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Servlet

Servlet. How can a HTML page, displayed using a browser, cause a program on a server to be executed?. Server side processing. Traditionally us e the CGI (Common Gateway Interface) mechanism or a server side scripting language.

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Servlet

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  1. Servlet • How can a HTML page, displayed using a browser, cause a program on a server to be executed?

  2. Server side processing • Traditionally use the CGI (Common Gateway Interface) mechanism or a server side scripting language. • CGI - calls an executable program on the server which takes information passed to it from the browser, processes it and returns a result (which can take the form of HTML sent to the browser). Common languages used to write programs called via CGI are: PERL, C, C++. • Server Side Scripting/ Includes - takes the request for the page, runs the page (which contains embedded instructions) through a program attached to the server to fill in the ‘blanks’ (the scripting instructions) and sends the completed page back to the browser. Common languages: asp, php, shtml, jsp.

  3. Servlets • There is a way for a Java program to respond to a call from a HTML page. This also solves two of the major problems associated with client side Java namely: • Not everyone is happy to download Java applets as it takes more time than a simple HTML page and some users feel that applets can carry a virus (an urban myth). • Not all browsers have VMs attached. If they do the VM may not be up to date (for example it may not be able to use SWING UIs). Microsoft introduced a policy of not packaging a VM with their browser (unlike AOL/ Netscape) to ‘discourage’ java.

  4. Servlets • A Browser can cause a Java program to be executed on a server if that Java program is written in the form of a ‘Servlet’. • A Servlet does not have a conventional GUI (like an applet) but sends output to and receives results from a browser • The Browser calls or starts the Servlet via a web server which is configured to know when a web server is required to perform a task. Recognition is usually via part of the URL (/servlet/,/jsp/,/jsv/)

  5. Servlet execution steps Step 1: User gives URL to browser http://www.ucc.ie/servlet/MyServlet

  6. Step 2: WebServer realises URL is not an ordinary file request www.ucc.ie /servlet/MyServlet

  7. Step 3: WebServer starts VM which loads Servlet class file MyServlet.class VM

  8. Step 4: Servlet object created, runs, sends output back to browser via WebServer html html VM MyServlet object

  9. Servlet Entry Points • Two methods: • doGet – called if browser submitted a ‘get information’ request to the web server (e.g. the user typed in a URL) • doPost – called if the browser submitted data to the web server, for example sent it the results of a HTML form. • Note that both methods are protected and ‘throw’ two exceptions

  10. How does a servlet communicate with the browser • Two objects are passed to the relevant method in the servlet: • HttpServletRequest – object contains information sent to the servlet from the browser • HttpServletResponse – object to facilitate transmission of information back to browser

  11. Using the response object you can • Redirect the browser (the redirection does not have to be to a particular type of web resource) response.sendRedirect("http://www.ucc.ie/php/go.php?flag=true"); • Open a connection to the browser for transmission of html response.setContentType("text/html"); // Tell the browser whats coming java.io.PrintWriter out = response.getWriter(); // Connect to the browser • Once a connection is open and the browser is aware of what is being transmitted, can send html to the browser for rendering. out.println("<html>");

  12. Writing a Servlet class • Have the class extend HttpServlet • Import javax.servlet.*, javax.servlet.http.* and java.io.* • Write an init method which takes a parameter of type ServletConfig. This init method is executed only once when the servlet starts (Question: Why is there an x in javax.servlet?)

  13. Servlet characteristics • When a web server causes a servlet to start, a new unique servlet object is automatically created (like an applet) • A servlet object has all the security clearance of an application therefore it can communicate with other computers, access databases, create/ alter files, start programs written in other languages (via RunTime object), etc. THIS makes servlets unpopular with sysadmins and allows languages like php to flourish on basic ISPs • When the servlet object is finished processing it is not destroyed. It remains alive so the web server does not have to wait for a new servlet object to be created and initialised when a new user sends a request to the server. • If more that one request is received for a servlet then the browser queues the eldest request. Queueung and loadbalancing can pose major problems for popular servlets.

  14. Dynamic Server Reloading • NOTE the above, if you change and recompile a servlet in an IDE or a deployment environment you must start and stop the web server so that it erases the old servlet object and picks up the new code. • The exception to this rule is if your IDE or deployment environment (web server) is told to scan servlet directories for new code then they automatically destroy the old objects and load the new ones. You must never assume this happens automatically – always check dynamic checking is switched on

  15. How can many different browsers use the same servlet object? • The web server associates a session id with each browser. The session id is available to the servlet object so that it can tell if a visiting browser has used the servlet object during the lifetime of this servlet object (so for example it can maintain a shopping cart). • One or more session objects can be associated with each session id so data can be kept regarding each session. This data is usually discarded when the session is finished. Though you can specify that data is to be saved in a destroy method.

  16. Session - Using • The command HttpSession session = request.getSession(true); will create an object called session which identifies each individual browser. • To tell if a browser has visited the servlet before use: session.isNew() which returns true if the visit is the first one.

  17. Session - Using • To store information for a particular session: session.setAttribute(tag,object); • Where a tag (key) is a String and object is a reference to an object (often a String). Example session.setAttribute(“username”,”jim”);

  18. Session - Using • To obtain information associated with a browser: session.getAttribute(tag) • Which will return the reference to the object stored (i.e. an address of type Object). Therefore the result of the call must be cast. Example String result = (String) session.getAttribute("username"); or String result = “”+ session.getAttribute("username"); This returns the String “jim”

  19. Example • servletSession.java

  20. HTML Forms • A user can fill in an HTML form on a browser. • The browser can send the HTML form to a servlet which can extract the contents of the form.

  21. HTML Forms (Hard Wired) • The easiest way to prepare a form is to use a HTML editor (e.g. FrontPage) and copy and paste the output into the servlet. • You must make sure the action associated with a form is the servlet: out.println("<FORM ACTION=\""+servletURL+"\" METHOD=\"POST\">"); • You can have the servlet automatically fill in its URL: out.println("<FORM ACTION=\""+ request.getRequestURI() +"\" METHOD=\"POST\">");

  22. HTML Forms (Hard Wired) • Each form element should be named: out.println("<input type=\"submit\" value=\"buttonName\" name=\""+buttonName+"\">"); • The contents of the form element can then be extracted by the servlet using the elements name: String val=request.getParameter(“username") If (val==null) val= “”; • If the element is empty then this returns null

  23. Example • processFormServlet.java

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