Here is the difference:

• addFlashAttribute() actually stores the attributes in a flashmap (which is internally maintained in the users session and 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

denis-anisimov:

This looks like a bug in Spring: spring-projects/spring-boot#27147

JS WebSocket class can't connect to the server and there is nothing to fix on our side.
The same happens with:

• console code ws = new WebSocket("ws://localhost: 35729");
• Live reload Chrome extension: it doesn't work with 2.5.2

pleku:
Fixed in Spring Boot 2.4.9 & 2.5.3

Live reload is broken starting with Spring Boot 2.5.2 #11375
DevTools' LiveReload support's HTTP header handling is case sensitive #26813