History of web application
Earlier in client- server computing, each application had its own client program and it worked as a user interface and need to be installed on each user's personal computer. Most web applications use HTML/XHTML that are mostly supported by all the browsers and web pages are displayed to the client as static documents. A web page can merely displays static content and it also lets the user navigate through the content, but a web application provides a more interactive experience.
Any computer running Servlets or JSP needs to have a container. A container is nothing but a piece of software responsible for loading, executing and unloading the Servlets and JSP. While servlets can be used to extend the functionality of any Java- enabled server. They are mostly used to extend web servers, and are efficient replacement for CGI scripts. CGI was one of the earliest and most prominent server side dynamic content solutions, so before going forward it is very important to know the difference between CGI and the Servlets.
Common Gateway Interface (CGI)
The Common Gateway Interface, which is normally referred as CGI, was one of the practical technique developed for creating dynamic content. By using the CGI, a web server passes requests to an external program and after executing the program the content is sent to the client as the output. In CGI when a server receives a request it creates a new process to run the CGI program, so creating a process for each request requires significant server resources and time, which limits the number of requests that can be processed concurrently. CGI applications are platform dependent. There is no doubt that CGI played a major role in the explosion of the Internet but its performance, scalability issues make it less than optimal solutions.
Java Servlet is a generic server extension that means a java class can be loaded dynamically to expand the functionality of a server. Servlets are used with web servers and run inside a Java Virtual Machine (JVM) on the server so these are safe and portable. Unlike applets they do not require support for java in the web browser. Unlike CGI, servlets don't use multiple processes to handle separate request. Servets can be handled by separate threads within the same process. Servlets are also portable and platform independent.