Servlet Tutorial

Revision as of 00:00, 28 September 2012

Servlets are the pure Java solution to handle web requests. Many application will use servlets instead of JSP and others will use servlets in conjunction with JSP. Experienced JSP programmers use servlets in conjunction with JSP to create clearer and simpler applications. The servlets handle Java processing: form handing, calculation and database queries. JSP formats the results.

Servlets belong in WEB-INF/classes. On this machine, the source is in Java source in /var/www/webapps/ROOT/WEB-INF/classes. WEB-INF/classes is the standard location for servlets and other Java classes. Resin automatically reloads and recompiles servlets, beans, and classes placed in WEB-INF/classes. You should make some changes and add errors to become familiar with Resin's recompilation and the error reporting.

Create the following servlet in WEB-INF/classes/test/HelloServlet.java with your favorite editor: eclipse, notepad, emacs, vi, or whatever.

WEB-INF/classes/test/HelloServlet.java

 package test;

 import java.io.*;

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

 public class HelloServlet extends HttpServlet {
   public void doGet (HttpServletRequest req,
                      HttpServletResponse res)
     throws ServletException, IOException
   {
     PrintWriter out = res.getWriter();

     out.println("Hello, world!");
     out.close();
   }
 }

Now browse the servlet at http://localhost:8080/hello. Resin will automatically compile the servlet for you. Browsing servlets differs from page browsing because you're executing a servlet class, not looking at a page. The /hello URL is configured for the hello, world servlet below.

Configuration

Configuration for the servlet is in the WEB-INF/web.xml file.

The servlet needs to be configured and it needs to be mapped to a URL. The <servlet> tag configures the servlet. In our simple example, we just need to specify the class name for the servlet.

The <servlet-mapping> tag specifies the URLs which will invoke the servlet. In our case, the /hello URL invokes the servlet.

WEB-INF/web.xml

 <web-app xmlns="http://java.sun.com/xml/ns/j2ee" version="2.4"
          xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
          xsi:schemaLocation="http:/java.sun.com/dtd/web-app_2_3.dtd">
   <servlet>
     <servlet-name>hello</servlet-name>
     <servlet-class>test.HelloServlet</servlet-class>
   </servlet>

   <servlet-mapping>
     <servlet-name>hello</servlet-name>
     <url-pattern>/hello</url-pattern>
   </servlet-mapping>
 </web-app>

Resin allows a short cut for the XML configuration in the example above; you can use XML attributes in place of elements. The Servlet 2.4 standard uses only elements. So the servlet-mapping configuration following the Servlet 2.4 standard would look like:

WEB-INF/resin-web.xml

 <web-app xmlns="http://caucho.com/ns/resin">
    <servlet servlet-name="hello"
             servlet-class="test.HelloServlet"/>

    <servlet-mapping url-pattern="/hello"
             servlet-name="hello"/>
 </web-app>

The two are entirely equivalent. For larger configurations, using attributes makes the resin.conf or web.xml more readable.

The xmlns="http://caucho.com/ns/resin" lets Resin validate the web.xml configuration. The validator will catch most errors in the web.xml.