Enhancing%20a%20Web%20Server%20with%20Servlets

1
Enhancing a Web Server with Servlets
2
Servlets

• Here focus on both sides of a client-server
  relationship.
• The client requests that some action be performed
  and the server performs the action and responds
  to the client.
• This request-response model of communication is
  the foundation for the highest-level view of
  networking in Javaservlets.

3
Servlets

• A common implementation of the request-response
  model is between World Wide Web browsers and
  World Wide Web servers.
• When a user selects a Web site to browse through
  their browser (the client application), a request
  is sent to the appropriate Web server (the server
  application).
• The server normally responds to the client by
  sending the appropriate HTML Web page.

4
Servlets

• A servlet extends the functionality of a server.
  The javax.servlet package and the
  javax.servlet.http package provide the classes
  and interfaces to define servlets.

5
Servlets

• Servlet technology today is primarily designed
  for use with the HTTP protocol of the Web, but
  servlets are being developed for other
  technologies.
• Servlets are effective for developing Web-based
  solutions that help provide secure access to a
  Web site, that interact with databases on behalf
  of a client, ...

6
Servlets

• ... that dynamically generate custom HTML
  documents to be displayed by browsers and that
  maintain unique session information for each
  client.

7
Servlets

• Many developers feel that servlets are the right
  solution for database-intensive applications that
  communicate with so-called thin
  clientsapplications that require minimal
  client-side support.

8
Servlets

• The server is responsible for the database
  access. Clients connect to the server using
  standard protocols available on all client
  platforms.
• Thus, the logic code can be written once and
  reside on the server for access by clients.

9
Servlets

• Our servlet example will make use of JDBC (Java
  Database Connectivity) database facilities to
  build a multi-tier client-server application that
  access a database.

10
Servlets

• The Servlet APIs are now developed by the Apache
  group (www.apache.org).
• Before you can program with servlets, you must
  download and install the Apache group's
  implementation of servlets called Tomcat.
• You may download Tomcat at no charge from Sun
  Microsystems at the Web site
• java.sun.com/products/jsp/tomcat

11
Servlets

• You may download Tomcat at no charge from Sun
  Microsystems at the Web site
• java.sun.com/products/jsp/tomcat

12
Servlets

• After downloading Tomcat, install it on your
  system and carefully read the readme file
  supplied in the doc directory.
• It explains how to set up Tomcat and discusses
  how to start the server that can be used to test
  servlets if you do not have a Web server that
  supports servlets.

13
Servlets

• To develop servlets, you also need to copy the
  servlet.jar file containing the servlet class
  files from the installation directory to your JDK
  extensions directory (the directory
  c\jdk1.3\jre\lib\ext on Windows or the directory
  /jdk1.3/jre/lib/ext on UNIX).

14
Overview of Servlet Technology

• Servlets are the analog on the server side to
  applets on the client side. Servlets are normally
  executed as part of a Web server.
• In fact, servlets have become so popular that
  they are now supported by most major Web servers,
  ...

15
Overview of Servlet Technology

• ... including the Netscape Web servers,
  Microsoft's Internet Information Server (IIS),
  the World Wide Web Consortium's Jigsaw Web server
  and the popular Apache Web server.

16
Overview of Servlet Technology

• The servlets in this chapter show the
  communication between clients and servers via the
  HTTP protocol of the World Wide Web.
• A client sends an HTTP request to the server. The
  server receives the request and directs it to be
  processed by appropriate servlets.

17
Overview of Servlet Technology

• The servlets do their processing (which often
  includes interacting with a database), then
  return their results to the clientnormally in
  the form of HTML documents to display in a
  browser, but other data formats, such as images
  and binary data, can be returned.

18
The Servlet API
19
The Servlet API

• We discuss at a high level the servlet-related
  classes, methods and exceptions. Architecturally,
  all servlets must implement the Servlet
  interface.
• The methods of interface Servlet are invoked
  automatically (by the server on which the servlet
  is installed).

20
The Servlet API

• The servlet packages define two abstract classes
  that implement the interface Servletclass
  GenericServlet (from the package javax.servlet)
  and class HttpServlet (from the package
  javax.servlet.http).

21
The Servlet API

• These classes provide default implementations of
  all the Servlet methods.
• Most servlets extend either GenericServlet or
  HttpServlet and override some or all of their
  methods with appropriate customized behaviors.

22
Servlets

• Methods of
• Interface Servlet

23
MethodDescription

• void init( ServletConfig config )
• This method is automatically called once during a
  servlet's execution cycle to initialize the
  servlet. The ServletConfig argument is supplied
  automatically by the server that executes the
  servlet.

24
MethodDescription

• ServletConfig getServletConfig()
• This method returns a reference to an object that
  implements interface ServletConfig. This object
  provides access to the servlet's configuration
  information such as initialization parameters and
  the servlet's ServletContext, which provides the
  servlet with access to its environment (i.e., the
  server in which the servlet is executing).

25
MethodDescription

• void service( ServletRequest request,
  
• ServletResponse
  response )
• This is the first method called on every servlet
  to respond to a client request.

26
MethodDescription

• String getServletInfo()
• This method is defined by a servlet programmer to
  return a String containing servlet information
  such as the servlet's author and version.

27
MethodDescription

• void destroy()
• This "cleanup" method is called when a servlet is
  terminated by the server on which it is
  executing.
• This is a good method to use to deallocate a
  resource used by the servlet (such as an open
  file or an open database connection).

28
The Servlet API

• The example we present extends class HttpServlet,
  which defines enhanced processing capabilities
  for servlets that extend the functionality of a
  Web server.
• The key method in every servlet is method
  service, which receives both a ServletRequest
  object and a ServletResponse object.

29
The Servlet API

• These objects provide access to input and output
  streams that allow the servlet to read data from
  the client and ...
• ... send data to the client. These streams can be
  either byte-based streams or character-based
  streams.

30
The Servlet API

• If problems occur during the execution of a
  servlet, either ServletExceptions or IOExceptions
  are thrown to indicate the problem.

31
HttpServlet Class
32
HttpServlet Class

• Web-based servlets typically extend the class
  HttpServlet.
• This class HttpServlet overrides the method
  service to distinguish between the typical
  requests received from a client Web browser.

33
HttpServlet Class

• The two most common HTTP request types (also
  known as request methods) are get and post.
• A get request gets (or retrieves) information
  from the server.
• Common uses of get requests are to retrieve an
  HTML document or an image.

34
HttpServlet Class

• A post request posts (or sends) data to the
  server.
• Common uses of post requests are to send the to
  Web-server, information from an HTML form in
  which the client enters data, ...
• ... to send to the server, information so that it
  can search on the Internet, ...
• ... or query a database for the client, ...
• ... to send authentication information to the
  server, etc.

35
HttpServlet Class

• The class HttpServlet defines the methods doGet
  and doPost, which respond to get and post
  requests from a client,respectively.
• These methods are called by the HttpServlet
  class's service method, which is called when a
  request arrives at the server.
• The method service first determines the request
  type, then calls the appropriate method.

36
HttpServlet Class

• Methods doGet and doPost receive as arguments an
  HttpServletRequest object and an
  HttpServletResponse object that enable
  interaction between the client and the server.

37
HttpServlet Class

• The methods of HttpServletRequest make it easy to
  access the data supplied as part of the request.
• The HttpServletResponse methods make it easy to
  return the servlet's results in HTML format to
  the Web client.

38
HttpServlet Class

• The interfaces HttpServletRequest and
  HttpServletResponse are discussed now.

39
HttpServletRequest Interface

• Important methods of interface HttpServletRequest
  Method
• Descriptions

40
HttpServletRequest Interface

• Every call to doGet or doPost for an HttpServlet
  receives an object that implements interface
  HttpServletRequest.
• The Web server that executes the servlet creates
  an HttpServletRequest object and passes this to
  the servlet's service method (which, in turn,
  passes it to doGet or doPost).

41
HttpServletRequest Interface

• This object contains the request from the client.
  A variety of methods are provided to enable the
  servlet to process the client's request. Some of
  these methods are from interface
  ServletRequestthe interface that
  HttpServletRequest extends.

42
HttpServletRequest Interface

• String getParameter( String name )
• Returns the value associated with a parameter
  sent to the servlet as part of a GET or POST
  request. The name argument represents the
  parameter name.

43
HttpServletRequest Interface

• Enumeration getParameterNames()
• Returns the names of all the parameters sent to
  the servlet as part of a POST request

44
HttpServletRequest Interface

• String getParameterValues( String name )
• Returns a String array containing the values for
  a specified servlet parameter.

45
HttpServletRequest Interface

• Cookie getCookies()
• Returns an array of Cookie objects stored on the
  client by the server. Cookies can be used to
  uniquely identify clients to the servlet.

46
HttpServletRequest Interface

• HttpSession getSession( boolean create )
• Returns an HttpSession object associated with the
  client's current browsing session.
• An HttpSession object can be created by this
  method (true argument) if an HttpSession object
  does not already exist for the client.
• HttpSession objects can be used in similar ways
  to Cookies for uniquely identifying clients.

47
HttpServletResponse Interface

• Important methods of interface HttpServletResponse
  Method
• Descriptions

48
HttpServletResponse Interface

• Every call to doGet or doPost for an HttpServlet
  receives an object that implements interface
  HttpServletResponse.
• The Web server that executes the servlet creates
  an HttpServletResponse object and passes this to
  the servlet's service method (which, in turn,
  passes it to doGet or doPost).

49
HttpServletResponse Interface

• This object contains the response to the client.
  A variety of methods are provided to enable the
  servlet to formulate the response to the client.
  Some of these methods are from interface
  ServletResponsethe interface that
  HttpServletResponse extends.

50
HttpServletResponse Interface

• A few key methods used in this interface are
  presented as follow

51
HttpServletResponse Interface

• void addCookie( Cookie cookie )
• Used to add a Cookie to the header of the
  response to the client. The Cookie's maximum age
  and whether the client allows Cookies to be saved
  determine whether or not Cookies will be stored
  on the client.

52
HttpServletResponse Interface

• ServletOutputStream getOutputStream()
• Obtains a byte-based output stream enabling
  binary data to be sent to the client.
• PrintWriter getWriter()
• Obtains a character-based output stream enabling
  text data to be sent to the client.

53
HttpServletResponse Interface

• void setContentType( String type )
• Specifies the MIME type of the response to the
  browser. The MIME type helps the browser
  determine how to display the data (or possibly
  what other application to execute to process the
  data).
• For example, MIME type "text/html" indicates that
  the response is an HTML document, so the browser
  displays the HTML page.

54
Multi-tier Client/Server Application

• with Servlets

55
Multi-tier Client/Server Application with
Servlets

• Servlets can communicate with databases via JDBC
  (Java Database Connectivity).
• JDBC provides a uniform way for a Java program to
  connect with a variety of databases in a general
  manner without having to deal with the specifics
  of those database systems.

56
Multi-tier Client/Server Application with
Servlets

• Many of today's applications are three-tier
  distributed applications, consisting of a user
  interface, business logic and database access.
• The user interface in such an application is
  often created using HTML (as shown in this
  chapter) or Dynamic HTML.

57
Multi-tier Client/Server Application with
Servlets

• In some cases, Java applets are also used for
  this tier.
• HTML is the preferred mechanism for representing
  the user interface in systems where portability
  is a concern.
• Because HTML is supported by all browsers,
  designing the user interface to be accessed
  through a Web browser guarantees portability
  across all platforms that have browsers.

58
Multi-tier Client/Server Application with
Servlets

• Using the networking provided automatically by
  the browser, the user interface can communicate
  with the middle-tier business logic.
• The middle tier can then access the database to
  manipulate the data. All three tiers may reside
  on separate computers that are connected to a
  network.

59
Multi-tier Client/Server Application with
Servlets

• In multi-tier architectures, Web servers are
  increasingly used to build the middle tier.
• They provide the business logic that manipulates
  data from databases and that communicates with
  client Web browsers.

60
Multi-tier Client/Server Application with
Servlets

• Servlets, through JDBC, can interact with popular
  database systems.
• Developers do not need to be familiar with the
  specifics of each database system.
• Rather, developers use SQL-based queries and the
  JDBC driver handles the specifics of interacting
  with each database system.

61
Multi-tier Client/Server Application with
Servlets

• The servlet of Fig 27.32 (a servlet) and the HTML
  document of Fig 27.33 (the client) show a
  three-tier distributed application that displays
  the user interface in a browser using HTML.

62
Multi-tier Client/Server Application with
Servlets

• The middle tier is a Java servlet that handles
  requests from the client browser and provides
  access to the third tiera Microsoft Access
  database (set up as an ODBC data source) accessed
  via JDBC.

63
Multi-tier Client/Server Application with
Servlets

• The servlet in this example is a guest book
  servlet that allows the user to register for
  several different mailing lists.
• When the servlet receives a post request from the
  HTML document of Fig 27.33, it ensures that the
  required data fields are present, then stores the
  data in the database and sends a confirmation
  page to the client.

64
Multi-tier Client/Server Application with
Servlets

• The class GuestBookServlet extends class
  HttpServlet (line 9) so it is capable of
  responding to GET and POST requests.
• Servlets are initialized by overriding method
  init (line 14).
• Method init is called exactly once in a servlet's
  lifetime and is guaranteed to complete before any
  client requests are accepted.

65
Multi-tier Client/Server Application with
Servlets

• The method init takes a ServletConfig argument
  and throws a ServletException.
• The argument provides the servlet with
  information about its initialization parameters
  (i.e., parameters not associated with a request,
  but passed to the servlet by the Web server for
  initializing servlet variables).

66
Multi-tier Client/Server Application with
Servlets

• In this example, the servlet's init method
  performs the connection to the Microsoft Access
  database.
• The method loads the JdbcOdbcDriver at line 20
  with
• Class.forName( "sun.jdbc.odbc.JdbcOdbcDriver" )
  

67
Multi-tier Client/Server Application with
Servlets

• Lines 21 and 22
• connection     DriverManager.getConnection( UR
  L, "", "" )
• attempt to open a connection to the Guests
  database.

68
Multi-tier Client/Server Application with
Servlets

• The string jdbcodbcGuests stored in URL
  specifies the database URL (Uniform Resource
  Locator) that helps the program locate the
  database (possibly on a network or in the local
  file system of the computer).
• The URL specifies the protocol for communication
  (jdbc), the subprotocol for communication (odbc)
  and the name of the database (Guests).

69
Multi-tier Client/Server Application with
Servlets

• The URL specifies the protocol for communication
  (jdbc), the subprotocol for communication (odbc)
  and the name of the database (Guests).
• The subprotocol odbc indicates that the program
  will be using jdbc to connect to a Microsoft ODBC
  data source (see appendix H for information on
  setting up an ODBC data source).

70
Multi-tier Client/Server Application with
Servlets

• ODBC is a technology developed by Microsoft to
  allow generic access to disparate database
  systems on the Windows platform (and some UNIX
  platforms).
• The Java 2 Software Development Kit (J2SDK) comes
  with the JDBC-to-ODBC-bridge database driver to
  allow any Java program to access any ODBC data
  source.

71
Multi-tier Client/Server Application with
Servlets

• The driver is defined by class JdbcOdbcDriver in
  package sun.jdbc.odbc. The second and third
  arguments to getConnection represent the username
  and password (in this example the database does
  not have a username and password).

72
Multi-tier Client/Server Application with
Servlets

• Lines 23-25
• statement  connection.prepareStatement(   "I
  NSERT INTO Guests values ( "       "?, ?, ?, ?, 
  ?, ?, ?, ? )" )
• create a PreparedStatement that will be used to
  insert a record into the database.

73
Multi-tier Client/Server Application with
Servlets

• The question mark characters in the string
  represent the placeholders for values that will
  be inserted.
• These values are specified with PreparedStatement
  set method calls before executing the insert
  operation.
• The eight placeholders in this example represent
  the user's email address, first name, last name,
  company and the four mailing lists the user would
  like to register to receive.

74
Multi-tier Client/Server Application with
Servlets

• When a post request is received from the HTML
  document in Fig 27.33, method doPost (line 33)
  responds by reading the HTML form field values
  from the post request, setting the parameters for
  the INSERT INTO operation on the database (lines
  54-65) and executing the insert operation (line
  66).
• Line 67 closes the statement to ensure that the
  insert operation is committed to the database.

75
Multi-tier Client/Server Application with
Servlets

• The if structure at lines 44-50 determines if the
  email, first name or last name parameters are
  empty Strings. If so, the servlet response asks
  the user to return to the HTML form and enter
  those fields.
• Line 83 defines method destroy to ensure that the
  database connection is closed before the servlet
  terminates.

76
Multi-tier Client/Server Application with
Servlets

• Figure 27.33 defines the HTML document that
  presents the guest book form to the user and
  POSTs the information to the servlet of Fig
  27.32.

77
Multi-tier Client/Server Application with
Servlets

• Lines 9-11 specify that the form's action is to
  post information to the GuestBookServlet.
• The screen captures show the form filled with one
  set of information (the first screen) and the
  confirmation Web page that was sent back to the
  client as the response to the post request.

78
Servlets

• Server-Side Java Programming

79
Servlets and Server-Side Java Programming

• java.sun.com/products/servlet/index.html
• The servlet page at the Sun Microsystems, Inc.
• Java Web site provides access to the latest
  servlet information, servlet resources and the
  Java Servlet Development Kit (JSDK).

80
Servlets and Server-Side Java Programming

• theserverside.com/home/index.jsp
• This Web site is an excellent resource for anyone
  doing server-side Java development and
  development with the Java 2 Enterprise Edition.

81
Servlets and Server-Side Java Programming

• www.servlets.com
• This is the Web site for the book Java Servlet
  Programming published by O'Reilly.
• The book provides a variety of resources.
• This book is an excellent resource for
  programmers who are learning servlets.

82
Servlets and Server-Side Java Programming

• www.servletcentral.com
• Servlet Central is an online magazine for
  server-side Java programmers.
• This includes technical articles and columns,
  news and "Ask the Experts."
• Resources include books, servlet documentation
  links on the Web, a servlet archive, a list of
  servlet-enabled applications and servers and
  servlet development tools.

83
Servlets and Server-Side Java Programming

• http//www.servletsource.com/
• is a general servlet resource site containing
  code, tips, tutorials and links to many other Web
  sites with information on servlets.
• http//www.cookiecentral.com/
• A good all-around resource site for cookies.

84
Servlets and Server-Side Java Programming

• www.purpletech.com/java/servlet-faq/
• The Purple Servlet FAQ is a great resource
  with dozens of links to tutorials, other servlet
  FAQs, mailing lists and newsgroups, articles, web
  servers, whitepapers and Java e-mail resources.

85
Servlets and Server-Side Java Programming

• http//www.servletforum.com/
• is an on-line newsgroup dedicated to Java
  Servlets. Post your own questions or check out
  the archived list of previously asked questions.

86
Servlets and Server-Side Java Programming

• www.enhydra.org/
• Enhydra is an open source Java/XML
  application server and development environment
  available for free download.
• www.locomotive.org/locolink/disp?home
• The Locomotive Project is an open source,
  servlet-compatible, web application server
  available for free download.

87
Servlets and Server-Side Java Programming

• www.servlet.com/srvpages/srvdev.html
• The Servlet, Inc. Servlet Developer's Forum has
  links to numerous web resources, examples,
  products that use servlets and server-enabled web
  servers.

88
Summary

• About Servlets

89
Summary about Servlets

• The request-response model of communication is
  the foundation for the highest-level view of
  networking in Javaservlets.
• A servlet extends the functionality of a server.

90
Summary about Servlets

• The javax.servlet package and the
  javax.servlet.http package provide the classes
  and interfaces to define servlets.

91
Summary about Servlets

• A common implementation of the request-response
  model is between World Wide Web browsers and
  World Wide Web servers.
• When a user selects a Web site to browse through
  their browser (the client application), a request
  is sent to the appropriate Web server (the server
  application).
• The server normally responds to the client by
  sending the appropriate HTML Web page.

92
Summary about Servlets

• All servlets must implement the Servlet
  interface. The methods of interface Servlet are
  invoked automatically (by the server on which the
  servlet is installed).
• An HttpServlet defines enhanced processing
  capabilities for servlets that extend the
  functionality of a Web server.

93
Summary about Servlets

• Class HttpServlet overrides method service to
  distinguish between the typical requests received
  from a client Web browser.
• The two most common HTTP request types (also
  known as request methods) are get and post.

94
Summary about Servlets

• Class HttpServlet defines methods doGet and
  doPost to respond to get and post requests from a
  client, respectively.
• Servlets can communicate with databases via JDBC
  (Java Database Connectivity).
• JDBC provides a uniform way for a Java program to
  connect with a variety of databases in a general
  manner without having to deal with the specifics
  of those database systems.

95
Summary about Servlets

• To connect to a database, you must first load the
  database driver, then connect to the database
  with class DriverManager's getConnection method.
• The URL to connect to a database specifies the
  protocol for communication, the subprotocol for
  communication and the name of the database.

96
Summary about Servlets

• The subprotocol odbc indicates that the program
  will be using jdbc to connect to a Microsoft ODBC
  data source.
• ODBC is a technology developed by Microsoft to
  allow generic access to disparate database
  systems on the Windows platform (and some UNIX
  platforms).

97
Summary about Servlets

• A PreparedStatement creates an SQL statement that
  will be executed at a later time.
• The parameters to the PreparedStatement are
  specified with set method calls before executing
  the PreparedStatement.