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1 An introduction to Java Servlet Alessandro Marchetto Fondazione Bruno Kessler-IRST,
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An introduction to Java Servlet
Alessandro MarchettoFondazione Bruno Kessler-IRST,
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Outline
Web applications Java Servlet
Servlet engine How to write a servlet How to Process a request (Form data) Servlet session tracking
Just a quick introduction After this lecture you will be able to
read and write “simple” Servlets
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Web applications The Web server
distributes pages of formatted information to clients that request it.
HTML Pages are transmitted using the http protocol
The browser renders (static) HTML pages.
HTML pages are stored in a file system (database) contained on the web server.
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Dynamic Web applications
In some cases the content of a page (HTML) is not necessarily stored inside a file.
The content can be assembled at run-time according to user inputs and informations stored in a database. It can come from the output of a program (server script).
Servlets and JavaServer Pages are the Java technology’s answer to dynamic Web sites. They are programs that run on a Web server and build Web pages.
Client
Web Server
http request http response
DB
Script/program
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HTML and DOM
The HTML Document Object Model (HTML DOM) defines a standard way for accessing and manipulating HTML documents.
The DOM presents an HTML document as a tree-structure (a node tree), with elements, attributes, and text.
<html><head>
<title>My title</title></head><body>
<h1>My header</h\><a href=“”>My link</a>
</body></html>
http://www.w3.org/TR/html401/ http://www.w3.org/TR/DOM-Level-2-HTML/html.html
its DOM
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Outline
Just a quick introduction After this lecture you will be able to
read and write “simple” servlets
Web applications Java Servlet
Servlet engine How to write a servlet How to Process a request (i.e., Form data) Servlet session tracking
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Servlet Engine A servlet engine (or servlet
container) provides the run-time environment in which a servlet is executed.
The servlet engine manages the life-cycle of servlets (i.e., from their creation to their destruction).
The servlet engine: loads, executes and destroyes servlets
Apache Tomcat is a servlet container http://tomcat.apache.org
Relationships between Web server, Servlet engine and Servlets.
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The Life-Cycle of a Servlet Servlet are Java classes. To execute them it is necessary
compiling them in bytecode. The servlet engine perform the following tasks:
it loads the servlet when it is requested (only the first time).
it calls the init() method of the servlet. it handles the requests calling the service()
method of the servlet. at the end, it calls the destroy() method.
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How to write a Java servlet All servlets implement the Servlet interface or extend a
class the implements it. We will use HttpServlet and these methods:
doGet doPost init destroy getServletInfo
To write a servlet some of these methods must be overridden .....
HttpServlet implements the Servlet interface …
http://java.sun.com/javaee/5/docs/api/
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import java.io.*;import javax.servlet.*;import javax.servlet.http.*;
public class ServletTemplate extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Use “request” to read incoming HTML form data (e.g. submitted data) // Use “response” to specify the HTTP response status (e.g., content type) PrintWriter out = response.getWriter(); // Use “out” to send content to browser }}
Generic Template
The generic template to write Servlet
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Helloworld
First example of servlet We override doGet() The execution of it produces a HTML
response
import java.io.*;import javax.servlet.*;import javax.servlet.http.*;
public class HelloClientServlet extends HttpServlet {
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
response.setContentType(“text/html”); PrintWriter out = response.getWriter(); out.println(“<HTML><HEAD><TITLE>Hello Client!</TITLE>”+ “</HEAD><BODY>Hello Client!</BODY></HTML>”);
out.close(); }}
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Context The Context can be used to store “global”
information shared among the servlets of a given application.
The Context is initialized when the Servlet container starts and destroyed when it shuts down. For this reason, it is suited for keeping the “long lived information”
It is implemented with: javax.servlet.ServletContextThe methods: -getServletContext(): obtain the context-getAttribute(String key): get an attribute from the context-setAttribute(String key, Object value): set an attribute
ServletContext
Common Data
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1
Servlets
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First Par
Second Par
Third Par
Submit
~hall
~gates
~mcnealy
<form method=“GET/POST” action=“ThreeParams”> First Par: <input type=“text” name=“param1” /> Second Par: <input type=“text” name=“param2” /> Third Par: <input type=“text” name=“param3” /> <input type=“submit” value=“Submit” /></form>
Forms and Servlets (1)
How to exchange data between client and server?
GET: In a URL the part after the question mark (?) is known as form data, and is a way to send data from a Web page to a server-side program http://host/ThreeParams
POST: a data packege is created and attached to the HTTP requests sent to the server
? param1= ~hall¶m2= ~gates¶m3= ~mcnealy
import java.io.*;import javax.servlet.*;import javax.servlet.http.*;import java.util.*;
public class ThreeParams extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); String title = "Reading Three Request Parameters"; out.println( ServletUtilities.headWithTitle(title) + "<BODY>\n" + "<H1 ALIGN=CENTER>" + title + "</H1>\n" + "<UL>\n" + " <LI>param1: " + request.getParameter("param1") + "\n" + " <LI>param2: " + request.getParameter("param2") + "\n" + " <LI>param3: " + request.getParameter("param3") + "\n" + "</UL>\n" + "</BODY></HTML>"); }
Forms and Servlets (2)
~hall
~ gate
~mcnealy
HTM
L
CO
DE
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Forms and Servlets (3)
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HTTP - Stateless protocol (1)
The Web server can’t remember previous transactions since HTTP is a “stateless” protocol.
E.g., a shopping cart:
• it contains all items that have been selected from an online store's catalog by a customer;
• the customer can check the content of the cart at any time during the session;• thus, the server must be able to maintain the cart of the user across several
Web page requests.
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There are four ways to maintain the state:1. Cookies
javax.servlet.http.Cookie(String nome, String val)methods: setName(String cookieName), setValue(String cookieValue), etc.
2. Hidden fields in forms<input type=“hidden” value=“<valore>” />
3. URL rewritinge.g., http://host/serveltPages/ShowSession;jsessionid=E4D371A0710
String encodeURL(String url) of the class HttpServletResponse where: url is the original l’URL and the output is the rewritten one
4. Servlets (HttpSession API)
HTTP - Stateless protocol (2)
How to maintain the state
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Saving the state in general
First request
The Server marks the client withan unique ID and creates a memorypartition that will contain the data (state)
Every subsequent connection must send the ID. In this way the serverrecognize/identify the client
xyz5
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Servlet Sessions (3)
Assuming ShoppingCart is some class you have defined yourself that
stores information on items being purchasedHttpSession session = request.getSession(true);ShoppingCart previousItems = (ShoppingCart)session.getAttribute("previousItems"); if (previousItems == null) { previousItems = new ShoppingCart(...); } String itemID = request.getParameter("itemID"); previousItems.addEntry(Catalog.getEntry(itemID)); session.setAttribute("previousItems", previousItems);
Recover the sessionconnected to the client
Recover the cart
Add a new product to the cart
Store the cart in the session
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References
Tutorial about Servlets and JSP www.apl.jhu/%7Ehall/java/Servlet-Tutorial Understanding “Architecture 2” www.javaworld.com/javaworld/jw-12-1999/jw-12-ssj-jspmvc.html Sun http://java.sun.com/products/servlet/
Sun (Servlet and JSP) API http://java.sun.com/javaee/5/docs/api/ Sun (Servlet) Documentation http://java.sun.com/products/servlet/docs.html Sun J2EE Tutorial http://java.sun.com/javaee/5/docs/tutorial/doc/
Apache Tomcat http://tomcat.apache.org/ Tomcat (servlet) API http://tomcat.apache.org/tomcat-5.0-doc/servletapi/index.html Tomcat (jsp) API http://tomcat.apache.org/tomcat-5.0-doc/jspapi/index.html Tomcat in Eclipse http://www.sysdeo.com/eclipse/tomcatplugin (a Tomcat installation is
required) Tutorial about how to intregrate Tomcat in Eclipse
http://plato.acadiau.ca/courses/comp/dsilver/2513/EclipseAndTomcatTutorial/
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Tomcat configuration (1)
Tomcat directory organization:
In the directory: $TOMCAT_HOME$/webapps
./myApplication
./myApplication/*.html
./myApplication/WEB-INF/
./myApplication/WEB-INF/web.xml
./myApplication/WEB-INF/classes/*.class
./myApplication/WEB-INF/lib/*.jar
[./myApplication/WEB-INF/src/*.java]
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web.xml:– XML file used by Tomcat– it is a deployment descriptor– used to set specific parameters for the current application, such as:
• name of the servlet (that can be reached though HTTP)• URL corresponding to servlet files (e.g., into /WEB-INF/classes/)• sessions “timeout” • etc.
– it is loaded during the application installation
It is composed of the tags:– servlet: set alias and parameters to initialize a given servlet– servlet-mapping: set URL(s) to a given servlet
Tomcat configuration (2)
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<?xml version="1.0" encoding="ISO-8859-1"?><web-app xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_5.xsd" version="2.5">
<servlet> <servlet-name>HelloServlet</servlet-name> <servlet-class>HelloClientServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>HelloServlet</servlet-name> <url-pattern>/HelloClientServlet</url-pattern> </servlet-mapping>
</web-app>
Sample of web.xml
Alias to use in HTTP request
Name of the class (with package)
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Steps to build a new Web application using Tomcat in Eclipse:
1. Define a new Tomcat Project2. Write our servlet file in the subdirectory WEB-INF/src3. Write the servlet configutation file (web.xml) in the subdirectory WEB-INF4. Start Tomcat or in the right-menu clcik on “Tomcat project“ and reload the
context5. Open a browser, and call the servelt such as
http://localhost:8080/ProofProject/proofServlet
Servlet in Eclipse+Tomcat
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How to compile and execute a Servlet?How to use Eclipse to done this task?
... Helloworld demo ...
![[Servlet]Java Servlet Specification v2.2](https://static.documents.pub/doc/80x56/577d352c1a28ab3a6b8fb6f6/servletjava-servlet-specification-v22.jpg)
