'Why is Spring boot User Session stored in ThreadLocal a bad pratice?

I'm new to an ongoing project/company and this project is using ThreadLocal to store user session data. It's a Springboot project.

I already know that this is BAD design, but i'm trying to find documentation to explain then why this will probably fail badly when the number of users/requests grow.

Can someone help me with links that explain the relation of Threads and Spring Sessions or provide an explanation on how this is a design flaw?



Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source