1. Web Application Development 1

2. Outline Web application architecture HTTP CGI Java Servlets Apache Tomcat Session maintenance 2

3. Architecture

4. 4 HTTP Created by Tim Berners-Lee at CERN –Defined 1989-1991 Standardized and much expanded by the IETF Rides on top of TCP protocol –TCP provides: reliable, bi-directional, in-order byte stream Goal: transfer objects between systems –Do not confuse with other WWW concepts: HTTP is not page layout language (that is HTML) HTTP is not object naming scheme (that is URLs) Text-based protocol –Human readable

5. Client-Server Paradigm Client:  initiates contact with server (“speaks first”)  typically requests service from server,  for Web, client is implemented in browser; for e-mail, in mail reader Server:  provides requested service to client  e.g., Web server sends requested Web page, mail server delivers e- mail PC running Explorer Apache Web server Mac running Safari HTTP request HTTP response http request http response

6. HTTP 6

7. 7 HTTP Request Message Format HTTP method URL spHTTP versioncr lf header field name : field value crlf request line sp header field name : field value crlf crlf Entity Body header lines …

8. 8 HTTP Request Example GET /somedir/page.html HTTP/1.0 User-agent: Mozilla/4.0 Accept: text/html, image/gif,image/jpeg Accept-language:fr (extra carriage return, line feed) request line (GET, POST, HEAD commands) header lines Carriage return, line feed indicates end of message

9. 9 HTTP Response Message Format HTTP version status code spstatus phrasecr lf header field name : field value crlf response line sp header field name : field value crlf crlf Response Body header lines …

10. 10 HTTP Response Example HTTP/1.0 200 OK Date: Thu, 25 Aug 2001 12:00:15 GMT Server: Apache/1.3.0 (Unix) Last-Modified: Mon, 22 Aug 2001 …... Content-Length: 6821 Content-Type: text/html data data data data data... header lines status line: (protocol status code status phrase) data, e.g., requested html file Carriage return, line feed indicates end of message

11. CGI Architecture

12. Objectives Standardizes execution of server-side applications/handlers Specifies: –Interface between client and handler –Interface between web server and hander

13. Web Server-Handler Interaction Handler Input –Meta-variables –Stdin (message body) Handler Output –Stdout

14. Client-Handler Interaction Clients request the execution of a handler program by means of a: –A request method (e.g., GET, POST) –Universal Resource Identifier (URI) Identifies handler –Extra arguments

15. URI Syntax :// / "?" http://finance.yahoo.com/add?op1=10&op2=20 http finance.yahoo.com 80 null add op1=10, op2=20

16. Query String Encoding RFC 2396 Why encode?  Can think of a CGI script as a function, send arguments by specifying name/value pairs.  Way to pack and unpack multiple arguments into a single string

17. Encoding Rules  All arguments  Single string of ampersand (&) separated name=value pairs name_1=value_1&name_2=value_2&...  Spaces  Replaced by a plus (+) sign  Other characters (ie, =, &, +)  Replaced by a percent sign (%) followed by the two-digit hexadecimal ASCII equivalent

18. Method GET –Arguments appear in the URL after the ? –Can be expressed as a URL –Limited amount of information can be passed this way –URL may have a length restriction on the server –Arguments appear on server log POST  Arguments are sent in the HTTP message body  Cannot be expressed as URL  Arbitrarily long form data can be communicated (some browsers may have limits (i.e. 7KB)).  Arguments usually does not appear in server logs.

19. Forms and CGI Specify request method in “method” attribute Automatically encodes all named field values

20. Example: add.html

21. 21 Servlets Generic Java2EE API for invoking and connecting to “mini-servers” (lightweight, short-lived, server- side interactions). Mostly HTTPServlets –Generate HTTP responses (as for CGI) –Servlet code snippets can be embedded directly into HTML pages: JSP = Java Server Pages Competitor: Microsoft ASP = Active Server Pages

22. 22 Servlet Architecture  Web server: Apache, Netscape Enterprise, IIS  Servlet container: Running JVM, hooked into the web server, loads and maintains servlets, session info, object store  Servlet: A Java class used to handle a specific request.

23. 23 Main Process JVM Java Servlet-Based Web Server Servlet1 Request for Servlet1 Request for Setvlet2 Request for Servlet1 Servlet2 Thread

24. 24 Servlet API (javax.Servlet) Servlet Container Servlet Interface HttpServletResponce HttpServletRequest ServletContext

25. 25 Servlet Container  Contains and manages servlets through their lifecycle  Provides the network services over which requests and responses are sent  Decodes requests  Formats MIME based responses

26. 26 Servlet Interface  Central abstraction of the servlet API  All servlets either implement this interface, or extend a class that implements this interface  GenericServlet and HttpServlet two classes in the servlet API that implement the Servlet interface

27. 27 HttpServlet  Web developers extend HttpServlet to implement their servlets.  Inherited from GenericServlet  Init()  Destroy()  Defined by HTTPServlet  doGet()  doPost()

28. 28 doGet() and doPost() protected void doGet(HttpServletRequest req, HttpServletResponse resp)HttpServletRequestHttpServletResponse protected void doPost(HttpServletRequest req, HttpServletResponse resp)HttpServletRequestHttpServletResponse

29. 29 HttpServletResponse  Encapsulates all information to be returned from the server to the client  In the HTTP protocol, this information is transmitted from the server to the client either by HTTP headers or the message body of the request.  Headers: Can set/manipulate http headers  primitive manipulation: setStatus(), setHeader(), addHeader()  convenience methods: setContentType(), sendRedirect(), sendError()  Body: Obtain a PrintWriter, used to return character data to the client  getWriter()

30. 30 Example: Hello World

31. 31 HttpServletRequest  Encapsulates all information from the client request  In the HTTP protocol, this information is transmitted from the client to the server in the HTTP headers and the message body of the request.  Manipulate 'form' variables: getParameter() getParameterNames() getParameterValues()

32. 32 Example: Form

33. Environment Variables DOCUMENT_ROOT root directory of your server HTTP_COOKIE visitor's cookie, if one is set HTTP_HOST hostname of the page HTTP_REFERER HTTP_USER_AGENT browser type of the visitor HTTPS "on" if called through a secure server QUERY_STRING query string REMOTE_ADDR IP address of the visitor REMOTE_HOST hostname of the visitor REMOTE_USER visitor's username REQUEST_METHOD GET or POST REQUEST_URI SCRIPT_FILENAME The full pathname of the current CGI SERVER_SOFTWARE server software (e.g. Apache 1.3)

34. 34 Example: Environment Variables

35. 35 Tomcat Configuration Tomcat webapps csc309Servlets Root index.jspindex.html conf server.xml WEB-INF classes web.xml HelloWorld.class web.xml tomcat_users.xml SomeOther.class

36. 36 Deployment Descriptor (web.xml) $CATALINA_HOME/conf/web.xml –Default for all web applications $CATALINA_HOME/webapp/ece1779Servlets/WEB_INF/web.xml –Specific to ece779Servlets SampleServerName HelloWorld SampleServletName /index.html http://127.0.0.1:8080/ece1779Servlets/http://127.0.0.1:8080/ece1779Servlets/index.html Maps to: $CATALINA_HOME/webapp/ece1779Servlets/WEB_INF/classes/HelloWorld.class

37. 37 $CATALINA_HOME/conf/tomcat_users.xml

38. 38 Tomcat Web Application Manager Deploy applications Start/stop/reload

39. 39 Servlet Debugging Logs –$CATALINA_HOME/logs Interactive debugger –Use Java JPDA interface Allows an external program to “attach” to process –Start Tomcat in debugging mode $CATALINA_HOME/bin/catalina.sh jpda start –Start IDE debugger –Attach debugger to running Tomcat instance

40. 40 Session Maintenance  HTTP is stateless by default  No memory of browsers state  Preserving state between browser/web server interactions  Behavior reflects previous forms filled in and pages viewed  Kept at the browser or/and web server.  Example: Want to allow access to a given script only once a user is logged in. Don't want to require a user to login at every page.

41. 41 Store State At Server  Current state is stored at the server (i.e., in a file, database, or in JVM’s memory)  Each request includes a token identifying the browsers session (tokens can be passed via cookies, hidden variables, URL rewriting).  At each request, the executing servlet uses the token to fetch session state  Careful: Browser back button problem. The page the user is viewing may not reflect state stored at the server.

42. 42 HttpSession  One instance for each active session  Accessing/creating a session HttpSession session = request.getSession();  Session attributes Object o = session.getAttribute("name"); session.setAttribute("name",o);

43. 43 Session Duration Default: expire a session if idle (no http requests) for 30 minutes –Delete all session related attributes from server –Does not remove cookie from client Changing session timeout –Configuration file (web.xml) 30 –Run-time session.getMaxInactiveInterval() session.setMaxInactiveInterval() Terminating a session –session.invalidate() –Deletes session related attributes form server –Removes cookie from client

44. 44 Session Example

45. 45 Other Issues Multithreading –HttpSession is not thread safe –Use proper synchronization to prevent concurrent property modifications Persistent sessions –Tomcat has support for persistent sessions Survive server restart –Saves session data to a file –Serializes properties Property values need to be serializable Session event listeners –Possible to create servlets that execute when a session times out –Can be use to garbage collect resources outside of the JVM

46. 46 ServletContext  Interface defines a servlet’s view of the web application within which the servlet is running.  Container Provider is responsible for providing an implementation of the ServletContext interface in the servlet container.  Using the ServletContext object, a servlet can  log events  obtain URL references to resources  set attributes that other servlets in the context can access.

47. 47 ServletContext Continued One instance of ServletContext per web application per container ServletContext context = getServletContext(); Provides access to Initialization Parameters  Defined in the deployment descriptor  A file called web.xml in the WEB-INF subdirectory of the app directory  Methods: getInitParameter(), getInitParameterNames()

48. 48 ServletContext Attributes  Context level shared objects. Can be bound into the context and accessed by any servlet that is part of the application.  Methods: setAttribute() getAttribute() getAttributeNames() removeAttributes()  More than 1 thread may be accessing a shared object.  Application developer responsible for synchronizing access to shared objects

49. 49 ServletContext Resources  Gives servlet access to local documents associated with the application. (i.e. static web pages, images)  Access to server logs  Methods: getResource() getResourceAsStream() getResourcePaths() log(“Error! File does not exist”);

50. ServletContextExample 50

51. 51 JSP Servlets disadvantage –A little java in a whole lot of HTML Java Server Pages –Java embedded in HTML Deployed in textual form (as opposed to byte code) Translated to into a servlet on the fly $CATALINA_HOME/webapps/ece1779/hello.jsp $CATALINA_HOME/work/Catalina/localhost/ece1779/org/apache/jsp/hello_jsp.java

52. 52 JSP Life Cycle

53. JSP Expressions Java expression that evaluates into a String Translated into out.print() statement Scriplets <% java.util.Date currentTime = new java.util.Date; String strDate = currentTime.toString(); %> One or more Java statements Left in place Directives Tags

54. Movieplex Example

55. Design Content All data stored in Movieplex.java - Array of movies - Array of theatres - Array of showtimes Initialization Initialization.java - Servlet that creates Movieplex object and stores in ServletContent Regular operation ListTheaters.jsp - Retrieves movie data from servlet context - Renders HTML page with list of all available theatres ShowTheater.jsp - Takes theaterid as parameter - Renders HTML listing all the movies currently shown at a theatre

56. Data Structures public class Movie { public int id; public String name; public String rating; public String starring; public String director; public String synopsis; public String poster; public String url; } public class Theater { public int id; public String name; public String address; public String phone; } public class Showtime { public int id; public int theaterid, movieid; public int year, month, day; // date public int hour, minute; // time of day public float price; }

57. Data Structures public class Movieplex { public Theater [] theaters; public Movie [] movies; public Showtime [] showtimes; public Movieplex() { Theater [] t = { new Theater(1,"Canada Square","2190 Yonge Street, Toronto, ON, M4S2C6", "416-6460444"), new Theater(2,"SilverCity","2300 Yonge Street, Toronto, ON, M4P1E4","416- 5441236") }; this.theaters = t; Movie [] m = { new Movie(1,"Casino Royale", "PG-13","Daniel Craig, Judi Dench, Mads Mikkelsen, Eva Green, Jeffrey Wright",......), new Movie(2,"Babel", "R", "Brad Pitt, Cate Blanchett, Gael Garcia Bernal, Elle Fanning, Koji Yakusho",... ) }; this.movies = m; Showtime [] s = { new Showtime(1,1,1,2007,3,23,18,0,8), new Showtime(2,1,1,2007,3,18,20,0,8) }; this.showtimes = s; }

58. 58 Three Tier Architecture

59. 59 RDBMS Relational Database Management Systems A way of saving and accessing data on persistent (disk) storage.

60. 60 Why Use an RDBMS Data Safety –data is immune to program crashes Concurrent Access –atomic updates via transactions Fault Tolerance –replicated dbs for instant fail-over on machine/disk crashes Data Integrity –aids to keep data meaningful Scalability –can handle small/large quantities of data in a uniform manner Reporting –easy to write SQL programs to generate arbitrary reports

61. 61 RDBMS Technology Client/Server Databases –Oracle, Sybase, MySQL, SQLServer Personal Databases –Access Embedded Databases –Pointbase

62. 62 Server server process disk i/o Client/Server Databases client client processes tcp/ip connections

63. 63 client application code Inside the Client Process tcp/ip connection to server db library API

64. 64 JDBC  JDBC (Java DataBase Connectivity)  Standard SQL database access interface.  Allows a Java program to issue SQL statements and process the results.  DB independent. Can replace underlying database with minimal code impact.  Defines classes to represent constructs such as database connections, SQL statements, result sets, and database metadata.

65. 65 A JDBC Application

66. 66 JDBC Pieces  Java client: code implementing the application  JDBC API. Provides DB independent abstraction to  establish a connection with a database  send SQL statements  process the results  JDBC Driver  Translates API calls to requests made against the specific database.  Specific driver for the each database.  Installed on the client. Usually a set of class files placed in the class path.  e.g., mysql-connector-java-5.1.7-bin.jar  All large databases are now supported.  Database server:  The actual database engine  Pointbase, Oracle, MSAccess, SQL Server, Postgresql etc.

67. 67 JDBC Package java.sql java.sun.com/products/jdk/1.2/docs/api/java/sql/package-summary.html Classes and interfaces –DriverManagergetConnection() –ConnectionTransactions commit(), rollback(), setAutoCommit() SQL statements createStatement(), prepateStatement() –StatementexecuteQuery(), executeUpdate() –ResultSetScrolling over tuples next(), previous(), first(), last(), isLast() Values of attributes getString(int index), getTime(String name) –PreparedStatementexecuteQuery(), executeUpdate() setInt (int index, int x)

68. 68 API: Establish Connection 1Class.forName(com.mysql.jdbc.Driver) make the driver class available 2String url = “jdbc:mysql://sysnet.cs.toronto.edu/databasename"; This is the connect string. The connect string, mentions the driver as well as the database. For the example above, the driver is the jdbc:pointbase:embedded bridge driver. The database is sample. 3Connection con=DriverManager.getConnection(url, uID, pw); Get a connection (session) with a specific database. Within the context of a Connection, SQL statements are executed and results are returned. A Connection can be used to obtain information about the DB By default a Connection automatically commits changes after each statement. Typically, setting up a connection is an expensive operation.

69. 69 API: Executing Queries  A query can return many rows, each with many attributes  Steps are 1Send query to the database 2Retrieve one row at a time 3For each row, retrieve attributes

70. 70 Trivial Example

71. 71 API: Updates Int rowsAffected= stmt.executeUpdate( "DELETE * FROM ACCOUNTS;"); Executes a SQL INSERT, UPDATE or DELETE statement. Returns the number of rows affected.

72. 72 API: Prepared Statements  Is a parameterized SQL statement.  Used to speedup query parsing (statement does not need to be reparsed)  Used to simplify development (clumsy strings do not have to be created and recreated for each update). Example: String insert="INSERT INTO ACCOUNT(NAME,AMOUNT)VALUES(?,?);"; PreparedStatement ps =con.prepareStatement(insert); ps.setString(1,"Charlie"); // Fill in the first ? ps.setDouble(2,23.45); // Fill in the second ? rowsEffected=ps.executeUpdate(); ps.setInt(1,"Arnold"); // Fill in the first ? ps.setInt(2,102.23); // Fill in the second ? rowsEffected=ps.executeUpdate();

73. 73 Scope of DB Connections 1.Request: Open/close connection on each action invocation 2.Session: Keep 1 open connection associated with the session: HttpSession.getParameter(“dbCon”) 3.Web App: Keep 1 open connection for the web application and re-use it

74. 74 Connection Pooling The Solution: –Maintain a pool of open connections that time themselves out The SharedPoolDataSource allocates a new Connection object –It wraps it in a PoolingConnection object that delegates calls to its enclosed Connection object and returns it to the client. –It sets a timeout that will reclaim the Connection for a pool of free connections If client accesses PoolingConnection after timeout, PoolingConnection will request a fresh connection from the pool. Client returns Connection to the pool when done with it. –SharedPoolDataSource does not close the Connection, rather saves it for the next request Client may block awaiting a freed connection if some maximum upper limit of open connections is reached.

75. 75 Connection Pooling –Init method of main webapp servlet: // Create a data source DriverAdapterCPDS ds = new DriverAdapterCPDS(); ds.setDriver("com.mysql.jdbc.Driver"); ds.setUrl("jdbc:mysql://servername/databasename"); ds.setUser("public"); ds.setPassword("public"); // Create a connection pool SharedPoolDataSource dbcp = new SharedPoolDataSource(); dbcp.setConnectionPoolDataSource(ds); //Add to webapp context getServletContext().setAttribute(“dbConPool”,dbcp);

76. 76 Connection Pooling –Each servlet doGet() or doPost() calls: SharedPoolDataSource dbcp = getServletContext().getAttribute(“dbConPool”); Connection con = dbcp.getConnection(); … Statement s = con.createStatement(); … con.close();

77. 77 Indirection and Capabilities Servlet connection x connection Poolingconnection private connection x onTimeOut: x = null Servlet dbonnection y Cannot revoke capability Indirection enables revoking capability Capability: enforces access control by providing a handle to a resource.

78. 78 SQLGateway

79. 79 Atomic Transactions ATM banking example: –Concurrent deposit/withdrawal operation –Need to protect shared account balance What about transferring funds between accounts? –Withdraw funds from account A –Deposit funds into account B

80. 80 Properties of funds transfer Should appear as a single operation –Another process reading the account balances should see either both updates, or none Either both operations complete, or neither does –Need to recover from crashes that leave transaction partially complete

81. 81 Transactions Definition: A transaction is a collection of DB modifications, which is treated as an atomic DB operation. –Transactions ensure that a collection of updates leaves the database in a consistent state (as defined by the application program); all updates take place or none do. –A sequence of read and write operations, terminated by a commit or abort Definition: Committed –A transaction that has completed successfully; once committed, a transaction cannot be undone Definition: Aborted –A transaction that did not complete normally JDBC: By default the Connection automatically commits changes after executing each statement. If auto commit has been disabled, the method commit must be called explicitly; otherwise, database changes will not be saved.

82. 82 API: Transactions Example: // Change to transactional mode con.setAutoCommit(false); // Transaction A begins here stmt.executeUpdate("DELETE * FROM ACCOUNT...;");// 1 stmt.executeUpdate("INSERT INTO ACCOUNT...."); // 2 stmt.executeUpdate("INSERT INTO ACCOUNT...."); // 3 stmt.executeUpdate("INSERT INTO ACCOUNT...."); // 4 con.commit(); // Commit changes to database // All of 1,2,3,4 take effect

83. 83 API: Transactions Example: // Transaction B begins here stmt.executeUpdate("DELETE * FROM SALES...;"); // 5 stmt.executeUpdate("INSERT INTO SALES...."); // 6 stmt.executeUpdate("INSERT INTO SALES...."); // 7 stmt.executeUpdate("INSERT INTO SALES...."); // 8 con.rollback(); // Rollback to before transaction B began // None of 5,6,7,8 effects the DB

84. 84 Example Movie database Add an order and decrement # of available spaces