How to find out the type of device at the time of acceptance of the request?

I am making a website. the backend is made in java spring. In the idea, the user requests a page of the website and I have to find out the type of device (screen size is possible) and respond with html and css with a document for the desired screen type. How can one find out the type or size of the screen when making a get request? The first time you access the website.