Flash Attributes

Flash attributes provide a way for one request to store attributes that are intended for use in another. This is most commonly needed when redirecting — for example, the Post-Redirect-Get pattern. Flash attributes are saved temporarily before the redirect (typically in the session) to be made available to the request after the redirect and are removed immediately.

Spring MVC has two main abstractions in support of flash attributes. FlashMap is used to hold flash attributes, while FlashMapManager is used to store, retrieve, and manage FlashMap instances.

Flash attribute support is always “on” and does not need to be enabled explicitly. However, if not used, it never causes HTTP session creation. On each request, there is an “input” FlashMap with attributes passed from a previous request (if any) and an “output” FlashMap with attributes to save for a subsequent request. Both FlashMap instances are accessible from anywhere in Spring MVC through static methods in RequestContextUtils.

Annotated controllers typically do not need to work with FlashMap directly. Instead, a @RequestMapping method can accept an argument of type RedirectAttributes and use it to add flash attributes for a redirect scenario. Flash attributes added through RedirectAttributes are automatically propagated to the “output” FlashMap. Similarly, after the redirect, attributes from the “input” FlashMap are automatically added to the Model of the controller that serves the target URL.

Matching requests to flash attributes

The concept of flash attributes exists in many other web frameworks and has proven to sometimes be exposed to concurrency issues. This is because, by definition, flash attributes are to be stored until the next request. However the very “next” request may not be the intended recipient but another asynchronous request (for example, polling or resource requests), in which case the flash attributes are removed too early.

To reduce the possibility of such issues, RedirectView automatically “stamps” FlashMap instances with the path and query parameters of the target redirect URL. In turn, the default FlashMapManager matches that information to incoming requests when it looks up the “input” FlashMap.

This does not entirely eliminate the possibility of a concurrency issue but reduces it greatly with information that is already available in the redirect URL. Therefore, we recommend that you use flash attributes mainly for redirect scenarios.