Getting started with Servlets

Hello All, this time I’m going to give you a brief introduction on how to set-up and run a simple servlet. A long due part of my first introductoral tutorial (sorry for that).

 

A little Background Information:

 

  • “Remind me, What was a Servlet again?”: Servlets were introduced to allow to combine HTML pages with Java-Code and thus give them a dynamic character.
  • “Combine?”: Yes combine, i.e. to include the result of java-code into a HTML page.

 

Creating the foundation for your Servlet’s cosy little home:

 
Before you can start off with your first Servlet, you need to ensure that you are running a framework and that you have the right folder structure in your project.

  • Set-Up a Tomcat/JBoss (For detailed information on how to do so see Setting up Tomcat or JBoss)

  • Create an appropriate (project) File-Directory-Structure as follows:
    • <project-folder> (This can have any name, depending on what you call your project)

    • WEB-INF (!This folder is mandatory! – It also has the nice property, to “hide” what is inside of it and its sub-folder structures*)
      • lib (should you use any external libraries, that need to be accessible by your code, then store them here)

      • classes (here the “.class”es of your Servlet and maybe additional JavaBeans should be stored)

      • <_anything-else_>, as I already mentioned above, you can store any other folders and files within the WEB-INF folder (even your source files (which you shouldnt for safety reasons*)) and it would not be visible to an external accessee.

 

Example:

  • HelloWorldServlet
    • WEB-INF
      • web.xml
      • classes
        • beans
          • HelloWorldBean.class (NOTE: in our minimal example (Implementation) we won’t include the HelloWorldBean for simplicity)
        • servlets
          • HelloWorldServlet.class
      • sources (*)
        • beans
          • HelloWorldBean.java (NOTE: in our minimal example (Implementation) we won’t include the HelloWorldBean for simplicity)
        • servlets
          • HelloWorldServlet.java

 

Write your first amazing Servlet:

 
Now You can write your first Servlet, which I call (as ingeniously indicated above) HelloWorldServlet.java and it looks like this:

 

package servlets;

import java.io.IOException;
import java.io.PrintWriter;
import java.util.Date;

import javax.servlet.ServletException;
import javax.servlet.http.*;

public class HelloWorldServlet extends HttpServlet {
	public void service(HttpServletRequest request,
			 			 HttpServletResponse response)
			throws IOException, ServletException {
		// Set the content type of the response document.
		response.setContentType("text/html;charset=ISO-8859-1");

		// Retrieve the printwriter to write to the response document.
		PrintWriter out = response.getWriter();

		// Output / Respond an html document with a greeting and a date.
		out.println("<html><body>");
		out.println("	Hello World!     ");
		out.println("	It's about time: " + new Date());
		out.println("</html></body>");
	}
}

 

Compile your first amazing Servlet:

 

I suggest doing so using an ANT-script, as follows:

<project name="helloworldservlet" default="compile" basedir=".">
  <!-- Set global properties for this build -->
  <property name="src" location="sources"/>
  <property name="build" location="classes"/>
  <!-- Replace this with your Tomcat path. -->
  <property name="tomcat" location="C:\Program Files\Tomcat_7.0.2"/>

  <!-- Define the classpath. -->
  <path id="classpath">
	<!-- This is the jar for the Servlet-api -->
    <fileset dir="${tomcat}/lib">
    	<include name="**/servlet-api.jar"/>
    </fileset>
  </path>
	
  <!-- Create the build directory structure used by compile -->
  <target name="init">
    <mkdir dir="${build}"/>
  </target>

  <!-- The depends attribute indicates which target has to be executed before
	   this one. -->
  <target name="compile" depends="init" >
    <!-- Compile the java code from ${src} into ${build} -->
    <javac srcdir="${src}" destdir="${build}">
    	<classpath refid="classpath"/>
    </javac>
  </target>

  <!-- Delete the ${build} and ${dist} directory trees -->
  <target name="clean">
    <delete dir="${build}"/>
  </target>
</project>

 

Give your application the finishing touch:

 

  • Create a web.xml file
    • “What for?”: Basically the web.xml file – also called the Web Deployment Descriptor – is the configuration for your particular project.

    • “Oh, I see and what does it look like?”

      •  

      
<?xml version="1.0" encoding="UTF-8"?>

<!DOCTYPE web-app PUBLIC
"-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
"http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>
    <display-name>Eine erste Webanwendung</display-name>

    <servlet>
        <!-- generally: a unique name for your servlet -->
        <servlet-name>HelloWorldServlet</servlet-name>

        <!-- generally: the path to your servlet (from classes folder))
                  with subfolders separated by "."s -->
        <servlet-class>
            servlets.HelloWorldServlet
        </servlet-class>
    </servlet>

    <servlet-mapping>
        <!-- generally: the unique name of the servlet you are referring to -->
        <servlet-name>HelloWorldServlet</servlet-name>

        <!-- The path to this Servlet relative to the project folder.
                  Or more generally: the Servlet you want to be associated with
                  the given pattern whereas a "*" can function as a wildcard
                  (i.e. a placeholder for any kind of description).-->
        <url-pattern>/HelloWorldServlet</url-pattern>

    </servlet-mapping>
</web-app>

 

Look what you’ve done!

 

  • “Load” your application. It is up to you in which order you want to do the following two actions:
    • A. Start either one of the servers:
      • For Tomcat that is (01.10.2010): $TOMCAT_HOME_FOLDER$/bin/startup.bat
      • For JBossthat is (01.10.2010): $JBOSS_HOME_FOLDER$/bin/run.bat
    • B. Move the File-Directory to the right spot:
      • “And where might that be?”: It depends:
        • For Tomcat that is (01.10.2010): $TOMCAT_HOME_FOLDER$/webapps/
        • For JBossthat is (01.10.2010): $JBOSS_HOME_FOLDER$/server/default/deploy/
        • NOTE: The $ annotated parts represent variables, please insert your respective (here) folder or (generally) value there.
  • Finally, access the HelloWorldServlet page: By typing the following URL: http://localhost:8080/$project_name$/HelloWorldServlet (in our example: http://localhost:8080/HelloWorldServlet/HelloWorldServlet) into your internet browser’s address bar.

Congratulations you are looking at your first Servlet :)!

NOTE: More about Servlets can be read in Thomas Stark’s Java EE 5.

* Please NOTE: Even though WEB-INF is secured against external access within the Container, it is still accessible by the underlying filesystem! So DO NOT store your source files there!

Best Regards,

David