Here is the difference:
addFlashAttribute()actually stores the attributes in a flashmap (which is internally maintained in the users
sessionand removed once the next redirected request gets fulfilled)
addAttribute()essentially constructs request parameters out of your attributes and redirects to the desired page with the request parameters.
So the advantage of
addFlashAttribute() will be that you can store pretty much any object in your flash attribute (as it is not serialized into request params at all, but maintained as an object), whereas with
addAttribute() since the object that you add gets transformed to a normal request param, you are pretty limited to the object types like
String or primitives.
answerd by Biju Kunjummen https://stackoverflow.com/questions/14470111/spring-redirectattributes-addattribute-vs-addflashattribute