Howto: A Hello World Servlet

Making sure the server works

Making sure your web page works

The first step is to make sure your web page works.

Check to make sure you have a working web page by visiting http://students.cs.ndsu.nodak.edu/~username. See instructions here for help with setting up your web page.

Making sure Tomcat works

You will also want to make sure that your account is configured to use Tomcat. Take the simple JSP page below and put it in your public_html directory somewhere and browse to it.

For example, if I were to place the sample JSP page at /home/helsene/public_html/java.jsp, I could access it as http://students.cs.ndsu.nodak.edu/~helsene/java.jsp

<html>
<head> <title> Test JSP </title> </head>
<body> <%= new java.util.Date().toString() %> </body>
</html>

So your JSP works now? Great! If this simple test page doesn't work, contact the departmental administrator to get your account configured to use Tomcat. If you are enrolled in CSCI 366, your account has already been configured with Tomcat access.

Compiling a basic servlet

The program

This servlet is a short little 'Hello World'

We will be creating a java class file that acts as our servlet, but before we compile it, we need to write it first!

import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;

public class HelloWorld extends HttpServlet {

        public void doGet(HttpServletRequest request, HttpServletResponse response)
        throws IOException, ServletException
        {
                response.setContentType("text/html");
                PrintWriter out = response.getWriter();
                out.println("<html>");
                out.println("<head>");
                out.println("<title>Hello World!</title>");
                out.println("</head>");
                out.println("<body>");
                out.println("<h1>Hello World</h1>");

                out.println("<br/>");
                out.println("This HTML is generated by a servlet, the code for it is located <a href=\"/~username/HelloWorld.java\">here.</a>");
                out.println("</body>");
                out.println("</html>");
        }
}

Save this class as HelloWorld.java in any folder of your choosing.

Compiling a servlet

Compiling is a bit trickier. You will need the servlet api jar file in order to compile. This file is located in /opt/lablibs/servlet-api.jar in the lab, so if you're using the lab to compile, you can just use: javac -cp “./:/opt/lablibs/servlet-api.jar” <name>.java.

If you're not in the lab, you'll need to adjust your classpath or import the .jar into Netbeans of Eclipse. Instructions for doing that can be found here.

Download servlet-api.jar from the link above and save it somewhere handy. Assuming I saved the servlet-api.jar and the HelloWorld.java files in my home directory (/home/username/) I could compile the HelloWorld.java file using the command:

javac HelloWorld.java

Now we've got a HelloWorld.class, now what do we do with it?

Setting up Tomcat to use our servlet

Putting your class in the right folder

So now we need to make a folder called classes. This folder needs to be in public_html/WEB-INF/. All of your servlet classes go in here, so copy the compiled HelloWorld.class file into public_html/WEB-INF/classes/.

At this point you should have the files/folders:

/home/username/public_html/
/home/username/public_html/WEB-INF/
/home/username/public_html/WEB-INF/classes/
/home/username/public_html/WEB-INF/classes/HelloWorld.class

The web.xml file

Tomcat needs to look for servlets in a special WEB-INF folder. You should create this in your public_html folder.

The WEB-INF folder needs to contain a web.xml file which tells Tomcat where and how to access the servlet.

Example web.xml

For this howto, we can use this web.xml file:

<web-app>
   <servlet>
      <servlet-name>Hello World</servlet-name>
      <servlet-class>HelloWorld</servlet-class>
   </servlet>
   <servlet-mapping>
      <servlet-name>Hello World</servlet-name>
      <url-pattern>/servlet/HelloWorld</url-pattern>
   </servlet-mapping>
</web-app>

XML tag name	Purpose
servlet	This defines the servlet, you can set a name and the classfile to be associated with that name
servlet-mapping	This maps a URL to a servlet using the servlet's name (that you defined in <servlet>)
servlet-name	An arbitrary name to define your servlet (must be consistent throughout the entire web.xml)
servlet-class	The name of the class file, if your class is called 'HelloWorld.class', you will use 'HelloWorld'
url-pattern	The path portion of the URL to access your servlet. If you want to access your servlet at 'http://students.cs.ndsu.nodak.edu/~username/servlet/HelloWorld', then this will be '/servlet/HelloWorld'

This file defines a servlet called Hello World, tells Tomcat to use the class HelloWorld and says that it will be accessed by the URL /servlet/HelloWorld. The full URL in this case will be: http://students.cs.ndsu.nodak.edu/~username/servlet/HelloWorld

You can copy this and use this web.xml in your servlet, just change the values to match your servlet. Servlet-name can be any name of your choosing, servlet-class needs to be the package and class name of the class you want to use, and url-pattern defines which URL you want to use to access the servlet.

Where does web.xml go?

This web.xml file needs to be placed at public_html/WEB-INF/web.xml for Tomcat to see it. So make the web.xml file and use the example above

The files/folders we should have now look like:

/home/username/public_html/
/home/username/public_html/WEB-INF/
/home/username/public_html/WEB-INF/web.xml
/home/username/public_html/WEB-INF/classes/
/home/username/public_html/WEB-INF/classes/HelloWorld.class

You should check out http://students.cs.ndsu.nodak.edu/~username/servlet/HelloWorld, replacing “username” with your actual username. If this doesn't work, double check the steps above. If everything looks fine and it doesn't work after 5 minutes, then Tomcat did not load your servlet correctly. Send me an email and I can investigate.

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