Follow the steps here: https://spring.io/quickstart
Add spring dev tools as a dependency during project configuration here https://start.spring.io/
Use 2.5.2 version
Install Live reload extension for Chrome: https://chrome.google.com/webstore/detail/livereload/jnihajbhpnppcggbcgedagnkighmdlei/related
Run the app using mvn spring-boot:run.
Open browser http://localhost:8080/hello.
Click on the Live reload extension button.
There will be an error shown in the popup : see attached image.
The problem is : JS WebSocket class can't connect to the server.
If I write the code in the Chrome console ws = new WebSocket("ws://localhost:35729"); then it fails with error.
Everything works fine with 2.5.1 version.
Comment From: wilkinsona
Thanks for the report and sorry for the inconvenience. I suspect that this regression is caused by the changes for https://github.com/spring-projects/spring-boot/pull/26813. Specifically, I think the lower-casing has corrupted the value of the sec-websocket-key header.
Comment From: denis-anisimov
Most likely yes.