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Dynamic Web Applications with Servlets

Learn how to generate dynamic responses using servlets in web applications. Understand the servlet life cycle, request handling, and response generation. Manage session and shared states.

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Dynamic Web Applications with Servlets

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  1. Lecture 8 Servlets (Based on Møller and Schwartzbach, 2006, Chapter 9) CIS336Website design, implementation and management(also Semester 2 of CIS219, CIS221 and IT226) David Meredith d.meredith@gold.ac.uk www.titanmusic.com/teaching/cis336-2006-7.html

  2. Web applications and servlets • Last week we saw how to build a simple Web server • Could only generate static pages • This week we look at how to generate responses dynamically using servlets that have been plugged into the server • Servlet framework is a Java-based API for programming Web applications • A servlet is a program written for this API

  3. Web applications: Basic concepts • Web server responds to http requests • Responses can be generated by Web applications • contain programs that are plugged into the Web server • One server typically runs many concurrent threads of each Web application • Each thread handles one HTTP request or one client • A series of response-request interactions between a server and a client is called a session

  4. Sessions • Session is a sequence of related interactions between a client and a server • It is a thread of execution managed by a server • Session thread stops when client logs out or times out after client has stopped issuing requests • HTTP is stateless, so session states have to be managed at a higher level • Three categories of state: • Shared state (global data) • shared between all sessions and stored in a database on the server • Session state (local data) • private to a particular session thread • e.g., • contents of a shopping cart • whether client has logged in • "program pointer" of thread • Transient state • data that is only used in a single interaction

  5. A "Hello World!" Servlet • Servlet can be run with the Apache Tomcat server • Servlet API consists of two packages • javax.servlet • javax.servlet.http • contains functionality specific to HTTP • Servlet is usually a subclass of HttpServlet • doGet method is invoked in response to a GET method • takes two arguments: • HttpServletRequest object holds information about the request • HttpServletResponse object used to generate response

  6. Servlet life cycle • init method used to initialize servlet when it is constructed • e.g., connecting to a database • GET request causes doGet method to be invoked • POST request causes doPost method to be invoked • Variables in doGet and doPost correspond to transient state • i.e., private to session • When server shut down, destroy method invoked on each servlet • getServletInfo method should be implemented to return a short descriptive string • log method writes messages to the log

  7. Requests • HttpServletRequest parameter of doGet and doPost contains information about incoming request • Most important methods of HttpServletRequest are • getHeader • returns the value of the specified header name as a string • e.g., request.getHeader("User-Agent") • getParameter • returns decoded value of a form field as a string • e.g., request.getParameter("name") • returns the value provided by the user to a GUI form element whose name attribute has the value "name" • works for GET and POST requests • works for different encodings in POST requests • provides useful level of abstraction • getInputStream • returns an input stream for accessing HTTP request body • getRemoteHost, getRemoteAddr, getRemotePort • return information about the client end of the TCP connection

  8. Requests • Note use of • getRemoteHost (l.13) • getHeader (l.14) • getParameter (l.17) • POST requests are redirected to doGet • htmlEscape escapes special characters in the value entered in the "name" control • Never assume that values from expected form fields or request header lines are present (see lines 15, 18)

  9. Responses • HttpServletResponse parameter of doGet and doPost used to construct outgoing response • Main methods in HttpServletResponse are • setStatus • sets response status code (default is 200) • addHeader and setHeader • adds or overwrites header fields • getOutputStream • returns output stream for writing response body • getWriter • returns a PrintWriter which wraps the output stream and sends characters rather than bytes • setContentType • sets the Content-Type response header field • sets character encoding for PrintWriter • e.g., response.setContentType("text/xml;charset=UTF-8") • sendError and sendRedirect • simplify construction of error responses and temporary redirects

  10. Servlet contexts and Shared State • Each Web application has a ServletContext object that holds information about the context of a servlet • accessible via Servlet class getServletContext method • ServletContext has getServerInfo method that returns information about the server • e.g., Apache Tomcat/5.0.30 • Each Web application may contain many servlets and other resources (e.g., CSS, GIFs, etc. ) • Each Web application is associated with a context path • Context path is base of URIs for resources in application • Determined by deployment configuration (see later) • ServletContext getRealPath method computes absolute path of resource from context path and relative path of resource • ServletContext can be used to store shared state • setAttribute method binds an object to an attribute name • getAttribute returns object bound to an attribute name • Data can be communicated between different Web applications running on the same server by using the getContext method which returns the ServletContext of a servlet

  11. A Polling Service • This Web application consists of • an HTML file called QuickPollQuestion.html which contains a form for entering the poll question • a servlet called QuickPollSetup for receiving data from the form in QuickPollQuestion.html • a servlet called QuickPollAsk for making an HTML page containing the question and a form which allows the user to vote 'Yes' or 'No' • a servlet called QuickPollVote for receiving a vote • a servlet called QuickPollResults for showing the results

  12. QuickPollQuestion.html • Deployment configuration (see later) defines mapping from URI setup in action attribute of form element to the QuickPollSetup servlet Deploy

  13. QuickPollSetup Servlet • String q set to equal value entered by user into GUI element named "question" in QuickPollQuestion.html • Attributes question, yes and no set in servlet context, c • Message sent back to user to indicate that question has been registered

  14. QuickPollAsk Servlet • Uses question stored in servlet context to make a new HTML form with two radio buttons that allow the user to vote on the question • getAttribute returns an Object which has to be cast to a String

  15. QuickPollVote Servlet • Receives form data from voting form • Increments either yes or no servlet context attributes accordingly • Sends an acknowledgement to the user

  16. QuickPollResults Servlet • Uses tables within tables to draw a barchart that represents the responses to the poll • Set Cache-Control so as to disable caching to ensure that new, up-to-date form is loaded, not cached version • Use POST in cases where servlet has side-effects, GET when it doesn't

  17. HttpSession objects • HttpSession object is an abstraction of a session that hides implementational details of session management • HttpServletRequest has a getSession method which returns the current HttpSession object • Session terminated by • an invocation of the invalidate method • inactivity for a certain time interval, determined by setMaxInactiveInterval method • HttpSession object can be used to store session state using setAttribute and getAttribute methods

  18. A Shopping Cart Servlet (1/2) • cart is stored in a TreeMap that maps items to amounts • cart is stored as an attribute of the session • If the request method is POST, then an item is added to the cart (causes side-effect on server) • Returns 400 Bad Request if cannot parse amount

  19. A Shopping Cart Servlet (2/2) • Prints out form for adding new item • Prints out table listing contents of cart • URLs passed through the encodeURL method to ensure special characters are escaped

  20. Web applications and deployment • In servlets, files always organised in a common directory structure as follows • Assuming context path is myapp, • myapp/ and all subdirectories except WEB-INF contain static resources • e.g., HTML, images, stylesheets, JSP files • myapp/WEB-INF/ contains the deployment descriptor (see later) • myapp/WEB-INF/classes contains all servlet classes and auxiliary classes used by servlets • myapp/WEB-INF/lib contains additional jar files • Class files must be placed in subdirectories that match their package names • e.g., if MyServlet.class belongs to inc.widget package, then it is placed in myapp/WEB-INF/classes/inc/widget/ • An whole Web application can be bundled in a single Web Archive file (.WAR) using the jar tool

  21. Deployment descriptors • Every Web application contains a deployment descriptor • An XML file called web.xml stored in myapp/WEB-INF/ • Configures application by providing • mapping from URI paths to application resources • initialization parameters • error-handling configuration • ...

  22. Example Deployment Descriptor • display-name element provides name for the application • web-app element can contain more than one servlet and servlet-mapping elements • servlet element (servlet declaration) • associates a name with a servlet class • can be many servlet declarations for a given servlet class • each declaration results in an instance of the servlet being constructed • servlet-mapping element associates servlet name with a URI pattern • pattern is relative to context path • e.g., if context path is myapp, then complete path here would be myapp/hello/ and this would cause the HelloWorld servlet to be executed • namespace declaration identifies deployment descriptor language which is defined by an XML Schema definition

  23. Initialization parameters • Context initialization parameters apply to whole application • accessed using getInitParameter method of ServletContext object • in deployment descriptor above, context initialization parameter, admin, is added with the value john.doe@widget.inc • Servlet initialization parameters apply to individual servlet instances • accessed using getInitParameter method of HttpServlet object • in deployment descriptor above, servlet initialization parameter, verbose, added with value true

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