the browser.

4. In the address bar, we are able to see the new redirected address. It’s not transparent.

5. sendRedirect is slower because one extra round trip is required, because a completely new request is created and the old request object is lost. Two browser request are required.

6. But in sendRedirect, if we want to use the same data for a new resource we have to store the data in session or pass along with the URL.

Which one is good?

Its depends upon the scenario for which method is more useful.

If you want control is transfer to new server or context, and it is treated as completely new task, then we go for sendRedirect. Generally, a forward should be used if the operation can be safely repeated upon a browser reload of the web page and will not affect the result.

Source

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.

The main important difference between the forward() and sendRedirect() method is that in case of forward(), redirect happens at server end and not visible to client, but in case of sendRedirect(), redirection happens at client end and it's visible to client.