Springboot cannot reference css and js files when using thymeleaf3.0

I found some articles on the Internet, but many of them were useless. I tried various methods of modifying application.properties on cdns and then changing the startup class file, but it didn't work.
Finally, after spending a whole day watching related videos on the Internet, I discovered that I had made a mistake in how I opened the HTML. The correct operation should be to enter the address in the browser to open it instead of opening it in the idea. Because I am used to using Eclipse, and I learned it by reading books, I am not very good at new development tools, which led to errors.