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CGI programming. Common Gateway Interface interface between web server and other programs (cgi scripts) information passed as environment variables passed.

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Presentation on theme: "CGI programming. Common Gateway Interface interface between web server and other programs (cgi scripts) information passed as environment variables passed."— Presentation transcript:

1 CGI programming

2 Common Gateway Interface interface between web server and other programs (cgi scripts) information passed as environment variables passed to standard input (STDIN) script outputs to standard output (STDOUT) output is http response message

3 CGI Environment Web Server defines –working directory –preset variables –filehandles (links to resources on the server) CGI script must produce –minimal set of response headers e.g. Content-Type: text/html –content of http response

4 Environment Variables provide info about the web server and the client information drawn from http request headers SERVER_NAME REMOTE_ADDR CONTENT_LENGTH CONTENT_TYPE

5 Server-Script interface STDIN –Web server launches CGI program and provides standard input STDOUT –CGI program outputs response to web server STDERR –Web server handles CGI program error output –Apache appends it to error log

6 CGI Output headers: –Content-Type print Content-Type:text/html\n\n; –Location print Location:someFile.html\n\n; –Status print 503 Service unavailable;

7 CGI Example


9 Ice Cream Stand Design Browser Web Server CGI Script Present order form and response Handle request and response Produce order form Process order form

10 CGI script design Input –Form data Output –Order form –Order response Self-referencing form

11 ice cream stand CGI script


13 CGI is programmer-oriented HTML embedded in the program HTML generated as a series of function calls requires –knowledge of HTML tags –programming skills

14 Does CGI implement M-V-C? No! Data processing (model) is inseparable from response page generation (view) Also contains elements of controller –Handles request headers and creates response headers

15 CGI security problems scripts can be corrupted by user data –hidden fields –arbitrary commands embedded in text fields file permissions file locations trust relationships between web server and other machines

16 speed of CGI each request creates a new process overhead of communication through CGI overhead of interpretation and compilation Possible solutions (only partly effective) –code optimisation –Fast CGI –mod_perl with Apache

17 Alternatives to CGI Java servlets JSP - Java Server Pages PHP ASP - Active Server Pages Coldfusion

18 Java Servlets

19 Servlets add functionality to a web server comparable to CGI –More tightly defined –Separate http handling from middleware –Deployed in a web container (see later) vendor and platform independent (Java) integrate with other Java technologies –J2EE framework

20 Servlets efficient –permanently available, no compile overhead robust –encapsulation, type-checking, error handling secure –specialised interfaces to other server resources that are not vulnerable to attack

21 Servlets implement javax.servlet.Servlet interface public void init(ServletConfig c) run on initialisation public void service (ServletRequest req, ServletResponse res) runs for each request and response public void destroy () end of servlet life

22 Web Server Servlet Class init(ServletConfig c) service(ServletRequest r, ServletResponse s) destroy() once at first request or at server start every request once when server shuts down webcontainer

23 HTTP servlets Most commonly used servlet subclass –javax.servlet.http.HttpServlet implements additional methods to handle http functionality service() method passes handling to more specific sub-class methods –doGet, doPost …

24 The Hello World servlet import*; import javax.servlet.*; import javax.servlet.http.*; public class HelloWorld extends HttpServlet{

25 The Hello World servlet public void doGet (HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { res.setContentType(text/html); Printwriter out = res.getWriter();

26 The Hello World servlet out.println ( ); out.println (Hello World); out.println ( ); out.println ( Hello World ); out.println ( ); }

27 Servlets vs CGI similar idea –web container like CGI environment –request and response objects vs std I/O servlet compilation once only –much faster, even though run in JVM security problems greatly reduced –web container is much more secure but still HTML embedded in code

28 Java Server Pages

29 Java Server Pages (JSP) Template for page generation Separates code from HTML HTML with additional jsp tags processed on server side links to other Java entities for more complex processing/ database access platform independent

30 JSP elements A JSP is a template for generating a web page –Response to an http request JSP elements are tags embedded in HTML JSP scripting elements –Specify Java code to be run when template is requested –Separate the coding from HTML content Fits with M-V-C philosophy

31 JSP Digital Clock Date and Time Simple JSP Example


33 JSP scripting elements Three different kinds of scripting, determining when each is executed: Insert snippets of Java code embed a code expression, which evaluates in the response (no ;) declare variables and methods

34 Examples

35 result

36 JSP and Servelets

37 How does JSP work? NOT a Java scripting language NOT like php –JSP are NOT parsed on request Java code must involve classes, creation of objects, etc… JSP is a designer-friendly way of writing servlets

38 Clock example

39 Server with Tomcat Web Container client translation request processing GET clock.jsp 1 clock.jsp read 2 Servelet generate 3 clock.class compile and deploy 4 execute 5 http response 6

40 public class clock implements Servlet { public void service (ServletRequest r, ServletResponse s) throws ServletException, IOException { s.setContentType (text/html); PrintWriter out = s.getWriter (); out.println ( ); out.println ( JSP… ); out.println ( );

41 out.println ( Date and Time ); out.println (new java.util.Date.toString()); out.println ( ); }

42 JSP directive elements applied when the JSP is compiled into a servelet –Only executed once (on compilation) –Do not affect the response Used to set up resources such as –Java classes –inclusions

43 JSP directive elements specify page information (static) scripting language, error page includes a file, e.g. an applet declare a tag library (custom actions)

44 JSP and http

45 A JSP is a servelet Permanently resident in server memory Multi-threaded Request and response objects Sessions and cookies

46 Accessing request information Methods of the request object provide all request information object is called request public String getParameter (String name) public String getMethod () public String getHeader (String name) public Cookie [] getCookies ()

47 javax.servelet.http.Cookie class getName () –the name of the cookie getValue(), setValue (String value) –gets/sets the value of a cookie getDomain(), setDomain(String dName) –get/set the cookie domain name getPath(), String setPath(String path) –get/set the request path the cookie is associated with getMaxAge(), setMaxAge (int expiry) –get/set maximum age for the cookie

48 javax.servelet.http.HttpSession provides standard functionality for handling sessions handles cookies as standard but must be extended to handle URL rewriting holds client state info resident in memory –automatically times out abandoned sessions created/returned by HttpServeletRequest class getSession method

49 JSP and Java Beans

50 Java Beans ordinary Java classes with the following properties: –introspection –customization –events –properties –persistence

51 Java Beans introspection –an analyser can inspect how the Bean works properties –naming conventions for getter and setter methods persistence –implement the Serializable interface –Bean state can be stored

52 Example Java bean public class ExampleBean implements { private String name = null; private int score = 0; public ExampleBean() {} // Empty constructor /* Getter and Setter Methods */ public String getName() { return name; } public void setName(String s) { name = s; }

53 Example Java bean public int getScore() { return score; } public void setScore(int i) { score = i; } /* No method required to implement Serializable */ }

54 JSP action elements action elements –perform an action when page is requested uses a JavaBean component property from JavaBean used in the page sets a JavaBean property (possibly using request information)


56 The following information was saved: User Name: Address:

57 Other JSP action elements responses from other jsp pages or servelets forwards processing to other jsp or servelet passes a parameter with include or forward generates the HTML to embed an applet

58 Timetable change From 10 November: Two lectures moved into one slot: –Wednesday 11-1 –B39 –(lab with GE being moved) Labs will still be Thursday, 9-11

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