java - RequestDispatcher for remote server?

Question

I am trying to create a HttpServlet that forwards all incoming requests as is, to another serlvet running on a different domain.

How can this be accomplished? The RequestDispatcher's forward() only operates on the same server.

Edit: I can't introduce any dependencies.

Answer 1

You can't when it doesn't run in the same ServletContext or same/clustered webserver wherein the webapps are configured to share the ServletContext (in case of Tomcat, check crossContext option).

You have to send a redirect by HttpServletResponse.sendRedirect(). If your actual concern is reusing the query parameters on the new URL, just resend them along.

response.sendRedirect(newURL + "?" + request.getQueryString());

Or when it's a POST, send a HTTP 307 redirect, the client will reapply the same POST query parameters on the new URL.

response.setStatus(HttpServletResponse.SC_TEMPORARY_REDIRECT);
response.setHeader("Location", newURL);

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Update as per the comments, that's apparently not an option as well since you want to hide the URL. In that case, you have to let the servlet play for proxy. You can do this with a HTTP client, e.g. the Java SE provided java.net.URLConnection (mini tutorial here) or the more convenienced Apache Commons HttpClient.

If it's GET, just do:

InputStream input = new URL(newURL + "?" + request.getQueryString()).openStream();
OutputStream output = response.getOutputStream();
// Copy.

Or if it's POST:

URLConnection connection = new URL(newURL).openConnection();
connection.setDoOutput(true);
// Set and/or copy request headers here based on current request?

InputStream input1 = request.getInputStream();
OutputStream output1 = connection.getOutputStream();
// Copy.

InputStream input2 = connection.getInputStream();
OutputStream output2 = response.getOutputStream();
// Copy.

Note that you possibly need to capture/replace/update the relative links in the HTML response, if any. Jsoup may be extremely helpful in this.