Java Servlets and JSP

Server-side Java for the web

  • a servlet is a Java program which outputs an html page; it is a server-side technology

First java servlet


       import java.io.*;
       import java.util.*;
       import javax.servlet.*;
       import javax.servlet.http.*;
                 
       public class FirstServlet 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>First servlet</title>");
               out.println("</head>");
               out.println("<body>");
               out.println("It works...<hr>");
               out.println("</body>");
               out.println("</html>");
           }
       }

What is a java servlet ?

  • a java servlet is a java class extending HTTPServlet class
  • a java servlet class overrides the doGet(), doPost() or other equivalent HTTP method and (usually) prints at the standard output an html file
  • a java servlet class can contain any kind of java code the JDK can compile
  • a java servlet class needs to be compiled prior to using it; it must use servlet-api.jar

Apache Tomcat

  • the most well known servlet/jsp container
  • is a web server + implementation of Java Servlet and JSP (Java Server Pages) APIs
  • is developed by Apache Software Foundation
  • available at http://tomcat.apache.org/ under Apache Software License

Installing Tomcat

  1. Download the binary zip distribution (e.g. apache- tomcat-6.0.20.zip) in a local folder (we will use c:\temp in the rest of the guide)
  2. Set the environment variables JAVA_HOME=path_to_JDK_installation_folder CATALINA_HOME=path_to_tomcat_installation_folder
    either as Windows system variables or in the files startup.bat and shutdown.bat from the bin directory

    E.g. place the following lines in the beginning of startup.bat and shutdown.bat: 
    
       set JAVA_HOME=c:\progra~1\java\jdk1.6.0
       set CATALINA_HOME=c:\temp\apache-tomcat-6.0.20

Starting/shutting down Tomcat

  • start Tomcat from a cmd prompter (window): 
    
    c:\temp\apache-tomcat-6.0.20\bin\startup.bat
  • verify startup by pointing a browser to the url 
    
    http://localhost:8080
  • shutting down Tomcat from a cmd prompter (window): 
    
    c:\temp\apache-tomcat-6.0.20\bin\shutdown.bat

Tomcat standard folders

  • bin - contains executable files for controlling the server (start, shut down etc.)
  • conf - contains configuration files; most important server.xml for configuring the server and web.xml a general configuration file for web applications
  • lib - libraries (jars) used by tomcat and deployed web applications
  • logs - log files
  • temp - temporary files
  • webapps - contains the web applications deployed
  • work - contains files created by tomcat during running (e.g. it crates a servlet from each jsp file)

Format of a web application

  • web applications are stored in the webapps folder, either as a folder or as a .war archive
  • a web application (either a folder or a .war archive) must contain the following:
    • WEB-INF folder
    • WEB-INF\web.xml: a configuration file
  • optionally the WEB-INF folder can contain the following subfolders:
    • classes: which contain servlets
    • lib: which contains jars used by the web application
  • html, jsp and resource files can be placed anywhere in the web application home folder
  • servlets must be placed in the folder or subfolders of WEB-INF\classes

A very simple web.xml example


    <?xml version="1.0" encoding="ISO-8859-1"?>

    <web-app xmlns="http://java.sun.com/xml/ns/javaee"
             xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
             xsi:schemaLocation=http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd version="2.5">

    </web-app>

Configuring servlets

  • for JSPs (Java Server Pages) no additional configuration needs to be done
  • for java servlets additional lines must be placed in the web.xml file: 
    
    <servlet>
	    <servlet-name>ServletsName</servlet-name>
	    <servlet-class>The_Class_Name_of_the_Servlet</servlet-class>
    </servlet>
    <servlet-mapping>
         <servlet-name>ServletsName</servlet-name>
         <url-pattern>/URL_of_the_servlet</url-pattern>
    </servlet-mapping>

What is a Java Server Page (JSP)

  • an html file containing parts of java code; the java code is placed inside the "<% ... %>" tags or some other related tags
  • Tomcat will create a servlet from the jsp file (which will be saved in the work folder)
  • when the jsp is requested the servlet is executed and the output of the server is sent back to the client

First JSP file


    <html>
        <body>
        <%
            out.println("First JSP... It works<br/>");
        %>
        </body>
    </html>

Additional documentation