1. Web Application Development

2. Web Architecture Web Server HTML docs browser
Request:
Web
Server
Response: HTML Code
HTML
docs

3. Web Browser Universal user interface presenting web content
Internet Explorer
Netscape Communicator
Scripting languages like VBScript or JavaScript can be used to perform client-side data validation.
Provide some interactivity within the document.
DHTML is a combination of HTML, Cascading Style Sheets, the document object model (DOM), and scripting languages.
Other client-side technologies:
ActiveX
Java Applets

4. Web Server The heart of any web interaction.
A program running on the server that listens for incoming requests and service those requests.
Look for a web page or might execute a program on the server side.
Leading web servers today are:
Apache
Microsoft-IIS
Netscape-Enterprise
Rapidsite
Httpd
webStar

5. N-Tier Applications
Typical client/server systems have fallen into the category of a two-tiered architecture.
A way of designing an application in which clients contact well-known servers to access resources.
The WWW as a client-server application
Client?
Server?
Contact?
Well-known?
Resources?
Access?

6. Client/Server Architecture
Requests
Disk
Responses

7. Two-Tiered Application
Processing load is given to the PC while the more powerful server simply acts as a traffic controller between the application and the database.
Disadvantages:
Application performance suffers due to:
limited resources of the PC
Network traffic tends to increase as well
Maintenance
Solution?
Three-tiered Architecture

8. Three-tiered Architecture
Application is broken up into three separate logical layers, each with a well-defined set of interfaces.
Presentation tier – typically consists of a graphical user interface of some kind.
Middle tier – consists of the application logic and
Storage tier – contains the data that is needed for the application
Issues Addressed
Performance, network traffic, and maintenance

9. n-tier Architecture
Three-tier architecture lacks reusability and scalability.
Extend the middle tier by allowing for multiple application objects rather than just a single application.
Each object must have an interface which allows them to work together.
The interface can be thought of as a contract.

10. Web Application Architecture
Typically follow a three-tier architecture model.
Presentation layer – includes not only web browser but web server.
Responsible for assembling the data into presentable format.
Application layer – Consists of some sort of script or program
Third layer – provides the second tier with the data that it needs.

11. 3-tier Architecture Data Store Web Server Run-time Env. 2nd Tier
Request:
Run-time Env.
Browser
2nd Tier
Script or Program
Response: HTML Code
Data Store
3rd Tier
1st Tier

12. Web Application Roundtrip
Typical web application:
collects data from the user (first tier),
sends the request to the web server,
run the requested server program (second and third tiers),
package up the data to be presented in the web browser, and
send it back to the browser for display

13. Collecting Data
Involves collecting some kind of data from the user using HTML form.
Other methods:
Java Applets
ActiveX controls
Windows Forms

14. Sending a Request
The web browser packages up user data and issues an HTTP request to the web server to execute a server program.
An HTTP request consists of URL for the page or script, form data, and any additional header information.
Each request must specify which method the request is to use.
Common methods are GET and POST

15. GET Method
All of the form data that has been entered is appended to the request string using key=value pairs. For example:
– the web server to process the request
/cgi-bin/hello.cgi – name and location of the server resource
? – separates the location from data
Key=value – field names and associated values
& - separates key=value pairs
+ - replaces the space characters
Get method is used as default method for all web requests.

16. GET Method
Problems?
All of the form data packaged with request string, if some previous results for the exact same request URL exist in browser cache, then older results might be displayed.
Amount of data that can be passed is limited.

17. POST Method Package up the form data as request body.
The server program will be able to read the contents of the input and parse out the variable names and values.
Allows more data that can be passed and it will always send the request to the server.

18. Which Method Should Use?
GET request should be used to retrieve information.
POST request should be used if the request will actually modify the contents of a data store on the server.
A simple database search that returns a set of results should use a GET request.

19. Executing the Server Script
A web server passes a request to a specific script, or program, to be processed.
The web server first determines which type of operating environment it needs to load.
This is done through mapping.
Loads any required runtime environment.
Forward the request to the loaded environment.

20. Returning the results
Final step in a web application is to make some kind of response to the operation and return that to browser.
The server script specifies the content type and then writes the response to an output stream.
The browser first look at the response header and determine the mime type to render the data.
Most common content type is “text/html”.

21. Why Programmability? What’s the drawback to the simple document model?
Static
Assumes documents created before they are requested.
What are examples of information that might be part of web documents that may not be known before they are requested?

22. Developing Server Application
To make a program accessible to a web server, it must possess the following characteristics:
When a user issues a request from web browser, the web server has to be able to execute the requested program.
There must be a way in which the web server passes any form data to program.
Once the program is invoked, there has to be a standard entry point.
After the program has processed the input data, it has to package up the results and send them back to the web server which will, in turn, send them back to the web browser.

23. Server-Side Technologies
A few years back only solution was Common Gateway Interface.
Propriety APIs by Microsoft and Netscape (ISAPIs, NSAPIs)
Latest technologies being offered are Active Server Pages, Java Servlets and Java Server Pages, PHP, ASP.NET, Cold Fusion, etc.

24. Common Gateway Interface CGI
Simple way to create web application that accepts user inputs, queries a database, and returns some results back to the browser.
URL (the request) determines the name of a program to run.
CGI provides a way to specify a set of parameters to give the program.
Every web server in extension provides support for CGI programs.
A gateway between the user request and the data it requires.
Can be written in just about any language. Most popular is Perl.

25. How CGI Works Creates a new process in which the program will be run.
Load any required runtime environments as well as the program itself.
Finally it will pass in a request object and invoke the program.
When the program is finished, the server will read the response from stdout.
CGI request 1
CGI Based
Web Server
Process for CGI 1
CGI request 2
Process for CGI 2
CGI request 1
Process for CGI 1

26. Disadvantages Of CGI Does not scale well.
Each time a request is received by the web server, an entire new process is created.
Each process consists of its own set of environment variables, a separate instance of runtime environment is required, a copy of the program, and an allocation of memory for the program to use.
What might happen when a large number of requests are received immediately?

27. Java Servlets
A server side-program that services HTTP requests and returns results as HTTP response.
A Java version of CGI.
Can take advantage of any/all other Java packages and features.
Java objects which are based on servlet framework and APIs and extend the functionality of the web server.
Mapped to URLs and managed by container with a simple architecture.

28. CGI vs. Servlet Written in C/C++, Visual basic, and Perl.
Difficult to maintain, non-manageable, and non-scalable.
Prone to security problem of the programming language.
Resource intensive and inefficient.
Platform and application specific.
Written in Java.
Powerful, reliable, and efficient.
Improves scalability, reusability (component based).
Build-in security of java language.
Platform independent and portable.

29. Servlet-based Web Server
CGI vs. Servlet
CGI request 1
CGI Based
Web Server
Process for CGI 1
CGI request 2
Process for CGI 2
CGI request 1
Process for CGI 1
Servlet request 1
Servlet-based Web Server
Servlet request 2
JVM
Servlet 1
Servlet request 1
Servlet 2

30. Java Servlets
A server side-program that services HTTP requests and returns results as HTTP response.
A Java version of CGI.
Can take advantage of any/all other Java packages and features.
Java objects which are based on servlet framework and APIs and extend the functionality of the web server.
Mapped to URLs and managed by container with a simple architecture.

31. A Servlet’s Job Read explicit data sent by the client.
Read implicit data sent by the client (request header).
Generate the results.
Send the explicit data back to the client (html).
Send implicit data to the client (status code and response header).

32. HelloServlet public class HelloServlet extends HttpServlet {
public void doGet(
HttpServletRequest request,
HttpServletResponse response) {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println(
"<title>HelloWorld!</title>"); }
...

34. What does Servlet Do?
Receives client request (mostly in the form of HTTP request)
Extract some information from the request
Do content generation or business logic process (possibly by accessing database, invoking EJBs, etc)
Create and send response to client (mostly in the form of HTTP response) or forward the request to another servlet

35. Request & Responses What is a request? What is a response?
Information that is sent from client to a server
Who made the request
What user-entered data is sent
Which HTTP headers are sent
What is a response?
Information that is sent to client from a server
Text(html, plain) or binary(image) data
HTTP headers, cookies, etc

36. Servlet Classes & Interfaces

37. Servlet Life cycle

38. Life Cycle Methods

39. Life cycle Methods Invoked by container Defined in
Container controls life cycle of a servlet
Defined in
javax.servlet.GenericServlet class or
init(): Invoked once when the servlet is first instantiated
destroy(): Invoked before servlet instance is removed
service() - this is an abstract method
javax.servlet.http.HttpServlet class
doGet(), doPost(), doXxx()
service() - implementation

40. Life cycle methods: init()
public void init()
throws ServletException {
String driver = getInitParameter("driver");
String fURL = getInitParameter("url");
try {
openDBConnection(driver, fURL);
} catch (SQLException e) {
e.printStackTrace();
} catch (ClassNotFoundException e){ }

41. Life cycle methods: init()
public void init(ServletConfig config) throws ServletException {
super.init(config);
String driver =
getInitParameter("driver");
String fURL = getInitParameter("url");
try {
openDBConnection(driver, fURL);
} catch (SQLException e) {
e.printStackTrace();
} catch (ClassNotFoundException e){ }
public void destroy() {
bookDB = null;

42. service() & doGet()/doPost()
service() methods take generic requests and responses:
service(ServletRequest request,
ServletResponse response)
doGet() and doPost() take HTTP requests and responses:
doGet(HttpServletRequest request, HttpServletResponse response)
doPost(HttpServletRequest request, HttpServletResponse response)

43. Service() method

44. doGet() and doPost()

45. doGet() & doPost()
Extract client-sent information (HTTP parameter) from HTTP request
Set (save) and get (read) attributes to/from scope objects.
Perform some business logic or access database.
Optionally forward the request to other Web components (Servlet or JSP).
Populate HTTP response message and send it to client.

46. Example: doGet() import javax.servlet.*; import javax.servlet.http.*;
import java.io.*;
public class HelloServlet extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
// Just send back a simple HTTP response
response.setContentType("text/html");
PrintWriter out = response.getWriter();
out.println("<title>First Servlet</title>");
out.println(
"<big>Hello J2EE Programmers! </big>"); }

47. Steps of Populating HTTP Response
Fill Response headers
Set some properties of the response
Buffer size
Retrieve an output stream from the response
Write body content to the output stream

48. Example: Simple Response
public class HelloServlet extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
// Fill response headers
response.setContentType("text/html");
// Set buffer size
response.setBufferSize(8192);
// Retrieve an output stream from the response
PrintWriter out = response.getWriter();
// Write body content to output stream
out.println("<title>First Servlet</title>");
out.println("<big>Hello Servlet Programmers! </big>"); }

49. Scope Objects
Enables sharing information among collaborating web components via attributes maintained in Scope objects
Attributes of Scope objects are accessed with
getAttribute()
setAttribute()
4 Scope objects are defined
Web context, session, request, page

50. Four Scope Objects: Accessibility
Web context (ServletConext)
Accessible from Web components within a Web context
Session
Accessible from Web components handling a request that belongs to the session
Request
Accessible from Web components handling the request
Page
Accessible from JSP page that creates the object

51. Four Scope Objects: Class
Web context
javax.servlet.ServletContext
Session
javax.servlet.http.HttpSession
Request
subtype of javax.servlet.ServletRequest:
javax.servlet.HttpServletRequest
Page
javax.servlet.jsp.PageContext

52. Web Context
(ServletContext)

53. What is ServletContext For?
Used by servets to
Set and get context-wide object-valued attributes
Get request dispatcher
To forward to or include web component
Access Web context-wide initialization parameters set in the web.xml file
Access Web resources associated with the Web
context
Log
Access other misc. information

54. Scope of ServletContext
Context-wide scope
Shared by all servlets and JSP pages within a "web application"
Why it is called “web application scope”
A "web application" is a collection of servlets and content installed under a specific subset of the server's URL namespace and possibly installed via a *.war file
All servlets in BookStore web application share same ServletContext object
There is one ServletContext object per "web application" per JVM

55. ServletContext: Web Application Scope

56. How to Access ServletContext Object?
Within your servlet code, call getServletContext()
Example
public void doGet(HttpServletRequest req,
HttpServletResponse res)
throws ServletException {
ServletContext ctx = getServletContext();
String attr1 = ctx.getAttribute(“bookDB”);
if (bookDB == null)
throw new UnavailableException(
"Couldn't get database."); }
The ServletContext is contained in ServletConfig object, which the Web server provides to a servlet when the servlet is initialized
init (ServletConfig servletConfig) in Servlet interface

57. Getting and Using RequestDispatcher Objects
public void doGet (HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
HttpSession session = request.getSession(true);
ResourceBundle messages =
(ResourceBundle) session.getAttribute("messages");
//set headers and buffer size before accessing the Writer
response.setContentType("text/html");
response.setBufferSize(8192);
PrintWriter out = response.getWriter();
// then write the response
out.println("<html>" + "<head><title>" + messages.getString("TitleBookDescription") + "</title></head>");

58. //Get the dispatcher; it gets the banner to the user
ServletContext ctx = getServletContext();
RequestDispatcher dispatcher =
ctx.getRequestDispatcher("/banner");
if (dispatcher != null){
dispatcher.include(request, response);
...

59. Example: Logging public void doGet(HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
...
getServletContext().log(“Life is good!”);
getServletContext().log(“Life is bad!”,
someException);

60. Session (HttpSession)

61. Why HttpSession?
Need a mechanism to maintain client state across a series of requests from a same user (or originating from the same browser) over some period of time
Example: Online shopping cart
Yet, HTTP is stateless
HttpSession maintains client state
Used by Servlets to set and get the values of session scope attributes

62. How to Get HttpSession?
via getSession() method of a Request object (HttpRequest)

63. Example: HttpSession public class CashierServlet extends HttpServlet {
public void doGet (HttpServletRequest request,
HttpServletResponse response)
throws ServletException, IOException {
// Get the user's session and shopping cart
HttpSession session = request.getSession();
ShoppingCart cart =
(ShoppingCart)session.getAttribute("cart");
...
// Determine the total price of the user's books
double total = cart.getTotal();

64. Servlet Request (HttpServletRequest)

65. What is Servlet Request?
Contains data passed from client to servlet
All servlet requests implement ServletRequest interface which defines methods for accessing
Client sent parameters
Object-valued attributes
Locales
Client and server
Input stream
Protocol information
Content type
If request is made over secure channel (HTTPS)

66. Requests

67. Getting Client Sent Parameters
A request can come with any number of parameters
Parameters are sent from HTML forms:
GET: as a query string, appended to a URL
POST: as encoded POST data, not appeared in the URL
getParameter("paraName")
Returns the value of paraName
Returns null if no such parameter is present
Works identically for GET and POST requests

68. A Sample FORM
<HTML><HEAD><TITLE>Collecting Three Parameters</TITLE></HEAD>
<BODY BGCOLOR="#FDF5E6"><CENTER>
<FORM ACTION="/sample/servlet/ThreeParams“ METHOD=POST>
<TABLE>
<TR><TD COLSPAN=2>Please Enter Your Information</TR>
<TR><TD>First Name
<TD><INPUT TYPE="TEXT" NAME="param1"></TR>
<TR><TD>Last Name
<TD><INPUT TYPE="TEXT" NAME="param2"> </TR>
<TR><TD>Class Name
<TD><INPUT TYPE="TEXT" NAME="param3"></TR>
<TR><TD COLSPAN=2 ALIGN=CENTER>
<INPUT TYPE="SUBMIT" VALUE="Save"></TR>
</TABLE>
</FORM>
</CENTER></BODY></HTML>

69. A Sample FORM

70. A FORM Based Servlet import javax.servlet.*;
Import javax.servlet.http.*;
public class ThreeParams extends HttpServlet {
public void doPost(HttpServletRequest request,
HttpServletResponse response)
throws ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String title = "Your Information";
out.println("<HTML><BODY BGCOLOR=#CCCCCC><H1 ALIGN=CENTER>" +
title + "</H1><UL>\n" +
" <LI><B>First Name in Response</B>: “ +
request.getParameter("param1") + "\n" +
" <LI><B>Last Name in Response</B>: “ +
request.getParameter("param2") + "\n" +
" <LI><B>Nick Name in Response</B>: “ request.getParameter("param3") + "\n" +
"</UL></BODY></HTML>"); }

71. Getting Client Information
Servlet can get client information from the request
String request.getRemoteAddr()
Get client's IP address
String request.getRemoteHost()
Get client's host name

72. Getting Server Information
Servlet can get server's information:
String request.getServerName()
e.g. "
int request.getServerPort()
e.g. Port number "8080"

73. HTTP Request URL Contains the following parts
path]?[query string]

74. HTTP Request URL: [request path]
[request path] is made of
Context: /<context of web app>
Servlet name: /<component alias>
Path information: the rest of it
Examples

75. HTTP Request URL: [query string]
[query string] are composed of a set of parameters and values that are user entered
Two ways query strings are generated
It can explicitly appear in a web page
<a href="/bookstore1/catalog?Add=101">Add To Cart</a>
String bookId =
request.getParameter("Add");
appended to a URL when a form with a GET HTTP method is submitted
String userName =
request.getParameter(“username”)

76. Context, Path, Query, Parameter Methods
String getContextPath()
String getQueryString()
String getPathInfo()
String getPathTranslated()

77. HTTP Request Headers
HTTP requests include headers which provide extra information about the request
Example of HTTP 1.1 Request:
GET /search? keywords= servlets+ jsp HTTP/ 1.1
Accept: image/ gif, image/ jpg, */*
Accept-Encoding: gzip
Connection: Keep- Alive
Cookie: userID= id456578
Host:
Referer:
User-Agent: Mozilla/ 4.7 [en] (Win98; U)

78. HTTP Request Headers Accept Accept-Encoding Authorization
Indicates MIME types browser can handle.
Accept-Encoding
Indicates encoding (e. g., gzip or compress) browser can handle
Authorization
User identification for password- protected pages
Instead of HTTP authorization, use HTML forms to send username/password and store info in session object

79. HTTP Request Headers Connection Cookie Host
In HTTP 1.1, persistent connection is default
Servlets should set Content-Length with setContentLength (use ByteArrayOutputStream to determine length of output) to support persistent connections.
Cookie
Gives cookies sent to client by server sometime earlier. Use getCookies, not getHeader
Host
Indicates host given in original URL.
This is required in HTTP 1.1.

80. HTTP Request Headers If-Modified-Since Referer User-Agent
Indicates client wants page only if it has been changed after specified date.
Don’t handle this situation directly; implement getLastModified instead.
Referer
URL of referring Web page.
Useful for tracking traffic; logged by many servers.
User-Agent
String identifying the browser making the request.

81. HTTP Header Methods String getHeader(String name)
value of the specified request header as String
Enumeration getHeaders(String name)
values of the specified request header
Enumeration getHeaderNames()
names of request headers
int getIntHeader(String name)
value of the specified request header as an int

82. Showing Request Headers
public class ShowRequestHeaders extends HttpServlet {
public void doGet(HttpServletRequest request,
HttpServletResponse response) throws ServletException {
response.setContentType("text/html");
PrintWriter out = response.getWriter();
String title = "Showing Request Headers";
out.println("<HTML>” +
“<B>Request Method:</B>” + request.getMethod() "<BR>" +
"<B>Request URI:</B>" + request.getRequestURI() "<BR>\n" +
"<B>Request Protocol: </B>" + request.getProtocol() + "<BR><BR>" +
"<TH>Header Name<TH>Header Value");
Enumeration headerNames = request.getHeaderNames();
while(headerNames.hasMoreElements()) {
String headerName = (String)headerNames.nextElement();
out.println("<TR><TD>" + headerName);
out.println(" <TD>" + request.getHeader(headerName)); }

84. Thank You!