How does flash object map work inside grails framework
grails, grails-controller
Solution
Flash is actually a temporary storage map for the present request and the next request only. It won't retain the entries after the next request unless entries are repopulated in the next request (which would be current in future). Here is how it works in Grails:
`FlashScope` interface which extends Map itself has two methods `next()` and `getNow()` is implemented by `GrailsFlashScope`. All of which can be found in `grails-web-mvc`.
`GrailsFlasScope` mainly maintains two concurrent HasMap (one for current request and second for the next request) to hold the entries. It implements `next()` from `FlashScope` to do the cleaning and "restricting-to-next-request-only" part as:
a. clear current b. make next as current c. clear next
Next thing to focus will be `GrailsWebRequestFilter` (implements `OncePerRequestFilter`) which makes sure that there is always a single execution of request per dispatch.
All http servlet requests are filtered by `GrailsWebRequestFilter`. This filter sets the flash scope to `next` so that every time the latest and valid info is retrieved.
Now the question is how does `FlashScope` reconciles current and next map? Well, that is why `FlashScope` is extended from `Map`. FlashScope overrides `get(key)` from map to reconcile both the maps by making sure values are retrieved from `next` map otherwise switch to current map.
How is `flash` available to `controllers` by default? All controller inherit `ControllersApi` which inherits `CommonWebApi`.
I hope you get what you were looking for..
Problem
Flash, which is a temporary storage map for one request. I am wondering how this is implemented on the grails core framework In particular I'm interested in the class(es) responsible for putting the flash map in the request and then taking it out once the request has finessed processing.