Sunday, February 19, 2012

Spring MVC - Flash Attributes

Latest Spring Framework incarnation (3.1) brought interesting feature called Flash Attributes. It is remedy for the problem mentioned a long time ago, in one of my posts: Spring MVC - Session Attributes handling. This problem can be described in few words: if we want to pass the attributes via redirect between two controllers, we cannot use request attributes (they will not survive the redirect), and we cannot use Spring's @SessionAttributes (because of the way Spring handles it), only ordinary HttpSession can be used, which is not very convenient.

Below you will find an example of Flash Attributes usage, before you start reviewing it, read Using flash attributes section of Spring documentation.

Suppose that we have two controllers: AController and BController, first one will prepare some data and pass to the second using Flash Attributes after the form submission. On the AController we will have something like this:
@RequestMapping(method = RequestMethod.POST)
public String handleFormSubmission(..., final RedirectAttributes redirectAttrs) {
    ...
    redirectAttrs.addFlashAttribute("AttributeName", value);
    return "redirect:to_some_url_handled_by_BController";
}
When the form will be submitted, attribute value will be stored as Flash Attribute named "AttributeName", and thanks to the Spring, will be passed to BController, where it can be used for example in following way:
@Controller
...
@SessionAttributes("AttributeName")
public class SearchCriteriaHandler {
    ...
    @RequestMapping(method = RequestMethod.GET)
    public void handleGetRequest(@ModelAttribute("AttributeName") final SomeType value) {
        ...
    }
    ...
}

Before your handler method will be called, Spring Framework will populate the Model with the available Flash Attributes - at this point value passed from AController will become a model attribute for the BController. Note, that because we also defined this attribute as the Session Attribute, it will be automatically stored for future usage within this controller, after the GET request handling.

Let me say that I was waiting for this feature for the long time, ;)

Related Posts: Spring MVC - Session Attributes handling

4 comments:

  1. Any reason here to declare RedirectAttributes as final. Would like to know.
    Thanks for such a simple article.

    ReplyDelete
  2. @catchmahesh, @id - at least one :), but pretty important - re-assigning method parameters is bad practice. When you mark method parameters as final, you clearly express the intention, and avoid accidental reassignment :)

    ReplyDelete
  3. Thank you Michal - Found it useful today.

    ReplyDelete