As of writing this book, HTTP/1.1 is the primarily used version of HTTP. HTTP/2 targets the shortcomings of HTTP performance of web applications in the past. In particular, requesting several resources of a web based system could lead to suboptimal performance due to the numerous connections involved. Version 2 of HTTP aims to lower latency and maximize throughput by multiplexing, pipelining, compressing headers, and Server Push.

Most of the changes in HTTP/2 do not affect the engineers' work compared to 1.1. The servlet container deals with HTTP concerns under the hood. The exception to this is the Server Push feature.

Server Push works in such a way that the server directly sends HTTP responses of resources related to a client-requested resource, following the assumption that the client would need these resources as well. It allows the server to send resources which were not explicitly requested by a client. This is a performance optimization technique that in web pages mainly concerns style sheets, JavaScript code, and other assets.

The Servlet API supports Server Push messages by using the PushBuilder type that is instantiated with the HttpServletRequest.newPushBuilder() method.