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Server side

  1. 1. CSI 3140 WWW Structures, Techniques and Standards Server-side Programming: Java Servlets
  2. 2. Server-side Programming The combination of    HTML JavaScript DOM is sometimes referred to as Dynamic HTML (DHTML) Web pages that include scripting are often called dynamic pages (vs. static) Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 2
  3. 3. Server-side Programming Similarly, web server response can be static or dynamic   Static: HTML document is retrieved from the file system and returned to the client Dynamic: HTML document is generated by a program in 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 Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 3
  4. 4. Servlet Overview Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 4
  5. 5. Servlet Overview 1.When server starts it instantiates servlets 2.Server receives HTTP request, determines need for dynamic response 3.Server selects the appropriate servlet to generate the response, creates request/response objects, and passes them to a method on the servlet instance 4.Servlet adds information to response object via method calls 5.Server generates HTTP response based on information stored in response object Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 5
  6. 6. Hello World! Servlet Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 6
  7. 7. Hello World! Servlet All servlets we will write are subclasses of HttpServlet Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 7
  8. 8. Hello World! Servlet Server calls doGet() in response to GET request Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 8
  9. 9. Hello World! Servlet Interfaces implemented by request/response objects Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 9
  10. 10. Hello World! Servlet Production servlet should catch these exceptions Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 10
  11. 11. Hello World! Servlet JWSDP Tomcat server exception handling:   Stack trace appended to logs/jwsdp_log.*.txt HTML document returned to client may (or may not) contain partial stack trace Servlet output to System.out.print(), printStackTrace(), etc. is appended to logs/launcher.server.log Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 11
  12. 12. Hello World! Servlet First two things done by typical servlet; must be in this order Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 12
  13. 13. Hello World! Servlet Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 13
  14. 14. Hello World! Servlet HTML generated by calling print() or println() on the servlet’s PrintWriter object Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 14
  15. 15. Hello World! Servlet Good practice to explicitly close the PrintWriter when done Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 15
  16. 16. 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 Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 16
  17. 17. Running Servlets  Simple way to run a servlet (better later): 1. 2. 3. 4. 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 http://localhost:8080/servlet/ServletHello Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 17
  18. 18. Dynamic Content Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 18
  19. 19. Dynamic Content Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 19
  20. 20. Dynamic Content Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 20
  21. 21. 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 Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 21
  22. 22. 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 server shuts down Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 22
  23. 23. Servlet Life Cycle Example life cycle method: attempt to initialize visits variable from file Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 23
  24. 24. Servlet Life Cycle Exception to be thrown if initialization fails and servlet should not be instantiated Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 24
  25. 25. 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 Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 25
  26. 26. Parameter Data Parameter data is the Web analog of arguments in a method call: Query string syntax and semantics Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 26
  27. 27. 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 Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 27
  28. 28. Parameter Data Parameter names and values can be any 8bit characters URL encoding is used to represent nonalphanumeric characters: Value of arg is ‘a String’ URL decoding applied by server to retrieve intended name or value Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 28
  29. 29. Parameter Data URL encoding algorithm Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 29
  30. 30. Parameter Data Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 30
  31. 31. Parameter Data Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 31
  32. 32. Parameter Data Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 32
  33. 33. Parameter Data Must escape XML special characters in all user-supplied data before adding to HTML to avoid cross-site scripting attacks Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 33
  34. 34. Parameter Data Cross-site scripting Attacker Comment containing <script> element Blogging Web site Victim Document containing attacker’s comment (and script) Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 34
  35. 35. Parameter Data Also need to escape quotes within attribute values. Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 35
  36. 36. Parameter Data Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 36
  37. 37. Parameter Data A form automatically generates a query string when submitted  Parameter name specified by value of name attributes of form controls  Parameter value depends on control type Value for checkbox specified by value attribute Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 37
  38. 38. Parameter Data Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 38
  39. 39. Parameter Data username lifestory doit boxgroup1 (values same as labels) Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 39
  40. 40. Parameter Data Query string produced by browser (all one line): Checkbox parameters have same name values; only checked boxes have corresponding parameters Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 40
  41. 41. Parameter Data GET vs. POST method for forms:  GET:  Query string is part of URL  Length of query string may be limited  Recommended when parameter data is not stored but used only to request information (e.g., search engine query)  The URL can be bookmarked or emailed and the same data will be passed to the server when the URL is revisited Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 41
  42. 42. Parameter Data Browser content copyright 2004 Google, Inc. Used by permission. Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 42
  43. 43. Parameter Data GET vs. POST method for forms:  POST:  Query string is sent as body of HTTP request  Length of query string is unlimited  Recommended if parameter data is intended to cause the server to update stored data  Most browsers will warn you if they are about to resubmit POST data to avoid duplicate updates Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 43
  44. 44. Parameter Data Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 44
  45. 45. Parameter Data GET vs. POST in a Web application: According to the HTTP 1.1 specification (RFC 2616): In particular, the convention has been established that the GET and HEAD methods SHOULD NOT have the significance of taking an action other than retrieval A consequence of this is that “web accelerators” might prefetch your GET-based URL, with possible disastrous consequences if you actually use GET for processing. (note that because of this, accelerators such as Google’s won’t pre-fetch GET-based URLs with parameters) Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 45
  46. 46. Sessions Many interactive Web sites spread user data entry out over several pages:  Ex: add items to cart, enter shipping information, enter billing information Problem: how does the server know which users generated which HTTP requests?  Cannot rely on standard HTTP headers to identify a user Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 46
  47. 47. Sessions Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 47
  48. 48. Sessions Server sends back new unique session ID when the request has none Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 48
  49. 49. Sessions Client that supports session stores the ID and sends it back to the server in subsequent requests Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 49
  50. 50. Sessions Server knows that all of these requests are from the same client. The set of requests is known as a session. Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 50
  51. 51. Sessions And the server knows that all of these requests are from a different client. Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 51
  52. 52. Sessions Returns HttpSession object associated with this HTTP request. • Creates new HttpSession object if no session ID in request or no object with this ID exists • Otherwise, returns previously created object Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 52
  53. 53. Sessions Boolean indicating whether returned object was newly created or already existed. Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 53
  54. 54. Sessions Incremented once per session Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 54
  55. 55. Sessions Three web pages produced by a single servlet Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 55
  56. 56. Sessions Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 56
  57. 57. Sessions ,,, Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 57
  58. 58. Sessions ,,, Session attribute is a name/value pair Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 58
  59. 59. Sessions ,,, Session attribute will have null value until a value is assigned Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 59
  60. 60. Sessions ,,, Generate sign-in form if session is new or signIn attribute has no value, weclome-back page otherwise. Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 60
  61. 61. Sessions Sign-in form Welcome-back page Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 61
  62. 62. Sessions Second argument (“Greeting”) used as action attribute value (relative URL) Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 62
  63. 63. Sessions Form will be sent using POST HTTP method (doPost() method will be called) Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 63
  64. 64. Sessions Text field containing user name is named signIn Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 64
  65. 65. Sessions … Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 65
  66. 66. Sessions … Retrieve signIn parameter value Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 66
  67. 67. Sessions … Normal processing: signIn parameter is present in HTTP request Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 67
  68. 68. Sessions … Generate HTML for response Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 68
  69. 69. Sessions Thank-you page Must escape XML special characters in user input Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 69
  70. 70. Sessions … Assign a value to the signIn session attribute Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 70
  71. 71. Sessions Session attribute methods:   setAttribute(String name, Object value): creates a session attribute with the given name and value Object getAttribute(String name): returns the value of the session attribute named name, or returns null if this session does not have an attribute with this name Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 71
  72. 72. Sessions … Error processing (return user to sign-in form) Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 72
  73. 73. Sessions By default, each session expires if a serverdetermined length of time elapses between a session’s HTTP requests  Server destroys the corresponding session object Servlet code can:   Terminate a session by calling invalidate() method on session object Set the expiration time-out duration (secs) by calling setMaxInactiveInterval(int) Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 73
  74. 74. Cookies A cookie is a name/value pair in the SetCookie header field of an HTTP response Most (not all) clients will:   Store each cookie received in its file system Send each cookie back to the server that sent it as part of the Cookie header field of subsequent HTTP requests Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 74
  75. 75. Cookies Tomcat sends session ID as value of cookie named JSESSIONID Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 75
  76. 76. Cookies Cookie-enabled browser returns session ID as value of cookie named JSESSIONID Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 76
  77. 77. Cookies Servlets can set cookies explicitly    Cookie class used to represent cookies request.getCookies() returns an array of Cookie instances representing cookie data in HTTP request response.addCookie(Cookie) adds a cookie to the HTTP response Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 77
  78. 78. Cookies Cookies are expired by client (server can request expiration date) Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 78
  79. 79. Cookies Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 79
  80. 80. Cookies Return array of cookies contained in HTTP request Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 80
  81. 81. Cookies Search for cookie named COUNT and extract value as an int Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 81
  82. 82. Cookies Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 82
  83. 83. Cookies Send replacement cookie value to client (overwrites existing cookie) Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 83
  84. 84. Cookies Should call addCookie() before writing HTML Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 84
  85. 85. Cookies Privacy issues HTTP request to intended site Client HTTP response: HTML document including ad <img> Web site providing requested content HTTP request for ad image Image plus Set-Cookie in response: third-party cookie Web site providing banner ads Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 85
  86. 86. Cookies Privacy issues HTTP request to 2nd intended site Client HTTP response: HTML document including ad <img> Web site providing requested content Second Web site providing requested content HTTP request for ad image plus Cookie (identifies user) Image Web site providing banner ads Based on Referer, I know two Web sites that this user has visited Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 86
  87. 87. Cookies Privacy issues Due to privacy concerns, many users block cookies  Blocking may be fine-tuned. Ex: Mozilla allows  Blocking of third-party cookies  Blocking based on on-line privacy policy Alternative to cookies for maintaining session: URL rewriting Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 87
  88. 88. URL Rewriting Tomcat adds session ID within HTML document to all URL’s referring to the servlet Session ID = 4235 Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 88
  89. 89. URL Rewriting Subsequent request will contain session ID in the URL of the request Session ID = 4235 Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 89
  90. 90. URL Rewriting Next response must again add session ID to all URL’s Session ID = 4235 Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 90
  91. 91. URL Rewriting Original (relative) URL: href=“URLEncodedGreeting” URL containing session ID: href=“URLEncodedGreeting;jsessionid=0157B9E85” Path parameter Path parameter is treated differently than query string parameter  Ex: invisible to getParameter() Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 91
  92. 92. URL Rewriting Original servlet HttpServletResponse method encodeURL() will add session id path parameter to argument URL Relative URL of servlet Servlet using URL rewriting Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 92
  93. 93. URL Rewriting Must rewrite every servlet URL in every document Security issues URL with session ID 7152 Web site using URL rewriting User A Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 93
  94. 94. URL Rewriting Must rewrite every servlet URL in every document Security issues URL with session ID 7152 User A Web site using URL rewriting Email URL User B Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 94
  95. 95. URL Rewriting Must rewrite every servlet URL in every document Security issues URL with session ID 7152 User A Web site using URL rewriting Email URL Visit Web site with session ID 7152 User B Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 95
  96. 96. More Servlet Methods Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 96
  97. 97. More Servlet Methods Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 97
  98. 98. More Servlet Methods Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 98
  99. 99. More Servlet Methods Response buffer   All data sent to the PrintWriter object is stored in a buffer When the buffer is full, it is automatically flushed:  Contents are sent to the client (preceded by header fields, if this is the first flush)  Buffer becomes empty  Note that all header fields must be defined before the first buffer flush Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 99
  100. 100. More Servlet Methods Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 100
  101. 101. More Servlet Methods In addition to doGet() and doPost(), servlets have methods corresponding to other HTTP request methods    doHead(): automatically defined if doGet() is overridden doOptions(), doTrace(): useful default methods provided doDelete(), doPut(): override to support these methods Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 101
  102. 102. Data Storage Almost all web applications (servlets or related dynamic web server software) store and retrieve data    Typical web app uses a data base management system (DBMS) Another option is to use the file system Not web technologies, so beyond our scope Some Java data storage details provided in Appendices B (file system) and C (DBMS) One common problem: concurrency Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 102
  103. 103. Concurrency Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 103
  104. 104. Concurrency Tomcat creates a separate thread for each HTTP request Java thread state saved:    Which statement to be executed next The call stack: where the current method will return to, where that method will return to, etc. plus parameter values for each method The values of local variables for all methods on the call stack Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 104
  105. 105. Concurrency Some examples of values that are not saved when a thread is suspended:    Values of instance variables (variables declared outside of methods) Values of class variables (variables declared as static outside of methods) Contents of files and other external resources Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 105
  106. 106. Concurrency // Output HTML document Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 106
  107. 107. Concurrency Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 107
  108. 108. Concurrency Java support thread synchronization Only one thread at at time can call doGet()  Only one synchronized method within a class can be called at any one time Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 108
  109. 109. Concurrency Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 109
  110. 110. Concurrency Web application with multiple servlet classes and shared resource: Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 110
  111. 111. Concurrency Solution: create a shared class with synchronized static methods called by both servlets CounterReader readAndReset() CounterFile incr() CounterWriter File Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 111
  112. 112. Common Gateway Interface CGI was the earliest standard technology used for dynamic server-side content CGI basics:   HTTP request information is stored in environment variables (e.g., QUERY_STRING, REQUEST_METHOD, HTTP_USER_AGENT) Program is executed, output is returned in HTTP response Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 112
  113. 113. Common Gateway Interface Advantage:  Program can be written in any programming language (Perl frequently used) Disadvantages:   No standard for concepts such as session May be slower (programs normally run in separate processes, not server process) Guy-Vincent Jourdan :: CSI 3140 :: based on Jeffrey C. Jackson’s slides 113

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