Redirect after a POST vs redirect after a GET - spring

Question

I'm working on a Spring project. Here's my basic controller:
#Controller
public class Editor {
private static final String EDITOR_URL = "/editor";
#RequestMapping(value = EDITOR_URL, method = {POST, GET})
public ModelAndView edit(HttpServletResponse response,
HttpServletRequest request,
RedirectAttributes redirectAttributes,
#RequestParam Map<String, String> allRequestParams) {
// The code is trimmed to keep it short
// It doesn't really matter where it gets the URL, it works fine
String redirectURL = getRedirectUrl();
// redirectURL is going to be /editor/pad.html
return new ModelAndView("redirect:" + redirectUrl);
}
From web.xml:
<servlet-mapping>
<servlet-name>edm</servlet-name>
<url-pattern>/</url-pattern>
</servlet-mapping>
I have jetty embedded and I'm trying an integration test:
#Test
public void redirectToEditPadSuccess() throws Exception {
HttpHeaders requestHeaders = new HttpHeaders();
UriComponentsBuilder builder = UriComponentsBuilder.fromHttpUrl(END_POINT + "/edm/editor")
.queryParam("param1", "val1")
.queryParam("param2", "val2");
HttpEntity<?> entity = new HttpEntity<>(requestHeaders);
HttpEntity<String> response = restTemplate.exchange(
builder.build().encode().toUri(),
HttpMethod.POST,
entity,
String.class);
HttpHeaders httpResponseHeaders = response.getHeaders();
List<String> httpReponseLocationHeader = httpResponseHeaders.get("Location");
assertTrue(httpReponseLocationHeader.size() == 1);
String redirectLocation = httpReponseLocationHeader.get(0);
URL redirectURL = new URL(redirectLocation);
assertEquals("/edm/editor/pad.html", redirectURL.getPath());
}
So when I execute the above it works fine and I get a green OK sign.
Now, the controller accepts both POST and GET methods. If I execute the test using GET method (replacing HttpMethod.POST with HttpMethod.GET), the result is going to be a 404.
The logs reveal:
WARN org.springframework.web.servlet.PageNotFound - No mapping found for HTTP request with URI [/edm/editor/pad.html] in DispatcherServlet with name 'edm'
I tried to debug the application up to the DispatcherServlet and weird thing is that with GET, after the 302/redirect response the Dispatcher is being called again and turns this to a 200 - no idea how and why.

Answer 1

I'm going to try and explain what is going on, and then provide a solution.
First let's forget that you're running a rest case, and assume that the request is coming from a browser.
Scenario 1 : Browser issues a GET request, and the server responds with a redirect.
In this case, the browser reads the response status code as 302 and makes another request using the Location response header. The user sees a quick reload but doesn't notice anything wrong.
Scenario 2 : Browser issues a POST request, and the server responds with a redirect.
In this case, the browser does follow the response code and does issue a redirect, but, the second request is a GET request, and the original request body is lost in the second request. This is because strictly by HTTP standards, the browser cannot "re-post" data to the server, without an explicit request by the user. (Some browsers will prompt the user and ask them if they want to re-post)
Now in your code, RestTemplate is using what I presume to be a default HttpClientFactory, most likely this one: https://github.com/spring-projects/spring-framework/blob/master/spring-web/src/main/java/org/springframework/http/client/SimpleClientHttpRequestFactory.java.
This is how RestTemplate is handling the above two scenarios:
Scenario 1 : Rest Template issues a GET request, and the server responds with a redirect.
Here the Rest Template instance will work exactly as a browser would. That's the reason why two requests are being made, and the second one is looking for /edm/editor/pad.html
Scenario 2 : Rest Template issues a POST request, and the server responds with a redirect.
In this case, Rest Template will stop after the first call, because it cannot automatically override your request method and change it to GET, and it cannot prompt you for permission, like a browser would.
Solution: When creating an instance of RestTemplate, pass it an overridden version of the client factory, something like
new RestTemplate(new SimpleClientHttpRequestFactory() {
protected void prepareConnection(HttpURLConnection conn, String httpMethod) throws IOException {
super.prepareConnection(conn, httpMethod);
conn.setInstanceFollowRedirects(false);
}
});
This will instruct rest template to stop after the first request.
Sorry for the lengthy answer, but I hope this clarifies things.