The applications we developed so far were running on a single JVM. We already have some experience with concurrent programming and this is something that will come handy now. When we program a web application, a part of the code will run on the server and a part of the application logic will execute in the browser. The server part will be written in Java, the browser part will be implemented in HTML, CSS, and JavaScript. Since this is a Java book we will focus mainly on the server part, but we should still be aware of the fact that many of the functionalities can be and should be implemented to run in the browser. The two programs communicate with each other over the IP network, that is, the Internet, or in the case of an enterprise internal application, the network of the company.

Today, a browser is capable of running very powerful applications, all implemented in JavaScript. A few years ago, such applications needed client application implemented in Delphi...