Use of redirect: in Spring MVC

[Mike Tabak]

I am working with the following controller from the Spring In Practice book:

/* 
 * Copyright (c) 2013 Manning Publications Co.
 * 
 * Book: http://manning.com/wheeler/
 * Blog: http://springinpractice.com/
 * Code: https://github.com/springinpractice
 */
package com.springinpractice.ch09.article.web;

import javax.inject.Inject;
import javax.servlet.http.HttpServletRequest;
import javax.validation.Valid;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Controller;
import org.springframework.ui.Model;
import org.springframework.validation.BindingResult;
import org.springframework.web.bind.WebDataBinder;
import org.springframework.web.bind.annotation.InitBinder;
import org.springframework.web.bind.annotation.ModelAttribute;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RequestParam;

import com.springinpractice.ch09.article.model.ArticlePage;
import com.springinpractice.ch09.article.service.ArticleService;
import com.springinpractice.ch09.comment.model.Comment;

@Controller
@RequestMapping("/articles")
public final class ArticleController {
	private static final Logger log = LoggerFactory.getLogger(ArticleController.class);
	
	@Inject private ArticleService articleService;
	
	@Value("#{viewNames.articleList}") private String articleListViewName;
	@Value("#{viewNames.articlePage}") private String articlePageViewName;
	@Value("#{viewNames.postCommentFailed}") private String postCommentFailedViewName;
	
	@InitBinder
	public void initBinder(WebDataBinder binder) {
		binder.setAllowedFields(new String[] { "id", "name", "email", "web", "text" });
	}
	
	@RequestMapping(value = "", method = RequestMethod.GET)
	public String getArticles(Model model) {
		model.addAttribute(articleService.getAllArticles());
		return articleListViewName;
	}
	
	@RequestMapping(value = "/{articleName}/{pageNumber}", method = RequestMethod.GET)
	public String getArticlePage(
			@PathVariable String articleName,
			@PathVariable int pageNumber,
			Model model) {
		
		prepareModel(model, articleName, pageNumber);
		model.addAttribute(new Comment());
		return articlePageViewName;
	}
	
	@RequestMapping(value = "/{articleName}/comments", method = RequestMethod.POST)
	public String postComment(
			HttpServletRequest req,
			@PathVariable String articleName,
			@RequestParam("p") int pageNumber,
			Model model,
			@ModelAttribute @Valid Comment comment,
			BindingResult result) {
		
		if (result.hasErrors()) {
			log.debug("Comment validation error");
			result.reject("global.error");
			prepareModel(model, articleName, pageNumber);
			return postCommentFailedViewName;
		}
		
		// No validation errors; post comment.
		log.debug("Posting comment");
		comment.setIpAddress(req.getRemoteAddr());
		articleService.postComment(articleName, comment);
		return "redirect:" + pageNumber + "#comment-" + comment.getId();
	}

	/**
	 * @param articleName
	 * @param pageNumber
	 * @param model
	 */
	private void prepareModel(Model model, String articleName, int pageNumber) {
		ArticlePage page = articleService.getArticlePage(articleName, pageNumber);
		
		// articlePage.jsp expects this
		model.addAttribute(page);
		
		// list.jspf (comment list) expects this
		model.addAttribute(page.getArticle().getComments());
	}
}

The project is using an InternalResourceViewResolver. As you can see, the postComment() method is mapped to a URL of the form: "http://localhost:8080/sip/articles/{article-name}/commnets", where my web application context path is "/sip". When I hit this method (which saves a comment to an article), as you see above, the method does a redirect to view named by the string {pageNumber + "#comment-" + comment.getId()}, where pageNumber and comment.getId() are integers. This should form a string of the form: "1#commnet-5" for pageNumber = 1 and comment.getId() = 5. The return statement will then be: return "redirect:1#comment-5";. When I hit this method in the application with these values, it redirects to the URL "http://localhost:8080/sip/{article-name}/1?null". My understanding of redirect: was that it takes the view that follows ':' as the redirect URL, so it seems like it should have redirected to "http://localhost:8080/sip/1#comment-5". I don't see where the redirected URL is getting the {article-name} from and why it is dropping the "#comment-5". Also, as part of testing I changed the statement to: [return "redirect:";] and the URL I got was "http://localhost:8080/sip/#postComment". Could someone enlighten me as to what is going on. I am obviously not fully understanding how redirect: works in Spring. Thank you in advance.

[H Paul]

"I shall pass through this world but once. Any good, therefore, that I can show to any human being,
let me do it now. Let me not defer nor neglect it, for I shall not pass this way again."

GET-POST-REDIRECT pattern from http://en.wikipedia.org/wiki/Post/Redirect/Get

According to my understanding: See my down below Servlet3UploadController as a simple illustration concept-wise.
(Use case: The user get a upload form, post it, and recieve a status: success or failure)

For my example: I use TCP/IP monitor to see the traffic exchange between client and server

1. client From browser: http://localhost:18180/mvc/newupload (where mvc is my servlet context)
GET /mvc/newupload HTTP/1.1

server
HTTP/1.1 200 OK

2.
(client see the form and upload a file)

POST /mvc/newupload/form HTTP/1.1

3.
server
HTTP/1.1 302 Found
Location: http://localhost:18180/mvc/newupload;jsessionid=1AB62F1331AB48940C08CD248B75D486

(REDIRECT)

4. client see the form

@Controller
@RequestMapping(value = "/newupload")
public class Servlet3UploadController {
	private Logger logger = LoggerFactory
			.getLogger(Servlet3UploadController.class);

	@RequestMapping(method = RequestMethod.GET)
	public String showForm(HttpServletRequest request) {
		if (logger.isDebugEnabled()) {
			Map<String, ?> flashMap=RequestContextUtils.getInputFlashMap(request); 
			logger.debug("{}",flashMap);
		}
		
		return "uploadform";
	}

	@RequestMapping(value = "/form", method = RequestMethod.POST)
	public String handleFormUpload(NewFileUpload upload, RedirectAttributes redirectAttributes) {

		try {
			InputStream in = upload.getFile().getInputStream();
			logger.debug("file size {}", FileCopyUtils.copyToByteArray(in).length);
			
			//do something with it
			
			//show to the user
			redirectAttributes.addFlashAttribute("uploadstatus", "Upload succeeded.");
			
		} catch (IOException e) {
			e.printStackTrace();
			redirectAttributes.addAttribute("uploadstatus", "Upload failed.");

		}
		
		// This will IMPLICTLY handled by the server: method GET wth http://localhost:18180/mvc/newupload
		// use TCP/IP monitor 
		
		return "redirect:/newupload";  
	}
}

import org.springframework.web.multipart.MultipartFile;

public class NewFileUpload {
	private String name;
	private MultipartFile file;
...
}
	

uploadform.jsp

<form id="uploadForm" action="<c:url value="/newupload/form"/>" method="POST" enctype="multipart/form-data">
<fieldset>
<legend>Upload File</legend>
<input type="text" name="name"/>
<input type="file" name="file"/>
<input type="submit"/>
</fieldset>
</form>

<h1>${uploadstatus}</h1>

[Mike Tabak]

Thanks, I understand this. Actually my questions comes down to the following. Say I have a controller with the following method:

...
@RequestMapping(value="/rd/test", method = RequestMethod.GET)
public String rdtest(HttpServletRequest req) {
    return "redirect:rdtest";
}
.....

This will try to redirect to a view "/ctxroot/rd/rdtest" where ctxroot is the context root. So in the above since we don't have a leading slash in the redirect (redirect:/rdtest), the redirect is with respect to /ctxroot/rd. If we did include the slash we would have redirected to "/ctxroot/rdtest". So when the slash is missing, is this what is referred to as redirecting to the parent URL?

[H Paul]

Trust but verify.

A. I assume there is not request mapping at the class level

public class YOURCONTROLLER
{
@RequestMapping(value="/rd/test", method = RequestMethod.GET)  
public String rdtest(HttpServletRequest req) {  
    return "redirect:rdtest";  
}  
}

From Spring Framework Reference Documentation 4.0.4.RELEASE
Section:
16.5. Resolving views
RedirectView

"redirect:{url}" is a RedirectView. When Spring see The redirect: prefix, it will take it a REDIRECT with {url} is the redirect URL.

For your case:
1.
return "redirect:/rdtest";
will be mapped as:
http://server:port/ctxroot/rdtest

(relative to the current Servlet context ctxroot)

2. BUT

return "redirect:rdtest";

will be mapped as: http://server:port/ctxroot/rd/test/rdtest

This is the FUN part with a debugger. (Trust but verify.)

B. Just use a debugger and set a break at below method and put a trace for log.

package org.springframework.web.servlet;

public class DispatcherServlet extends FrameworkServlet {
...
	protected View resolveViewName(String viewName, Map<String, Object> model, Locale locale,
			HttpServletRequest request) throws Exception {

		for (ViewResolver viewResolver : this.viewResolvers) {
			View view = viewResolver.resolveViewName(viewName, locale);
			if (view != null) {
				return view;
			}
		}
		return null;
	}
...	
}	

Put a trace for log:

	<logger name="org.springframework.web">
		<level value="trace" />
	</logger>

Edited: Use TCP/IP mon to watch the exchange between client and server where you will see the absolute redirect URL: See header Location: {http url}