Which is the best server-side framework in java?

I would recommend Web Firm Framework. It provides Java classes for all HTML5 tags and attributes. Using it we can build the UI so the developer has full control over the UI building which is very important for building the best UI/UX. You don't have to write web services to send and receive data from the server, the framework has inbuilt support for it, you can develop the UI just like a client-side application.

The latest version supports multi-threading, i.e. you can use multiple threads to build different parts of the UI. It is also the best choice for building a highly reactive application.

Checkout its API doc for a full list of classes.

Is there any framework to handle authentication and authorization in Java?

Yes, Apache Shiro™ is the solution if you are looking for an independent framework, i.e. it has no dependency over the application framework. You can use it on any web or desktop application. Some server-side frameworks like Web Firm Framework highly recommend it, you can read it here.