Redirect to an external URL from controller action in Spring MVC

You can do it with two ways.

First:

@RequestMapping(value = "/redirect", method = RequestMethod.GET)
public void method(HttpServletResponse httpServletResponse) {
    httpServletResponse.setHeader("Location", projectUrl);
    httpServletResponse.setStatus(302);
}

Second:

@RequestMapping(value = "/redirect", method = RequestMethod.GET)
public ModelAndView method() {
    return new ModelAndView("redirect:" + projectUrl);
}

You can use the RedirectView. Copied from the JavaDoc:

View that redirects to an absolute, context relative, or current request relative URL

Example:

@RequestMapping("/to-be-redirected")
public RedirectView localRedirect() {
    RedirectView redirectView = new RedirectView();
    redirectView.setUrl("http://www.yahoo.com");
    return redirectView;
}

You can also use a ResponseEntity, e.g.

@RequestMapping("/to-be-redirected")
public ResponseEntity<Object> redirectToExternalUrl() throws URISyntaxException {
    URI yahoo = new URI("http://www.yahoo.com");
    HttpHeaders httpHeaders = new HttpHeaders();
    httpHeaders.setLocation(yahoo);
    return new ResponseEntity<>(httpHeaders, HttpStatus.SEE_OTHER);
}

And of course, return redirect:http://www.yahoo.com as mentioned by others.

Looking into the actual implementation of UrlBasedViewResolver and RedirectView the redirect will always be contextRelative if your redirect target starts with /. So also sending a //yahoo.com/path/to/resource wouldn't help to get a protocol relative redirect.

So to achieve what you are trying you could do something like:

@RequestMapping(method = RequestMethod.POST)
public String processForm(HttpServletRequest request, LoginForm loginForm, 
                          BindingResult result, ModelMap model) 
{
    String redirectUrl = request.getScheme() + "://www.yahoo.com";
    return "redirect:" + redirectUrl;
}