Servlets are simple java technology which allow handle, process or modify request and response with HTTP protocol.

Simply put, when your computer or any other want to load remote website, it will make request through network to server. On this remote server usually run some service or container, which act as Web Server. This Web Server does basically really simple thing. It listen on specific port, process incoming requests and usually return response. This request could be file, image, folder or something else.

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In many times we want Web Server to generate some dynamic content. Like file with current date, greetings to logged person, shop order and others. To achieve this we need to use some programing language to tell Web Server what to do.

We have several options:

Server	Language
Apache	PHP, Perl, Python, Ruby, Rails
MS Server	Asp.net
Tomcat 7	Java (Servlet, JSP)
NginX	Only static files

In this case, Tomcat 7 is not really WebServer (or HTTP server) but Servlet and JSP Container which is fully complaint with Java Servlet specification. There is many versions, which have different features, each version of Tomcat supports different one.

Servlet example:

package my.app;

import java.io.IOException;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

@WebServlet(
	asyncSupported = false, 
	name = "MainServlet",
	urlPatterns = {"/MainController"}
)
public class MainServlet extends HttpServlet {

	@Override
	protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
		resp.getWriter().write("Hello world!");
	}

}

Links

• Servlet Specification 3.0

Servlet history

VERSION	DATE	JAVA EE / JDK	FEATURES / CHANGES
Servlet 4.0	September 2017	JavaEE 8	HTTP/2
Servlet 3.1	May 2013	JavaEE 7	Non-blocking I/O, HTTP protocol upgrade mechanism
Servlet 3.0	December 2009	JavaEE 6, JavaSE 6	Pluggability, Ease of development, Async Servlet, Security, File Uploading
Servlet 2.5	September 2005	JavaEE 5, JavaSE 5	Requires JavaSE 5, supports annotation
Servlet 2.4	November 2003	J2EE 1.4, J2SE 1.3	web.xml uses XML Schema
Servlet 2.3	August 2001	J2EE 1.3, J2SE 1.2	Addition of Filter
Servlet 2.2	August 1999	J2EE 1.2, J2SE 1.2	Becomes part of J2EE, introduced independent web applications in .war files
Servlet 2.1	November 1998	Unspecified	First official specification, added RequestDispatcher, ServletContext
Servlet 2.0		JDK 1.1	Part of Java Servlet Development Kit 2.0
Servlet 1.0	June 1997