Server Side Development: �Servlets

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• Container Architecture
• Web Components
• Servlets and Servlet Applications
• Servlet API
• Javax.servlet
• Javax.servlet.http
• Deploying an Application

​

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Server Side Development�Outline

• The owl two-tiered client-erver model has been superceded by the multi-tiered architecture prevelant in the enterprise applications
• Allows each layer to communicate just with layers above and below it
• Benefits of having a tiered application
• Encapsulates rules and functionality together providing for easier maintenance & development
• Enhances flexibility and reusability of logic and software components
• Allows developers to focus on the area of their speciality e.g. database, servers, web page, etc.

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Server Side Development�Tiered Architecture

• A web server is a program running on the server that listens for incoming requests and services those requests as they come in.
• Once the web server receives a request, depending on the type of request the web server might look for a web page, or it might execute a program on the server.
• It will always return some kind of results to the web browser, even if its simply an error message saying that it couldn’t process the request.
• By default the role of a web server is to serve static pages using the http protocol
• Web servers can be made dynamic by adding additional processing capability to the server

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Server Side Development�Web Server

• Several different tools are available for extending the server capabilities
• Java enterprise architecture
• VB .Net architecture
• Active Server Pages (ASP)
• CGI-Perl scripting
• These tools process incoming requests from the user and generate custom html pages

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Server Side Development�Server Extensions

• Tomcat is a stand alone web server and a servlet container
• It is open source and free for usage
• It is written in Java
• You do not have to be a Java programmer to use it
• It’s web server is not as fully featured as others like Apache
• Installing Tomcat
• Make sure that jdk1.4 (or higher) is installed on your machine
• Download the latest windows version of Tomcat
• Run the installer by double clicking on the download
• The installer checks if JRE and JDK are available for Tomcat
• Accept the license agreement
• Installation directory: c:\Program Files\Apache Tomcat 4.0
• On installation you get a message Completed

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Server Side Development�Tomcat

HTTP

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• User applications implement this protocol
• Other protocols implemented by the OS.
• Different applications use different protocols
• Web Servers/Browsers use HTTP
• File Transfer Utilities use FTP
• Electronic Mail applications use SMTP
• Naming Servers use DNS
• Interacts with transport layer to send messages

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HTTP�Application Layer Protocol

• Two parameter required for identifying the receiving process
• Host machine identifier - IP Address (localhost or ip-address)
• Host machine process identifier - Port (80 or 8080 for web server)

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HTTP�Application Layer Protocol, cont’d.

Socket

Socket

Internet

• Lightweight protocol for the web involving a single request & response for communication
• Provides 8 methods
• Get: Used to request data from server

(By convention get will not change data on server)

• Post: Used to post data to the server
• Head: returns just the HTTP headers for a resource.
• Put: allows you to "put" (upload) a resource (file) on to a webserver so that it be found under a specified URI.
• Delete: allows you to delete a resource (file).
• Connect:
• Options: To determine the type of requests server will handle
• Trace: Debugging

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HTTP�HyperText Transfer Protocol

• GET and POST allow information to be sent back to the web server from a browser
• e.g. when you click on the “submit” button of a form the data in the form is send back to the server, as "name=value" pairs.
• Choosing GET as the "method" will append all of the data to the URL and it will show up in the URL bar of your browser.
• The amount of information you can send back using a GET is restricted as URLs can only be 1024 characters.
• A POST sends the information through a socket back to the webserver and it won't show up in the URL bar.
• This allows a lot more information to be sent to the server
• The data sent back is not restricted to textual data and it is possible to send files and binary data such as serialized Java objects.

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HTTP�GET and POST

• Contains information about client and the request
• Four categories of header information
• General Information: Date, caching information, warnings etc.
• Entity Information: Body of the request or response e.g. MIME type, length etc.
• Request Information: Information about client e.g. cookies, types of acceptable responses etc.
• Response Information: Information about server e.g. cookies, authentication information etc.
• General & Entity information used for both client & server
• Request information included by client
• Response information included by server

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HTTP�HTTP Headers

• HTTP is a stateless protocol
• Request/Response occurs across a single network connection
• At the end of the exchange the connection is closed
• This is required to make the server more scalable
• Web Sites maintain persistent authentication so user does not have to authenticate repeatedly
• While using HTTP persistent authentication is maintained using a token exchange mechanism
• HTTP 1.1 has a special feature (keep-alive) which allows clients to use same connection over multiple requests
• Not many servers support this
• Requests have to be in quick succession

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HTTP�Protocol

• Three types of tracking methods are used:
• Cookies: Line of text with ID on the users cookie file
• URL Session Tracking: An id is appended to all the links in the website web pages.
• Hidden Form Elements: An ID is hidden in form elements which are not visible to user
• Custom html page allows the state to be tracked

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HTTP�Tracking State

• When a server responds to a request it provides a status code
• Web Container automatically handles setting of status codes
• Five categories of status codes
• Informational
• Success
• Redirection
• Client error
• Server error
• Common Status Codes
• 200 – Request was processed normally
• 401 – Unauthorized access
• 403 – Forbidden
• 404 – Requested resource not found on server
• 405 – Method Not allowed
• 500 – Internal server error

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HTTP�HTTP Status Codes

J2EE Architecture

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• Application is considered as a collection of related yet independent components
• Container acts as an execution environment for the components
• Container Provides services to the components

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J2EE Architecture�J2EE – Container Architecture

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internet

internet

J2EE Architecture�J2EE – Container Architecture, cont’d.

Application Container

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J2EE Architecture�Client Tier

Applet Container

Applet

Application Container

Application

JDBC

JAXP

JAAS

J

M

S

ClientApp (JAR file)

Main AppClass

Public static void main (String args[ ] )

Java Packages,

Classes, Libraries

Deployment Descriptor

• Client Container has a contract with applications

to provide certain functionality to the components

in the application

• Web Container
• Manages execution of servlets and JSPs
• Part of web or application server
• Supports HTTP
• EJB Container
• Business Components that contain business logic or rules
• Two types of EJBs
• Session Beans – Logic Oriented and deal with handling client requests and data processing
• Entity Beand – Strongly coupled with data and deal with data access and persistence

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J2EE Architecture�Middle Tier Container

• Two distinct parts of the applications
• Shopping Cart: Handles consumer side of the store
• Order Manager: Handles back end processing

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J2EE Architecture�E-Commerce Scenario

WEB Container

WEB Container

OrderManager Application

WEB Container

WEB Container

Shopping Cart Application

Process Servlet

Order

J2EE Server

EJB Container

WEB Container

Order EJB

Order Manager EJBApplication

Supplier Server

Order Manager

Servlet

StockOrder

(XML)

Database

Catalog Servlet

Cart Servlet

Static

Pages

• Cart Application
• Catalog servlet gets product data from the database
• Cart servlet keeps track of the customer purchase
• Process servlet processes the order
• Order Process Application
• Processes customer order
• Checks inventory levels (orders new parts from Suppliers)
• Processes payments
• Sends acknowledgement to the client

​

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J2EE Architecture�E-Commerce

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J2EE Architecture�E-Auctions

• Two separate applications
• Auction: Deals with bidding and searching
• Payment – Deals with backend financial processing

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J2EE Architecture�E-Auctions – Container Ideas

• Client Side uses a web browser to view store
• Auction Application
• Registration Servlet: Registers new users
• Post servlet: Accepts new items for auction
• Search servlet: Allows buyers to search database
• Bid servlet: Allows users to bid on pending items
• Informs the seller of the bid (e-mail)
• Purchase servlet: Processes sales
• History Servlet: Allows bidder/seller to review history of any item on auction
• Payment Application
• Payment Servlet: Credits the buyer and Debits the seller (Credit card transactions)

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J2EE Architecture�E-Auctions

Servlets

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• Classes that dynamically process requests and construct responses
• Dynamically generate html pages in response to requests
• May also send data in other forms like XML or serialized Java objects
• Run in a servlet container and have access to services that the container provides
• In an application processing of each request will normally be done by a different servlet.
• e.g. search catalog, check out, confirm order etc.
• Client of the servlet can be any of the following
• Browser
• Applet
• Java Application

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Servlets�Introduction

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Servlets�Servlet Lifecycle

• Servlet can communicate with four different entities
• Client during request/response cycle
• With servlet container to get context/config information
• With other resources on server e.g. servlets, EJBs
• With external resources like databases, legacy systems, and EIS
• Client communication can be in many forms
• In Http communication
• Request – Information parameters (as name value pairs)
• Response
• HTML (Browsers)
• WML (Mobile Devices)
• CSV (Spreadsheets)
• XML (Communicating with non-java systems)
• Serialized Objects

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Servlets�Servlet Communication

Servlets API

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• Contained in two packages
• javax.servlet
• javax.servlet.Http
• Contains 20 interfaces and 16 classes
• Prevalence of interfaces allows servlet implementation to be customized to container

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Servlets�Servlet API

• Javax.servlet package can be extended for use with any application layer protocol
• http is the most popularly used protocol
• Javax.servlet.http package is extension of the javax.servlet package for http protocol
• The Servlet spec allows you to implement separate Java methods implementing each HTTP method in your subclass of HttpServlet.
• Override the doGet() and/or doPost() method to provide normal servlet functionality.
• Override doPut() or doDelete() if you want to implement these methods.
• There's no need to override doOptions() or doTrace().
• The superclass handles the HEAD method all on its own.

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Servlets�JAVA Servlets

• Provides the contract between the servlet/web application and the web container
• Used for creating protocol independent server applications
• Servlet interface defines the core of the entire package
• Other interfaces provide additional services to the developer
• Contains 12 interfaces
• 7 interfaces implemented by the package
• 5 interfaces implemented by the user

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Servlets�Javax.servlet Package

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Exception

ServletException

UnavailableException

interface

Servlet

interface

ServletConfig

Serializable

GenericServlet

interface

FilterConfig

interface

ServletContext

interface

FilterChain

EventObject

ServletContextEvent

ServletContextAttributeEvent

interface

ServletRequest

ServletRequestWrapper

interface

RequestDispatcher

OutputStream

ServletOutputStream

InputStream

ServletInputStream

EventListener

interface

ServletContextListener

interface

ServletResponse

ServletResponseWrapper

interface

SingleThreadModel

EventListener

interface

ServletContextAttributeListener

Servlets�Class Diagram

• Server implemented interfaces
• ServletConfig
• ServletContext
• ServletRequest
• ServletResponse
• RequestDispatcher
• FilterChain
• FilterConfig
• User implemented interfaces
• Servlet
• ServletContextListener
• ServletContextAttributeListener
• SingleThreadModel
• Filter

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Servlets�Interfaces

• Servlet Classes
• GenericServlet
• ServletContextEvent
• ServletContextAttriubuteEvent
• ServletInputStream
• ServletOutputStream
• ServletRequestWrapper
• ServletResponseWrapper
• Exception Classes
• ServletException
• UnavailableException

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Servlets�Classes

• GenericServlet is abstract class that implements servlet interface
• Requires implementing the service() method
• Servlets normally extend from this class
• Methods
• LifeCycle Methods
• init()
• service()
• destroy()
• Environment Methods
• getServletContext()
• getInitParameter(…)
• getInitParameterNames()
• Utility Methods
• log(…)

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Servlets�Generic Servlet Class

• Javax.servlet package provides interfaces and classes to service client requests in protocol independent manner.
• Javax.servlet.http package supports http-specific functions.
• Several of the classes are derived from the javax.servlet packaage
• Some methods from the javax.servlet package are also used
• Contains
• 8 interfaces
• 7 classes

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Servlets�javax.servlet.http

Interfaces

• HttpSession
• HttpServletRequest
• HttpServletResponse
• HttpSessionAttributeListener
• HttpSessionActivationListener
• HttpSessionBindingListener
• HttpSessionContext
• HttpSessionListener

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Servlets�Classes and Interfaces

Classes

• Cookie
• HttpServlet
• HttpServletRequestWrapper
• HttpServletResponseWrapper
• HttpSessionBindingEvent
• HttpSessionEvent
• HttpUtils

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GenericServlet Serializable HttpServlet

ServletRequest interface HttpServletRequest

ServletRequestWrapper HttpServletRequestWrapper

ServletRequestWrapper HttpServletRequestWrapper

ServletResponse interface HttpServletResponse

Object NoBodyResponse

Object HttpUtils

ServletOutputStream NoBodyOutStream

Servlets�Class Diagram

• Extends the Generic Servlet
• Inherits the init() and destroy methods()
• Overrides the service() method
• Service() method
• Signature: Protected void service(HttpServletRequest req, HttpServletResponse res)
• Forwards the request to the appropriate method
• Developer should not normally override this method
• The developer needs to implement the methods corresponding to the request
• doGet(), doPost(), doHead(), doPut()

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Servlets�HttpServlet Class

• Extends ServletRequest
• Inherited methods from ServletRequest
• getParameterNames()
• getParameter(String name)
• getParameterValues(String name)
• getServerName()
• getServerPort()
• getRequestDispatcher
• New methods defined
• getCookies()
• getHeader()
• getPathInfo()
• getContextPath()
• getQueryString()

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Servlets�HttpServletRequest Interface

• Extends ServletResponse
• Inherited methods from ServletResponse
• getoutputStream()
• getWriter(String name)
• flushBuffer()
• setContentType()
• New methods
• encodeURL(String url)
• encodeRedirectURL(String url)
• setDateHeader()
• setStatus()
• ………

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Servlets�HttpServletRequest Interface, cont’d.

• Constructor
• Cookie (String name, String value)
• Methods
• public void setMaxAge(int expiry)
• public void setValue(String newValue)
• Can be added to the response by using
• void addCookie(Cookie cookie) of HttpServletResponse
• Can be obtained from the request by using
• Cookie[] getCookies() method of the HttpServletRequest

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Servlets�Cookie Class

• Create a servletclass
• extend HttpServlet
• Implement the doGet() or doPost() method
• Both methods accept two parameters
• HttpServletRequest
• HttpServletResponse
• Obtain parameters from HttpServletRequest Interface using
• getParameter(String name)
• Obtain the writer from the response object
• Process input data and generate output (in html form) and write to the writer
• Close the writer

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Servlets�Writing a Servlet

Example 1

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package edu.albany.mis.goel.servlets;

import javax.servlet.http.*;

import java.io.*;

public class Login extends HttpServlet {

public void doPost(HttpServletRequest request, HttpServletResponse response) {

// Get the parameter from the request

String username = request.getParameter("username");

// Send the response back to the user

try {

response.setContentType("text/html");

PrintWriter writer = response.getWriter();

writer.println("<html><body>");

writer.println("Thank you, " + username + ". You are now logged into the system.");

writer.println("</body></html>");

writer.close();

} catch (Exception e) {

e.printStackTrace();

}

}

}

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Example 1�Login Servlet

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">

<html>

<head>

<title>Login</title>

</head>

<body>

<h1>Login</h1>

Please enter your username and password

<form action="servlet/edu.albany.mis.goel.servlets.Login" method="POST">

<p><input type="text" name="username" length="40">

<p><input type="password" name="password" length="40">

<p><input type="submit" value="Submit">

</form>

</body>

</html>

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Example 1�Login.html

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

<!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>Login Servlet</display-name>

<servlet>

<servlet-name>Login</servlet-name>

<servlet-class>edu.albany.mis.goel.servlets.Login</servlet-class>

</servlet>

<servlet-mapping>

<servlet-name>Login</servlet-name>

<url-pattern>/Login</url-pattern>

</servlet-mapping>

</web-app>

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Example 1�web.xml

• Compiling
• Makefile contains all the scripts for compiling and deployment of the servlet
• Needs to be modified for any give application
• Commands
• make shutdown: shuts down the tomcat server
• make clean: cleans up the current setup for the application
• make all: compiles code, creates war file and deploys war file on server
• make startup: starts the server again
• Running the servlet
• http://localhost:8080/login/login.html

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Example 1�Login Deployment

Example 2

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package edu.albany.mis.goel.servlets;

import javax.servlet.*;

import javax.servlet.http.*;

import java.io.IOException;

import java.io.PrintWriter;

import java.util.Enumeration;

import java.util.Date;

/**

* Description:

* @author Andrew Harbourne-Thomas

* @version 1.0

*/

public class HttpRequestResponseServlet extends HttpServlet {

private static int cookiesCreated = 0;

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Example 2�HttpRequestResponsServlet

/** Output a web page with HTTP request information and response data.

* @param request The object containing the client request

* @param response The object used to send the response back

*/

public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

StringBuffer httpRequestTable = getHttpRequestTable(request);

StringBuffer httpResponseTable = getHttpResponseTable(response);

response.setContentType("text/html");

PrintWriter out = response.getWriter();

//HTML page

out.println("<html><head><title>RequestResponseServlet</title></head><body>");

out.println("<h1>Request Information</h1>" + httpRequestTable + "<hr>");

out.println("<h1>Response Information</h1>" + httpResponseTable);

out.println("</body></html>");

out.close();

}

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Example 2�Servlet – doGet()

public class HTMLTable {

private StringBuffer head;

private StringBuffer rows;

private StringBuffer foot;

​

/** Initalises the StringBuffer Objects.

*/

public HTMLTable() {

head = new StringBuffer();

head.append("<table width=\"90%\" align=\"center\">");

head.append("<tr><th width=\"50%\" bgcolor=\"lightgrey\">Attribute</td>");

head.append("<th width=\"50%\" bgcolor=\"lightgrey\">Value</td></tr>");

rows = new StringBuffer();

foot = new StringBuffer();

foot.append("</table>");

}

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Example 2�HTMLTable Class

/** Appends the attribute and value in a row to the HTML table StringBuffer.

* @param attribute The first column value.

* @param value The second column value.

*/

public void appendTitleRow(String attribute) {

rows.append("<tr><td colspan=2><b><u>").append(attribute);

rows.append("</u></b></td></tr>");

}

/** Appends the attribute and value in a row to the HTML table StringBuffer.

* @param attribute The first column value.

* @param value The second column value.

*/

public void appendRow(String attribute, String value) {

rows.append("<tr><td>").append(attribute);

rows.append("</td><td><code>").append(value).append("</code></td></tr>");

}

/** Appends the attribute and value in a row to the HTML table StringBuffer.

* @param attribute The first column value.

* @param value The second column value.

*/

public void appendRow(String attribute, int value) {

appendRow(attribute, new Integer(value).toString());

}

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Example 2�HTMLTable Class, cont’d.

/** Appends the attribute and value in a row to the HTML table StringBuffer

* @param attribute The first column value.

* @param value The second column value.

*/

public void appendRow(String attribute, boolean value) {

appendRow(attribute, new Boolean(value).toString());

}

/** Overrides Object.toString method to present a String representation of the HTML table built up.

* @return value The second column value.

*/

public String toString() {

return head.append(rows).append(foot).toString();

}

/** Presents a StringBuffer representation of the HTML table built up.

* @return value The second column value.

*/

public StringBuffer toStringBuffer(){

return head.append(rows).append(foot);

}

}

​

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Example 2�HTMLTable Class, cont’d.

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/** Prepare a HTML table of information about the request made.

* @param request The object containing the client request

* @return String containing the table

*/

private StringBuffer getHttpRequestTable(HttpServletRequest request) {

HTMLTable table = new HTMLTable();

table.appendRow("HTTP Request Method", request.getMethod());

table.appendRow("Query String", request.getQueryString());

table.appendRow("Context Path", request.getContextPath());

table.appendRow("Servlet Path", request.getServletPath());

​

//additional info if required

/*

table.appendRow("Path Info", request.getPathInfo());

table.appendRow("Path Translated", request.getPathTranslated());

table.appendRow("Request URI", request.getRequestURI());

table.appendRow("Request URL", request.getRequestURL().toString());

*/

​

Example 2�Servlet - getHttpRequestTable

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// Get cookies from the user request

Cookie[] ourCookies = request.getCookies();

​

if (ourCookies == null || ourCookies.length == 0) {

table.appendRow("Cookies", "NONE");

} else {

for (int i = 0; i < ourCookies.length; i++) {

String cookieName = ourCookies[i].getName();

String cookieValue = ourCookies[i].getValue();

table.appendRow("Cookie: <code>" + cookieName + "</code>", cookieValue);

}

}

Enumeration e = request.getHeaderNames();

while (e.hasMoreElements()) {

String headerName = (String)e.nextElement();

String headerValue = request.getHeader(headerName);

table.appendRow("Header: <code>" + headerName + "</code>", headerValue);

}

return table.toStringBuffer();

}

Example 2�Servlet – getHttpRequestTable, cont’d.

/** Prepare a HTML table of information about the response made.

* @param response Gives access to the response object

* @return String containing the table

*/

private StringBuffer getHttpResponseTable(HttpServletResponse response) {

HTMLTable table = new HTMLTable();

int cookieCount = cookiesCreated++;

String name = Integer.toString(cookieCount);

String value = new Date(System.currentTimeMillis()).toString();

Cookie cookie = new Cookie(name, value);

response.addCookie(cookie);

table.appendRow("Cookie Added:<code>" + name + "</code>", value);

return table.toStringBuffer();

}

}

​

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Example 2�Servlet – getHttpRequestTable, cont’d.

Tracking State

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• A Cookie is data (String) that the server passes to the browser and the browser stores on the server
• Set of name value pairs
• Web servers place cookies on user machines with id to track the users
• Two types of cookies
• Persistent cookies: Stored on hard drive in text format
• Non-persistent cookies: Stored in memory and goes away after you reboot or turn off the machine

​

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Tracking State�Cookies

• Attributes of a cookie
• Name: Name of a cookie
• Value: Value of the cookie
• Comment: Text explaining purpose of cookie
• Max-Age: Time in seconds after which the client should not send cookie back to server
• Domain: Domain to which the cookie should be sent
• Path: The path to which the cookie should be sent
• Secure: Specifies if cookie should be sent via https
• Version: Cookie version

(0 – original Netscape version of Cookie

1 – cookies standardized via RFC 2109)

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Tracking State�Cookie Attributes

package edu.albany.mis.goel.servlets;

import java.io.IOException;

import java.io.PrintWriter;

import java.util.Random;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.http.Cookie;

import javax.servlet.ServletException;

public class CookieServlet extends HttpServlet

{

protected void doGet(HttpServletRequest request,HttpServletResponse response) throws ServletException, IOException

{

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Tracking State�Cookie Servlet

Cookie[] cookies = request.getCookies();

Cookie token = null;

if(cookies != null) {

for(int i = 0; i < cookies.length; i++)

{

if(cookies[i].getName().equals("token")) {

// Found a token cookie

token = cookies[i];

break;

}

}

}

response.setContentType("text/html");

PrintWriter writer = response.getWriter();

writer.println("<html><head><title>Tokens</title></head><body ");

writer.println("style=\"font-family:verdana;font-size:10pt\">");

String reset = request.getParameter("reset");

System.out.println("token = " + token);

if (token == null || (reset != null && reset.equals("yes"))) {

Random rand = new Random();

long id = rand.nextLong();

writer.println("<p>Welcome. A new token " + id + " is now established</p>");

// Set the cookie

token = new Cookie("token", Long.toString(id));

token.setComment("Token to identify user");

token.setMaxAge(-1);

token.setPath("/cookie/track");

​

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Tracking State�Cookies (Token)

response.addCookie(token);

} else {

writer.println("Welcome back. Your token is " + token.getValue() + ".</p>"); }

String requestURLSame = request.getRequestURL().toString();

String requestURLNew = request.getRequestURL() + "?reset=yes";

writer.println("<p>Click <a href=" + requestURLSame +

">here</a> again to continue browsing with the same identity.</p>");

writer.println("<p>Otherwise, click <a href=" + requestURLNew +

">here</a> again to start browsing with a new identity.</p>");

writer.println("</body></html>");

writer.close();

}

}

​

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Tracking State�Cookies, cont’d.

package edu.albany.mis.goel.servlets;

import java.io.IOException;

import java.io.PrintWriter;

import java.util.Random;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.http.Cookie;

import javax.servlet.ServletException;

public class CookieServlet extends HttpServlet {

protected void doGet(HttpServletRequest request,HttpServletResponse response)

throws ServletException, IOException {

Cookie[] cookies = request.getCookies();

Cookie token = null;

if(cookies != null) {

for(int i = 0; i < cookies.length; i++) {

if(cookies[i].getName().equals("token")) {

// Found a token cookie

token = cookies[i];

break;

}

}

}

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Tracking State�Cookies

response.setContentType("text/html");

PrintWriter writer = response.getWriter();

writer.println("<html><head><title>Tokens</title></head><body ");

writer.println("style=\"font-family:verdana;font-size:10pt\">");

String reset = request.getParameter("reset");

System.out.println("token = " + token);

if (token == null || (reset != null && reset.equals("yes"))) {

Random rand = new Random();

long id = rand.nextLong();

writer.println("<p>Welcome. A new token " + id + " is now established</p>");

// Set the cookie

token = new Cookie("token", Long.toString(id));

token.setComment("Token to identify user");

token.setMaxAge(-1);

token.setPath("/cookie/track");

response.addCookie(token);

}

else {

writer.println("Welcome back. Your token is " + token.getValue() + ".</p>");

}

• http:// www.address.edu:1234/path/subdir/file.ext?query_string
• Service 🡪 http
• Host 🡪 www. Address. edu
• Port 🡪 1234
• /path/subdur/file.ext 🡪 resource path on the server
• query_string 🡪 additional information that can be passed to resource
• Http allows name value pairs to be passed to the resource
• http:// www.test.edu/index.jsp?firstname=sanjay+lastname=goel
• The server can place the id of a customer along with the URL
• http://www.fake.com/ordering/id=928932888329938.823948
• This number can be obtained by guessing or looking over some one’s shoulder
• Timeout for the sessions may be a few hours
• User can masquerade as the owner of the id and transact on the web

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Tracking State�URL Encoding

package edu.albany.mis.goel.servlets;

import java.io.IOException;

import java.io.PrintWriter;

import java.util.Random;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.ServletException;

public class TokenServlet extends HttpServlet {

protected void doGet(HttpServletRequest request,HttpServletResponse response)

throws ServletException, IOException {

// Get the token from the request

String tokenID = request.getParameter("tokenID");

// Prepare for response

response.setContentType("text/html");

PrintWriter writer = response.getWriter();

writer.println("<html><head><title>Tokens</title></head><body ");

writer.println("style=\"font-family:verdana;font-size:10pt\">");

}

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Tracking State�URL Rewriting

if (tokenID == null) {

// Client did not sent any token

Random rand = new Random();

tokenID = Long.toString(rand.nextLong());

writer.println("<p>Welcome. A new token " + tokenID + " is now established</p>");

}

else {

// Client sent the token back

writer.println("<p>Welcome back. Your token is " + tokenID + ".</p>");

// Prepare links for sending requests back

String requestURLSame = request.getRequestURL().toString() + "?tokenID=" + tokenID;

String requestURLNew = request.getRequestURL().toString();

// Write the response and close

writer.println("<p>Click <a href=" + requestURLSame +

">here</a> again to continue browsing with the same identity.</p>");

writer.println("<p>Otherwise, click <a href=" + requestURLNew +

">here</a> again to start browsing with a new identity.</p>");

writer.println("</body></html>");

writer.close();

}

}

• HTML allows creation of hidden fields in the forms
• Developers use hidden fields to store information for their reference
• ID can be stored as a hidden form field
• <Input Type=Hidden Name=“Search” Value=“key”>
• <Input Type=Hidden Name=“id” Value=“123429823”>

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Tracking State�Hidden Form Fields

package edu.albany.mis.goel.servlets;

import java.io.IOException;

import java.io.PrintWriter;

import java.util.Random;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

import javax.servlet.ServletException;

public class HiddenFieldServlet extends HttpServlet {

protected void doGet(HttpServletRequest request,HttpServletResponse response)

throws ServletException, IOException {

// Get the token from the request

String token = request.getParameter("token");

// Prepare for response

response.setContentType("text/html");

PrintWriter writer = response.getWriter();

writer.println("<html><head><title>Tokens</title></head><body ");

writer.println("style=\"font-family:verdana;font-size:10pt\">");

if(token == null) {

// Client did not sent any token

Random rand = new Random();

token = Long.toString(rand.nextLong());

writer.println("<p>Welcome. A new token " + token + " is now established</p>");

}

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Tracking State�Hidden Form Field

else {

// Client sent the token back

writer.println("<p>Welcome back. Your token is " + token + ".</p>");

// Prepare a URL for sending requests back

String requestURL = request.getRequestURL().toString();

// Write a form with a hidden field

writer.println("<p>");

writer.println("<form method='GET' action='" + requestURL + "'>");

writer.println("<input type='hidden' name='token' value='" + token + "'/>");

writer.println("<input type='submit' value='Click Here'/>");

writer.println("</form>");

writer.println(" to continue browsing with the same identity.</p>");

// Write another form without the hidden field

writer.println("<p>");

writer.println("<form method='GET' action='" + requestURL + "'>");

writer.println("<input type='submit' value='Click Here'/>");

writer.println("</form>");

writer.println(" to start browsing with a new identity.</p>");

writer.println("</body></html>");

writer.close();

}

}

}

• Provides methods to establish session between client and server
• Session lasts for a specified time
• Allows binding of objects over multiple requests
• Important Methods
• getID()
• getAttribute(String name)
• getAttriubuteNames()
• setAttribute(String name, Object value)
• removeAttribute(String name)
• inValidate()

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Tracking State�HttpSession Interface

/** This is the main servlet of the application which reads the

* products from the product list and presents it to the user for

* selecting and addition to the shopping card. The data is read from

* an XML file and is added to a hashmap which is added to the

* ServletContext for future access.

* Steps:

* init()

* 1. Gets the servletcontext

* 2. Obtains the name of the product file from the context (init param)

* 3. Creates a DOM parser

* 4. Parses the product file and creates a document (xml data)

* 5. Adds the product information to a Hashmap called product

* 6. Adds the Hashmap to the context.

* doGetPost()

* 1. Reads the products from the Hashmap

* 2. Creates web page which contains standard header footer (dispatcher)

* 3. Adds products to the web page and links them to the cartServlet

*/

package edu.albany.mis.goel.store;

import java.io.*;

import java.util.*;

import javax.servlet.*;

import javax.servlet.http.*;

​

// JAXP packages

import javax.xml.parsers.*;

import org.xml.sax.*;

import org.xml.sax.helpers.*;

import org.w3c.dom.*;

​

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Store�MainServlet

public class MainServlet extends HttpServlet {

public void init() throws ServletException {

// Load the products from XML file provided by init parameter

ServletContext context = getServletContext();

InputStream productsFile = context.getResourceAsStream((String) context.getInitParameter("productsFile"));

DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();

DocumentBuilder db = null;

try { db = dbf.newDocumentBuilder();

} catch (ParserConfigurationException pce) {

throw new ServletException (pce.getMessage());

}

Document doc = null;

try { doc = db.parse(productsFile);

} catch (IOException ioe) {

throw new ServletException(ioe.getMessage());

} catch (SAXException se) {

throw new ServletException(se.getMessage()); }

NodeList productsList = doc.getElementsByTagName("product");

HashMap products = new HashMap();

Node product;

for (int ctr = 0; ctr < productsList.getLength(); ctr ++ ) {

product = productsList.item(ctr);

NamedNodeMap attribs = product.getAttributes();

Node attrib = attribs.getNamedItem("name");

String name = attrib.getNodeValue();

attrib = attribs.getNamedItem("price");

String price = attrib.getNodeValue();

Product p = new Product(ctr,name,price);

products.put(new Integer(ctr),p);

}

// Store products in the ServletContext

context.setAttribute("products",products);

}

public void doPost(HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException {

doGetOrPost(req,res);

}

public void doGet(HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException {

doGetOrPost(req,res);

}

private void doGetOrPost(HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException {

PrintWriter out = res.getWriter();

// Include standard header

RequestDispatcher dispatcher = req.getRequestDispatcher("/header.html");

dispatcher.include(req,res);

HashMap products = (HashMap) getServletContext().getAttribute("products");

// List the products, clickable to add to cart

Iterator it = products.values().iterator();

out.println("<table>");

while (it.hasNext()) {

out.println("<tr>");

Product product = (Product) it.next();

out.print("<td><a href='Cart?add=true&id=" + product.getId() +"'>");

out.print(product.getName() + "</a></td><td>" + product.getPrice());

out.println("</td>);

out.println("</tr>");

}

out.println("</table>");

// Include standard footer

dispatcher = req.getRequestDispatcher("/footer.html");

dispatcher.include(req,res);

}

}

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Store�MainServlet

package edu.albany.mis.goel.store;

import java.util.*;

public class Cart {

private HashMap items = new HashMap();

// Default Cart Constructor

public Cart() {

}

// Function to get items from the cart

public Iterator getItems() {

return items.values().iterator();

}

public void addItem(Product product) throws ItemAlreadyAddedException {

Integer id = new Integer(product.getId());

if (this.items.containsKey(id)) {

throw new ItemAlreadyAddedException();

}

this.items.put(id, product);

}

}

​

package edu.albany.mis.goel.store;

import javax.servlet.*;

public class ItemAlreadyAddedException extends ServletException {

}

​

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Store�Cart and Product

package edu.albany.mis.goel.store;

public class Product {

private String name;

private String price;

private int id;

​

public Product(int id, String name, String price) {

this.price = price;

this.name = name;

this.id=id;

}

​

public String getPrice() {

return this.price;

}

​

public String getName() {

return this.name;

}

​

public int getId() {

return this.id;

}

​

}

package edu.albany.mis.goel.store;

import java.io.*;

import java.util.*;

import javax.servlet.*;

import javax.servlet.http.*;

public class CartServlet extends HttpServlet {

public void doPost(HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException {

}

public void doGet(HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException {

doGetOrPost(req,res);

}

private void doGetOrPost(HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException {

// Get the cart if it exists

HttpSession session = req.getSession();

Cart cart = (Cart) session.getAttribute("cart");

if (cart == null) {

cart = new Cart();

}

// Check to see if we are adding to the cart or we want to dispay the cart

String adding = req.getParameter("add");

PrintWriter out = res.getWriter();

// Add to it

if (adding.equalsIgnoreCase("true")) {

addToCart(req, cart, out);

}

// Display its contents

displayCart(cart, out);

}

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Store�CartServlet

private void addToCart(HttpServletRequest req, Cart cart, PrintWriter out)

throws ItemAlreadyAddedException {

// Get the item to add from the request

// Get the products from the servletcontext

HashMap products = (HashMap) getServletContext().getAttribute("products");

// Find the one represented by the ID that we passed in

try {

Integer id = new Integer(Integer.parseInt(req.getParameter("id")));

Product p = (Product) products.get(id);

// Add it to the cart

cart.addItem(p);

// add the cart to the session

req.getSession().setAttribute("cart",cart);

out.println("<b>Succesfully added product to cart!</b><br>");

} catch (NumberFormatException nfe) {

out.println("<b>Can't add product</b><br>");

}

}

private void displayCart(Cart cart, PrintWriter out) {

Iterator items = cart.getItems();

out.println("<h1>Current Cart Contents:</h1>");

out.println("<table>");

while (items.hasNext()) {

out.println("<tr>");

Product p = (Product)items.next();

out.println("<td>"+p.getName()+"</td>"+"<td>"+p.getPrice() +"</td>");

out.println("<tr>");

}

out.println("</table>");

}

}

/** Checkout for the customer. This is also the place where the

* security check should be done to make sure that the customer is a

* registered customer. There are two ways of doing that. Currently

* security is not implemented

*

* 1. Declarative - Relies on the deployment

* 2. Programmatic - Internally codes the security

*

* Steps

* 1. Prints the contents of the shopping cart

* 2. Asks the user to confirm his/her selection

* 3. Sends the paget to the confirm page.

*/

package edu.albany.mis.goel.store;

​

import java.io.*;

import javax.servlet.*;

import javax.servlet.http.*;

import java.security.Principal;

​

public class CheckOutServlet extends HttpServlet {

public void doPost(HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException {

doGetOrPost(req,res);

}

​

public void doGet(HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException {

doGetOrPost(req,res);

}

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Tracking State�CheckoutServlet

private void doGetOrPost(HttpServletRequest req, HttpServletResponse res)

throws ServletException, IOException {

// Get the writer

PrintWriter out = res.getWriter();

// include the cart display, and ask to confirm check out.

System.out.println("Dispatching the request");

RequestDispatcher dispatcher = req.getRequestDispatcher("/Cart?add=false");

dispatcher.include(req,res);

out.println("<br>Please Click Confirm to check out");

out.println("<form action='confirmed.html'>" +

"<input type='submit' value='Confirm'></form>");

}

}

​

Application Deployment

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AppDir/

index.html

main.jsp

images/

company.jpg

divider.jpg

admin/

admin.jsp

WEB-INF/

web.xml

classes/edu/albany/mis/goel/servlets/

ShoppingCart.class

Catalog.class

lib/

xereces.jar

xalan.jar

edu/albany/mis/goel/servlets/

ShoppingCart.java

Catalog.java

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Application Deployment�Structure of Web Application

• Public Resources that are downloaded directly to the client without processing
• Lib files are standard libraries that the code may need
• JSP files are an exception since they are converted to servlets and not downloaded directly

• Files which the web container processes but not client
• Lib files are standard libraries that the code may need

• Source Files which are developed by the user
• Package directory reduces chances of name conflicts

• Web applications are deployed in the web applications directory of the web server
• In tomcat this directory is ${Tomcat_Home}/webapps
• Two separate ways of deploying web applications

Exploded Directory Format

• Development directory is copied to the application directory of the web server
• Used primarily in development mode when changes are frequent

Web Application Archive (WAR) Format

• Archived version of development directory is copied to application directory of web server
• Created using jar utility i.e. jar –cv0f SimpleWebApp.war .

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Application Deployment�Deployment of Web Applications

• If web application is in a location different than the webapps directory context is defined
• Location: ${Tomcat_Home}/conf/server.xml
• <context path=“/store” docBase=“/store.war” reloadable=“true>
• Context declares a context to exist with a base URL path of /store
• The application can be accessed at http://localhost:8080/store/.
• docBase tells tomcat where to find the web application
• Relative path (/store.war) tells Tomcat that store.war is at the top level of the webapps directory
• An absolute path can also be supplied I.e. c:/myapps/store.war
• Reloadable set to true indicates that if the class or lib files change the application detects the change

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Application Deployment�Deployment of Web Applications, cont’d.

• Each application in a web container is associated with a context
• All web resources are associated with the context.
• Servlet context is rooted at a known path within web container. (e.g. {Tomcat_Home}/webapps/store/home.html)
• Context for this application is /store
• User would access this as: http://localhost:8080/store/home.html
• There is a special object called servlet context.
• A sandbox for the application (prevents name clashes and efficient downloading of classes without having to set classpath)
• Allows servlets access container resources
• Primary use of servlet context is to share attributes between servlets in an application.
• Context may be defined explicitly in a web server
• Configuration Directory in Tomcat: ${Tomcat_Home}/conf/server.xml
• <context path=“/examples” docBase=“examples” debug=“0” reloadable=“true”>

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Application Deployment�ServletContext

• Conveys configuration information of a web application
• The primary elements of a deployment descriptor file
• Servlet definitions & mappings
• Servlet context initialization parameters
• Error pages
• Welcome pages
• File based security
• Rules for the deployment descriptor file
• Resides at the top level of the WEB-INF directory
• Must be a well formed XML file called web.xml
• Must conform to the dtd

(located at http://java.sun.com/dtd/web-app-2-3.dtd)

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Application Deployment�Deployment Descriptor

• Header denotes the version of XML

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

• Describes the the DTD for the application

<!DOCTYPE web-app

PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"

“http://java.sun.com/dtd/web-app_2_3.dtd">

• Description of the application enclosed in web-app tags

<web-app>

Contents of the file

<web-app>

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Application Deployment�Deployment Descriptors - Header

• Context parameters are parameters that are related to the entire application.
• Any number of initialization parameters can be provided in the context
• One initialization parameter for web application is shown below:

<context-param>

<param-name>

adminEmail

</param-name>

<param-vlaue>

admin@wrox.com

</param-value>

</context-param>

• ServletContext object is used to obtain context information

e.g. String adminEmail = getServletContext().getInitParameter(“adminEmail”);

• The methods in ServletContext are abstract, their implementations must be provided by the web container.

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Application Deployment�Deployment Descriptors - Context

• Servlet Description, e.g.

<servlet>

<servlet-name>storeservlet</servlet-name>

<servlet-class>edu.albany.mis.goel.servlets.storeservlet<servlet-class>

<init-param>

<param-name>version<param-name>

<param-value>0.1b<param-value>

<init-param>

</servlet>

• The above servlet is invoked by http://localhost:8080/store/home.html (Here store is the context of the application)
• The initialization parameters are used for the specific servlet
• They can be accessed using the ServletConfig object

e.g. String version = getServletConfig().getInitParameter(“version”);

​

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Application Deployment�Deployment Descriptors - Servlets

• Servlet mappings map servlets to specific URL pattern

<servlet-mapping>

<servlet-name>Servlet1</servlet-name>

<url-pattern>/home.html<url-pattern>

</servlet-mapping>

• Allows web container to send requests to specific servlet
• Why is servlet mapping required?
• A logical way to specify servlets would be to use context/servletname

(i.e. http://localhost:8080/store/storeservlet)

• Allows multiple urls to be mapped to same servlet
• Allows implementation details to be hidden
• Servlets can be mapped to more than one URL thro the use of wildcards in <url-pattern>

e.g. <servlet-mapping>

<servlet-name>ValadatorServlet<servlet-name>

<url-pattern>/*</url-pattern>

</servlet-mapping>

• The previous example maps every URL encountered to the same servlet

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Application Deployment�Deployment Descriptors - Servlets

• Error pages allow the application to specify pages to be shown when particular errors occur
• Used for Java Exceptions and Http Errors.
• The error page shown below is displayed when the server encounters a java.lang.ArithmeticException.

<error-page>

<exception-type> java.lang.ArithmeticExceception </exception-type> 🡨 Exception Type

<location>/error.html</location> 🡨 Resource to Show

</error-page>

• The error page shown below is displayed when the server encounters a an Http error

<error-page>

<error-code>404</error-code> 🡨 Http Error Code

<location>/404.html</location> 🡨 Resource to Show

</error-page>

​

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Application Deployment�Deployment Descriptors – Error Pages

• Application Name & Description

<web-app>

<display-name> Music Store</display-name>

<description>Application for Music Rentals</description>

</web-app>

• Welcome Pages

<welcome-file-list>

<welcome-file>index.html</welcome-file> 🡨 Welcome File URL

</welcome-file-list>

​

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Application Deployment�Deployment Descriptors - Miscellaneous

• Define Security Constraint (resource collection & authorization constraint)

<security-constraint>

<web-resource-collection>

<web-resource-name>CheckOutResource</web-resource-name>

<url-pattern>/CheckOutServlet/*</url-pattern>

<http-method>GET</http-method>

<http-method>POST</http-method>

</web-resource-collection>

<auth-constraint>

<role-name>storeuser</role-name> 🡨 Welcome File URL

</auth-constraint>

</security-constraint>

• Define Login Configuration

<login-config>

<auth-method>FORM</auth-method>

<realm-name>Wrox Store Checkout</realm-name>

<form-login-config>

<form-login-page>/login.html</form-login-page>

<form-error-page>/error.html</form-error-page>

</form-login-config>

</login-config>

• Define Users in Tomcat (Add users in ${Tomcat_Home}/conf/tomcat-users.xml)

<tomcat-users>

<user name=“tomcat” password=“tomcat” roles=“tomcat” />

<user name=“role1” password=“tomcat” roles=“role1” />

</tomcat-users>

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Application Deployment�Security Constraints

• ServletConfig Object is used to pass initialization parameters to a servlet
• Useful methods
• getServletName(): Returns name of servlet
• getServletContext(): Returns servletContext object
• getInitParameter(String name): returns value of the specified parameter (null if not present)
• getInitParameterNames(): Gets names of all the parameters in the initialization list.

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Application Deployment�ServletConfig Interface

• ServletContext is specific to a particular web application running in a JVM
• Each web application in a container will have a single servlet context associated with it.
• Allows you to maintain state across all servlets and clients in the application
• Also acts a shared repository for common attributes to all servlets
• Allows servlets to share data with each other
• ServletContext Object also used for communication with host server
• Allows servlet to get information about server on which it is running
• A typical use of this would be in a chat application

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Application Deployment�ServletContext Interface

• Methods
• getContext(String uripath)
• getMimeType()
• getResourcePaths()
• getRequestDispatcher()
• getRealPath()
• getServerInfo()
• getInitParameter()
• getAttribute()
• setAttribute()
• ...

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Application Deployment�ServletContext Interface, cont’d.

Session Management

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Session Management�Basics

• HTTP is a stateless protocol. Each re.quest and response stand alone
• Without session management, each time a client makes a request to a server, it’s brand new user with a brand new request from the server’s point of view.
• A session refers to the entire interaction between between a client and a server from the time of the client’s first request, which generally begins the session, to the time the session is terminated.

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Session Management�Creating and Using Sessions

• Two methods of the HttpServletRequest object are used to create a session:
• HttpSession getSession( );
• HttpSession getSession(boolean);
• Other methods for dealing with sessions:

​

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95

Session Management�What do you do with a session?

• Sessions are useful for persisting information about a client and a client’s interactions with an application.
• To do that, the HttpSession interface defines a number of mehods:
• setAttribute(String, Object)
• getAttribute(String)

​

​

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96

Forwarding and Including Requests

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97

Forwarding and Including Requests�Obtaining RequestDispatcher

• From ServletRequest
• RequestDispatcher getRequestDispatcher(String path)
• The path argument can be a relative path or absolute path
• If the path is absolute relative to application context it starts with a “/” e.g. /Login
• If the path if relative it is interpreted relative to the current web component location, e.g. if web component is /store then case would be considered /store/case
• From ServletContext
• ServletContext getServletContext()
• RequestDispatcher getNamedDispatcher(String name)
• RequestDispatcher getRequestDispatcher(String path)
• The path argument should always start with a / and is interpreted relative to the application context

​

​

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98

Forwarding and Including Requests�Using RequestDispatcher

• Forwarding Request
• void forward(ServletRequest request, ServletResponse response) throws ServletException, java.io.IOException
• Calling servlet should not write any data to the response stream before calling this method
• If response data is sent to the stream before calling forward an error is thrown
• Including Resource
• void include(ServletRequest req, ServletResponse res) throws ServletException, java.io.Exception
• You can safely write to the ResponseStream before calling the include function.

​

​

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99

Forwarding and Including Requests�Adding Parameters

• Parameters are added for use in the forwarded request
• Several methods defined in ServletRequest Interface
• Object getAttrubute(String name)
• Enumeration getAttributeNames()
• void setAttribute(String name, Object o)
• void removeAttribute(String name)
• The calling servlet can set the attributes
• The receiving servlet will use getAttribute(String) method to retrieve the values

​

​

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