1. Server Side Programming: Java Servlets

2. Server-side Programming
The combination of
HTML: HyperText Markup Language
JavaScript
DOM: Document Object Model (DOM) is an application programming interface (API) for valid HTML and well-formed XML documents. It is sometimes referred to as Dynamic HTML (DHTML)
Web pages that include scripting are often called dynamic pages (vs. static)

3. Server-side Programming
Similarly, web server response can be static or dynamic
Static: HTML document is retrieved from the file system of the WebServer and returned to the client
Dynamic: HTML document is generated by a program that is running on the server and builds a response to an HTTP request
Java servlets are one technology for producing dynamic server responses
Servlet is a class instantiated by the server to produce a dynamic response
ASP.net and PHP provide similar technologies

4. Servlet

5. Servlet Overview
Servlet technology is used to create web application that
resides at the server side and
generates dynamic web page
Before Servlet, CGI (Common Gateway Interface) scripting language was popular as a server-side programming language.
What is a Servlet?
A technology that is used to create dynamic web applications
An API that provides many interfaces and classes
An interface that must be implemented for creating any servlet
A class that extend the capabilities of the servers and respond to the incoming request.
It can respond to any type of requests
A web component that is deployed on the server to create dynamic web page

6. Servlet vs. CGI A Servlet has:
Better performance: because it creates a thread for each request not process.
Portability: because it uses java language.
Robustness: because it is managed by the JVM, thus we don't need to worry about memory leak, garbage collection etc.

7. What is a Java Servlet? Java Servlets are:
Technology for generating dynamic Web pages (like PHP, ASP, ASP.NET, ...)
Protocol and platform-independent server side components, written in Java, which extend the standard Web servers
Java programs that serve HTTP requests
The HttpServlet class
Provides dynamic Web content generation (HTML, XML, …)

8. Servlet Services Java Servlets provide many useful services
Provides low-level API for building Dynamic Internet services
Serves as foundation to Java Server Pages (JSP) and Java Server Faces (JSF) technologies
Can deliver multiple types of data to any client
XML, HTML, WML, GIF, etc...
Can serve as “Controller” of JSP/Servlet application in the MVC architectural model

9. Why Use Servlets? Portability Write once, serve everywhere Power*
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Why Use Servlets?
Portability
Write once, serve everywhere
Power
Can take advantage of all Java APIs
Elegance
Simplicity due to abstraction
Efficiency & Endurance
Highly scalable
(c) 2006 National Academy for Software Development -

10. Why Use Servlets? (2) Safety Integration Extensibility & Flexibility*
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Why Use Servlets? (2)
Safety
Strong type-checking
Memory management
Integration
Servlets tightly coupled with server
Extensibility & Flexibility
Servlets designed to be easily extensible, though currently optimized for HTTP uses
Flexible invocation of servlet
Server-Side Includes (SSI)
servlet-chaining,
Filters
etc.
(c) 2006 National Academy for Software Development -

11. Servlet Overview
Web Browser
Client
Web Server

12. Servlet Container
It is the part of web server which can be run in a separate process. We can classify the servlet container states in three types:
Standalone: It is typical Java-based servers in which the servlet container and the web servers are the integral part of a single program. For example:- Tomcat running by itself
In-process: It is separated from the web server, because a different program runs within the address space of the main server as a plug-in. For example:- Tomcat running inside the JBoss.
Out-of-process: The web server and servlet container are different programs which run in a different process.

13. Servlet Overview When server starts it instantiates servlets (Object)
Server receives HTTP request, determines the need for dynamic response
Server selects the appropriate servlet to generate the response,
creates request/response objects, and p
asses them to a method on the servlet instance
Servlet adds information to response object via method calls
Server generates HTTP response based on information stored in response object

14. Hello World! Servlet

15. Hello World! Servlet All servlets we will write are subclasses of
HttpServlet

16. Hello World! Servlet
Server calls doGet() in response to the HTTP GET request

17. Hello World! Servlet
Interfaces implemented by request/response objects

18. Hello World! Servlet
Production servlet should
catch these exceptions

19. Hello World! Servlet First two things done by typical servlet;
must be in this
order

20. Hello World! Servlet HTML generated by calling print() or println()
on the servlet’s PrintWriter object
Good practice to explicitly close
the PrintWriter when done

21. Servlets vs. Java Applications
Servlets do not have a main()
The main() is in the server
Entry point to servlet code is via call to a method (doGet() in the example)
Servlet interaction with end user is indirect via request/response object APIs
Actual HTTP request/response processing is handled by the server
Primary servlet output is typically HTML

22. Running Servlets Simple way to run a servlet (better later):
Compile servlet (make sure that JWSDP libraries are on path)
Copy .class file to shared/classes directory
(Re)start the Tomcat web server
If the class is named ServletHello, browse to

23. Dynamic Content

24. Dynamic Content

25. Dynamic Content

26. Dynamic Content Potential problems:
Assuming one instance of servlet on one server, but
Many Web sites are distributed over multiple servers
Even a single server can (not default) create multiple instances of a single servlet
Even if the assumption is correct, this servlet does not handle concurrent accesses properly
We’ll deal with this later in the chapter

27. Servlet Life Cycle

28. Servlet Life Cycle Servlet API life cycle methods
init(): called when servlet is instantiated; must return before any other methods will be called
service(): method called directly by server when an HTTP request is received; default service() method calls doGet() (or related methods covered later)
destroy(): called when and before server shuts down

29. *
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Servlets Life-Cycle
The Web container manages the life cycle of servlet instances
The life-cycle methods should not be called by your code
init()
...()
service()
doGet()
doPost()
doDelete()
destroy()
doPut()
New
Destroyed
Running
You can provide an implementation of these methods in HttpServlet descendent classes to manipulate the servlet instance and the resources it depends on
(c) 2006 National Academy for Software Development -

30. Life Cycle of a Servlet
The web container maintains the life cycle of a servlet instance. Let's see the life cycle of the servlet:
Servlet class is loaded.
Servlet instance is created.
init method is invoked.
service method is invoked.
destroy method is invoked

31. *
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The init() Method
Called by the Web container when the servlet instance is first created
The Servlets specification guarantees that no requests will be processed by this servlet until the init method has completed
Override the init() method when:
You need to create or open any servlet-specific resources that you need for processing user requests
You need to initialize the state of the servlet
(c) 2006 National Academy for Software Development -

32. *
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The service() Method
Called by the Web container to process a user request
Dispatches the HTTP requests to one of the :
doGet(…),
doPost(…),
etc.
depending on the HTTP request method:
GET,
POST,
and so on
Sends the result as HTTP response
Usually we do not need to override this method
(c) 2006 National Academy for Software Development -

33. *
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The destroy() Method
Called by the Web container when the servlet instance is being eliminated
The Servlet specification guarantees that all requests will be completely processed before this method is called
Override the destroy method when:
You need to release any servlet-specific resources that you had opened in the init() method
You need to persist the state of the servlet
(c) 2006 National Academy for Software Development -

34. Servlet Life Cycle Example life cycle method:
attempt to initialize visits variable
from file
Exception to be thrown
if initialization fails and servlet
should not be instantiated

35. Servlets Architecture
The HttpServlet class
Serves client's HTTP requests
For each of the HTTP methods, GET, POST, and others, there is corresponding method:
doGet(…) – serves HTTP GET requests
doPost(…) – serves HTTP POST requests
doPut(…), doHead(…), doDelete(…), doTrace(…), doOptions(…)
The Servlet usually must implement one of the first two methods or the service(…) method

36. Servlets Architecture (2)
The HttpServletRequest object
Contains the request data from the client
HTTP request headers
Form data and query parameters
Other client data (cookies, path, etc.)
The HttpServletResponse object
Encapsulates data sent back to client
HTTP response headers (content type, cookies, etc.)
Response body (as OutputStream)

37. Servlets Architecture (3)*
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Servlets Architecture (3)
The HTTP GET method is used when:
The processing of the request does not change the state of the server
The amount of form data is small
You want to allow the request to be bookmarked
The HTTP POST method is used when:
The processing of the request changes the state of the server, e.g. storing data in a DB
The amount of form data is large
The contents of the data should not be visible in the URL (for example, passwords)
Example of HTTP GET: Google search
Example of HTTP POST: Login page
(c) 2006 National Academy for Software Development -

38. Servlets API The most important servlet functionality:
Retrieve the HTML form parameters from the request (both GET and POST parameters)
Retrieve a servlet initialization parameter
Retrieve HTTP request header information
HttpServletRequest.getParameter(String)
ServletConfig.getInitParameter()
HttpServletRequest.getHeader(String)

39. Servlets API (2) Set an HTTP response header / content type
Acquire a text stream for the response
Acquire a binary stream for the response
Redirect an HTTP request to another URL
HttpServletResponse.setHeader(<name>, <value>) / HttpServletResponse.setContentType(String)
HttpServletResponse.getWriter()
HttpServletResponse.getOutputStream()
HttpServletResponse.sendRedirect()

40. Servlet API
The javax.servlet and javax.servlet.http packages represent interfaces and classes for servlet API.
The javax.servlet package contains many interfaces / classes used by the servlet or the web container (not protocol specific).
The javax.servlet.http package contains interfaces / classes responsible for http requests only.
Classes in javax.servlet package
GenericServlet
ServletInputStream
ServletOutputStream
ServletRequestWrapper
ServletResponseWrapper
ServletRequestEvent
ServletContextEvent
ServletRequestAttributeEvent
ServletContextAttributeEvent
ServletException
UnavailableException
Interfaces in javax.servlet package
Servlet
ServletRequest
ServletResponse
RequestDispatcher
ServletConfig
ServletContext
SingleThreadModel
Filter
FilterConfig
FilterChain
ServletRequestListener
ServletRequestAttributeListener
ServletContextListener
ServletContextAttributeListener

41. Servlet API Classes in javax.servlet.http package HttpServlet Cookie
HttpServletRequestWrapper
HttpServletResponseWrapper
HttpSessionEvent
HttpSessionBindingEvent
HttpUtils (deprecated now)
Interfaces in javax.servlet.http package
HttpServletRequest
HttpServletResponse
HttpSession
HttpSessionListener
HttpSessionAttributeListener
HttpSessionBindingListener
HttpSessionActivationListener
HttpSessionContext (deprecated now)

42. Parameter Data

43. Parameter Data
The request object (which implements HttpServletRequest) provides information from the HTTP request to the servlet
One type of information is parameter data, which is information from the query string portion of the HTTP request
Query string with
one parameter
Parameter data is the Web analog of arguments in a method call:
Query string syntax and semantics

44. Parameter Data Query string syntax and semantics
Multiple parameters separated by &
Order of parameters does not matter
All parameter values are strings
Value of arg is empty string
But the value of color is “red”

45. Parameter Data Parameter names and values can be any 8-bit characters
URL encoding is used to represent non-alphanumeric characters:
URL decoding applied by server to retrieve intended name or value
Value of arg is
‘a String’

46. Parameter Data

47. Parameter Data

48. Servlet’s Collaboration

49. What is servlet collaboration
When one servlet communicates to another servlet, it is known as servlet collaboration. There are many ways of servlet collaboration:
RequestDispacher interface
RequestDispatcher rd=request.getRequestDispatcher(“SecondServletName ");
rd.forward(request, response);
sendRedirect() method etc.
response.sendRedirect(“SecondServletName");
The RequestDispatcher interface provides the facility of dispatching the request to another resource
it may be html, servlet or jsp.
This interface can also be used to include the content of another resource also.
There are two methods defined in the RequestDispatcher interface.

51. RequestDispatcher rd=request.getRequestDispatcher(“WelcomeServlet");
//servlet2 is the url-pattern of the second servlet
rd.forward(request, response); //method may be include or forward
RequestDispatcher rd = request.getRequestDispatcher("/index.html");  
rd.include(request, response);  

52. SendRedirect in servlet
The sendRedirect() method of HttpServletResponse interface can be used to redirect response to another resource, it may be servlet, jsp or html file.
It accepts relative as well as absolute URL. It works at client side because it uses the url bar of the browser to make another request. So, it can work inside and outside the server.
Difference between forward() and sendRedirect() method
forward() method
sendRedirect() method
The forward() method works at server side.
The sendRedirect() method works at client side.
It sends the same request and response objects to another servlet.
It always sends a new request.
It can work within the server only.
It can be used within and outside the server.
Example: request.getRequestDispacher("servlet2").forward(request,response);
Example: response.sendRedirect("servlet2"); OR
response.sendRedirect("

53. ServletConfig and ServletContext
An object of ServletConfig is created by the web container for each servlet. This object can be used to get configuration information from web.xml file.
If the configuration information is modified from the web.xml file, we don't need to change the servlet. So it is easier to manage the web application if any specific content is modified from time to time.
Methods of ServletConfig interface
public String getInitParameter(String name)
Returns the parameter value for the specified parameter name.
public Enumeration getInitParameterNames()
Returns an enumeration of all the initialization parameter names.
public String getServletName()
Returns the name of the servlet.
public ServletContext getServletContext()
Returns an object of ServletContext.
ServletConfig config=getServletConfig();  

54. ServletConfig: init-param
<servlet> <init-param> <param-name>ParameterName</param-name> <param-value>ParameterValue</param-value> </init-param> </servlet>
    PrintWriter out = response.getWriter();  
    ServletConfig config=getServletConfig();  
    String pvalue=config.getInitParameter("ParameterName");  
    out.print(“initial value is : "+pvalue);  

55. ServletConfig and ServletContext
An object of ServletContext is created by the web container at time of deploying the project. This object can be used to get configuration information from web.xml file. There is only one ServletContext object per web application. If any information is shared to many servlet, it is better to provide it from the web.xml file using the <context- param> element.
Commonly used methods of ServletContext interface
public String getInitParameter(String name)
Returns the parameter value for the specified parameter name.
public Enumeration getInitParameterNames()
Returns the names of the context's initialization parameters.
public void setAttribute(String name,Object object)
sets the given object in the application scope.
public Object getAttribute(String name)
Returns the attribute for the specified name.
public void removeAttribute(String name)
Removes the attribute with the given name from the servlet context.
ServletContext application=getServletConfig().getServletContext(); OR
ServletContext application=getServletContext();  

56. ServletContext: init-param
<web-app> <context-param> <param-name>ParameterName</param-name> <param-value>ParameterValue</param-value> </context-param> </web-app>
    PrintWriter out = response.getWriter();  
    ServletContext context=getServletContext();  
    String pvalue=context.getInitParameter("ParameterName");  
    out.print(“initial value is : "+pvalue);