Java Servlet Technology

1. Java Servlet Technology

2. Introduction • Servlets are Java programs that run on a Web server, handle HTTP requests and build Web pages • Servlet specification versions • [version 1.0] June 1997 • [version 2.4]November 2003 • [version 2.5]September 2005

3. J2EE architecture Distributed component-based multi-tier enterprise application model

4. Servlets in J2EE Java Servlet - a Java program that extends the functionality of a Web server, generating dynamic content and interacting with Web clients using a request-response paradigm

5. What is Servlet? • Java™ objects which are based on servletframework and APIs and extend thefunctionality of a HTTP server • Mapped to URLs and managed bycontainer with a simple architecture • Available and running on all majorweb servers and application servers • Platform and server independent

6. A Servlet That Generates Plain Text import javax.servlet.*; import javax.servlet.http.*; import java.io.*; public class HelloServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponseresponse) throws ServletException, IOException { PrintWriter out = response.getWriter(); out.println("Hello World!“); } ... }

7. A Servlet That Generates HTML import javax.servlet.*; import javax.servlet.http.*; import java.io.*; public class HelloServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponseresponse) throws ServletException, IOException { response.setContentType("text/html“); PrintWriter out = response.getWriter(); out.println("<title>Hello World!</title>“); } ... }

8. Mapping URLs to Servlets • When a request is received by the web container it must determine which web component should handle the request • Need to add a servlet definition and a servlet mapping for each servlet to web.xml file <servlet> <servlet-name>HelloServlet</servlet-name> <servlet-class>com.servlet.HelloServlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>HelloServlet</servlet-name> <url-pattern>/hello</url-pattern> </servlet-mapping>

9. URL patterns • Four types of URL patterns • Exact match:<url-pattern>/dir1/dir2/name</url-pattern> • Path match:<url-pattern>/dir1/dir2/*</url-pattern> • Extension match:<url-pattern>*.ext</url-pattern> • Default resource:<url-pattern>/</url-pattern>

10. Servlets and JSP - Comparison JSP is a text-based document capable of returning dynamic content to a client browser Servlets typically are used for returning non-HTML data

11. Servlet Request & Response Model

12. What Does Servlet Do? • Receives client request (mostly in the form ofHTTP request) • Extract some information from the request • Do content generation or business logic process(possibly by accessing database, invoking EJBs,etc) • Create and send response to client (mostly in theform of HTTP response) or forward the request toanother servlet or JSP page

13. Requests and Responses • What is a request? • Information that is sent from client to a server • Who made the request • What user-entered data is sent • Which HTTP headers are sent • What is a response? • Information that is sent to client from a server • Text (html, plain) or binary (image) data • HTTP headers, cookies, etc

14. Servlet Interfaces & Classes interfaces classes

15. Servlet Life Cycle • The life cycle of a servlet is controlled by the container • When a HTTP request is mapped to a servlet, the container performs the following steps • If an instance of the servlet does not exist, the web container • Loads the servlet class • Creates an instance of the servlet class • Initializes the servlet instance by calling the init method • Invokes the service method, passing request and response objects

16. Servlet Life Cycle Methods

17. Servlet Life Cycle Methods • Invoked by container • container controls life cycle of a servlet • Defined in • javax.servlet.GenericServlet class • init() • destroy() • service() - this is an abstract method • javax.servlet.http.HttpServlet class • doGet(), doPost(), doXxx() • service() - implementation

18. init() & destroy() • init() • Invoked once when the servlet is first instantiated • Not called for each request • Perform any set-up in this method • setting up a database connection • destroy() • Invoked before servlet instance is removed • Not called after each request • Perform any clean-up • closing a previously created database connection

19. Example: init() public class CatalogServlet extends HttpServlet { private BookDB bookDB; // Perform any one-time operation for the servlet, // like getting database connection object. // Note: In this example, database connection object is assumed // to be created via other means (via life cycle event // mechanism) // and saved in ServletContext object. This is to share a same // database connection object among multiple servlets public void init() throws ServletException { bookDB = (BookDB)getServletContext(). getAttribute("bookDB"); if (bookDB == null) throw new UnavailableException("Couldn't get database"); } ... }

20. Example: destroy() public class CatalogServlet extends HttpServlet { private BookDB bookDB; public void init() throws ServletException { bookDB = (BookDB)getServletContext(). getAttribute("bookDB"); if (bookDB == null) throw new UnavailableException( "Couldn't get database"); } public void destroy() { bookDB = null; } ... }

21. service() • Is called in a new thread by server for each request • Dispatches to doGet, doPost, etc • service() methods take generic requests and responses: service(ServletRequest request, ServletResponse response) • Do not override this method!

22. doGet, doPost, doXxx • Handles GET, POST and other requests • Override these to provide desired behavior

23. Things to do in doGet() & doPost() • Extract client-sent information (HTTP parameter) from HTTP request • Set (save) and get (read) attributes to/from Scope objects • Perform some business logic or access database • Optionally forward the request to other Web components (Servlet or JSP) • Populate HTTP response message and send it to client

24. Steps of Populating HTTP Response • Fill Response headers • Set some properties of the response • Buffer size • Get an output stream object from the response • Write body content to the output stream

25. Example: Simple Response public class HelloServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Fill response headers response.setContentType("text/html"); // Set buffer size response.setBufferSize(8192); // Get an output stream object from the response PrintWriter out = response.getWriter(); // Write body content to output stream out.println("<title>First Servlet</title>"); out.println("<big>Hello J2EE Programmers!</big>") } }

26. Scope Objects • Enables sharing information among collaborating web components via attributes maintained in Scope objects • Attributes are name/object pairs • Attributes maintained in the Scope objects are accessed with • getAttribute() & setAttribute() • 4 Scope objects are defined • Web context, session, request, page

27. Four Scope Objects: Accessibility • Web context • Accessible from Web components within a Web context • Session • Accessible from Web components handling a request that belongs to the session • Request • Accessible from Web components handling the request • Page • Accessible from JSP page that creates the object

28. Four Scope Objects: Classes • Web context javax.servlet.ServletContext • Session javax.servlet.http.HttpSession • Request javax.servlet.http.HttpServletRequest • Page javax.servlet.jsp.PageContext

29. WebContext (ServletContext) • Used by servets to • Set and get context-wide (application-wide)object-valuedattributes • Get request dispatcher • To forward to or include web component • Access Web context-wide initialization parametersset in the web.xml file • Access Web resources associated with the Webcontext • Log • Access other misc information

30. Scope of ServletContext • There is one ServletContext object per "web application" per Java Virtual Machine

31. How to Access ServletContext? • Within your servlet code, call getServletContext() public class CatalogServlet extends HttpServlet { private BookDB bookDB; public void init() throws ServletException { // Get context-wide attribute value from // ServletContext object bookDB = (BookDB)getServletContext(). getAttribute("bookDB"); if (bookDB == null) throw newUnavailableException( "Couldn't get database"); }

32. Dispatching to Another Component public void doGet (HttpServletRequest request,HttpServletResponseresponse) throws ServletException, IOException { HttpSession session = request.getSession(true); ResourceBundle messages = (ResourceBundle)session.getAttribute("messages"); // set headers and buffer size before accessing the Writer response.setContentType("text/html"); response.setBufferSize(8192); PrintWriter out = response.getWriter(); // then write the response out.println("<html>head><title>" + messages.getString("TitleBookDescription") +"</title></head></html>"); // Get the dispatcher; it gets the banner to the user RequestDispatcher dispatcher = getServletContext().getRequestDispatcher("/banner"); if (dispatcher != null) dispatcher.include(request, response); ...

33. Logging public void doGet (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ... getServletContext().log(“Life is good!”); ... getServletContext().log(“Life is bad!”, someException);

34. Session Tracking • Why session tracking? • User registration • Web page settings • On-line shopping cart • HTTP is stateless! • Have to use specific methods: • Cookies • Hidden Form Fields • URL Rewriting

35. Cookies • Idea: associate cookie with data on server String sessionID = makeUniqueString(); HashMap sessionInfo = new HashMap(); HashMap globalTable = findTableStoringSessions(); globalTable.put(sessionID, sessionInfo); Cookie sessionCookie = new Cookie("JSESSIONID", sessionID); sessionCookie.setPath("/"); response.addCookie(sessionCookie); • Still to be done: • Extracting cookie that stores session identifier • Setting appropriate expiration time for cookie • Associating the hash tables with each request • Generating the unique session identifiers

36. Hidden Form Fields • Idea: <INPUT TYPE="HIDDEN" NAME="session" VALUE="..."> • Advantage • Works even if cookies are disabled or unsupported • Disadvantages • Lots of tedious processing • All pages must be the result of form submissions

37. URL Rewriting • Idea • Client appends some extra data on the end of each URL that identifies the session • Server associates that identifier with data it has stored about that session • E.g., http://host/path/file.html;jsessionid=1234 • Advantage • Works even if cookies are disabled or unsupported • Disadvantages • Must encode all URLs that refer to your own site • All pages must be dynamically generated • Fails for bookmarks and links from other sites

38. The Session Tracking API • Session tracking API built on top of URL rewriting or cookies • Look up HttpSession object associated with current request (or create new one) • All cookie/URL rewriting mechanics hidden • Look up information associated with a session • Associate information with a session

39. Example: Looking Up Session HttpSession session = request.getSession(true); ShoppingCart sc = (ShoppingCart) session.getAttribute("shoppingCart"); if (cart == null) { cart = new ShoppingCart(); session.setAttribute("shoppingCart", cart); } ... // do something with your shopping cart object

40. HttpSession Methods • getAttribute(String name) • Extracts a previously stored value from a session object. Returns null if no value is associated with given name. • setAttribute(name, value) • Associates a value with a name. Monitor changes: values implement HttpSessionBindingListener. • removeAttribute(name) • Removes values associated with name • getAttributeNames() • Returns names of all attributes in the session • getId() • Returns the unique identifier

41. HttpSession Methods • isNew() • Determines if session is new to client (not to page) • getCreationTime() • Returns time at which session was first created • getLastAccessedTime() • Returns time at which session was last sent from client • getMaxInactiveInterval, setMaxInactiveInterval • Gets or sets the amount of time session should go withoutaccess before being invalidated • invalidate() • Invalidates current session

42. Getting Request Parameters • A request can come with any number ofparameters • Parameters are sent from HTML forms: • GET: as a query string, appended to a URL • POST: as encoded POST data, not appeared in the URL • getParameter("paramName”) • Returns the value of paramName • Returns null if no such parameter is present • Works identically for GET and POST requests

43. Getting Information From Response • Client information • String request.getRemoteAddr() • String request.getRemoteHost() • Server information • String request.getServerName() • int request.getServerPort() • Misc • ServletInputStream getInputStream() • BufferedReader getReader() • String getProtocol() • String getContentType() • boolean isSecure()

44. Servlet 2.4 vs Servlet 2.5 • A new dependency on J2SE 5.0 • Support for annotations • @Resource, @PostConstruct and @PreDestroy, @EJB, @WebServiceRef, @DeclareRoles, @RunAs • Several web.xml conveniences • Servlet name wildcarding • Multiple patterns in mappings • A handful of removed restrictions • Error handling • Session tracking • Some edge case clarifications

45. References • Java Servlet Specification 2.4http://jcp.org/aboutJava/communityprocess/final/jsr154/index.html • J2EE Tutorial “Java Servlet Technology” http://java.sun.com/javaee/5/docs/tutorial/doc/bnafd.html • New features added to Servlet 2.5 http://www.javaworld.com/javaworld/jw-01-2006/jw-0102-servlet.html?page=3 • Servlet Basics presentation http://www.javapassion.com/j2ee/ServletBasics_speakernoted.pdf