In the Spring Boot make the frontend part update without restarting the application?
I'm an Eclipse user (via Spring Tool Suite) and I never have any problems with reloading static content if the app is running in debug mode. Just right click on the main class and "Debug As->Java App" (or "Spring Boot App", it's the same). You have to also ensure that the workspace is configured to "Build Automatically", but that is the default as far as I know.
I can't offer such specific advice for users of other IDEs, but I'm sure they all have the same feature.