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1. Filters, Listeners, Wrappers, Internationalization. By, Susnatsahu

2. Filters Filters concept has introduced in Servlet 2.3 version. A filter is a program that runs on the server before the Servlet or JSP page with which it is associated. A filter can be attached to one or more Servlet or JSP pages and can examine the request information going into these resources. Filters are used to preprocess the Request and post process the Response.

4. Logging,

5. Authorization,

7. Encryption of the response.

9. Session validation

10. Internationalization

11. Data compression

13. The filter class encapsulates the logic that has to be executed before or after the actual request processing, which is done by the requested resources.

14. The filter class is declared in the deployment descriptor.

16. The Filter Life Cycle

18. javax.servlet.FilterConfig

19. javax.servlet.FilterChainjavax.servlet.Filter Every Filter class must implement Filter interface either directly or indirectly.

21. The Servlet container calls the init method exactly once after instantiating the filter.

22. The web container cannot place the filter into service if the init method either1. Throws a ServletException 2. Does not return within a time period defined by the web container

23. void doFilter (ServletRequestrequest, ServletResponse response, FilterChain chain) Encapsulates service logic to be implemented on ServletRequest to generate the ServletResponse. The FilterChain referncepassed as an argument to forwardrequest/response pair to the filter or targetresource of chain. void destroy( ) Called by the web container to indicate to a filter that it is being taken out of service.

25. Used during initialization of filter.

26. This object is used to fetch configuration information specified in web.xml

27. String getFilterName() Returns the filter-name of this filter as defined in the deployment descriptor.

30. How to apply a filter – In web.xml Or <url-pattern>*</url-pattern> (in 2.5) <servlet-name>/---Action</servlet-name> <filter> <filter-name>LogFilter</filter-name> <filter-class>LogFilter</filter-class> <init-param> <param-name>uname</param-name> <param-value>scott</param-value> </init-param> </filter> <filter-mapping> <filter-name>LogFilter</filter-name> <url-pattern>/####.jsp</url-pattern> </filter-mapping>

31. What are the possible ways for a Servlet to get a request ? Direct end-user request (REQUEST) (2.3 ver) By RequestDispatcher forward (FORWARD) By RequestDispatcher include (INCLUDE) By error page call (ERROR) Java servlet 2.4 specification sun introduced <dispatcher> tag to extend filter concept for last 3 calls also.

32. REQUEST – This is default value. Filter will only execute only on end user request. FORWARD - Filter will only execute for RequestDispatcher forward calls. <filter-mapping> <filter-name>LogFilter</filter-name> <url-pattern>*</url-pattern> <dispatcher>REQUEST</dispatcher> <dispatcher>FORWARD</dispatcher> </filter-mapping>

33. INCLUDE – Filter will only execute for RequestDispatcher include calls. ERROR - Filter will only execute for error page calls. <filter-mapping> <filter-name>LogFilter</filter-name> <url-pattern>/report.jsp</url-pattern> <dispatcher>ERROR</dispatcher> </filter-mapping> <error-page> <exception-type>java.lang.ArithmeticException</exception-type> <location>/index.jsp</location> </error-page>

34. In the context of web application there may be chances of occurring several events like – Request object creation Session object destruction Context object creation Adding attribute in request scope Removing attribute from application scope etc.. Listeners The servlet specification includes the capability to track key events in your Web applications through event listeners.

38. HttpSessionAttributeListener

39. HttpSessionBindingListener

40. HttpSessionActivationListenerServletRequestListener : This listener listens lifecycle events of request object. It has two methods. void requestInitialized(ServletRequestEvent rre) : This method will executed automatically at the time of request object creation i.e. before starting service method

41. void requestDestroyed(ServletRequestEvent rre) This method will executed automatically at the time of request object destroyed. ServletRequestAttributeListener : This listener listens events related to request scoped attributes. It has 3 methods. void attributeAdded(ServletRequestAttributeEvent srae) void attributeRemoved(ServletRequestAttributeEvent srae) void attributeReplaced(ServletRequestAttributeEvent srae)

43. HttpSessionListener : Related to life cycle of the session objects. void sessionCreated(HttpSessionEvent se) void sessionDestroyed(HttpSessionEvent se)

44. public class Log4jListener implements ServletContextListener { public void contextDestroyed(ServletContextEvent arg0) { } public void contextInitialized(ServletContextEvent event) { String drive=System.getenv("HOMEDRIVE"); String separator=System.getProperty("file.separator"); String logfolder =drive+separator+"MyApps"+ separator+"logs"+ separator; File file=new File(logfolder); if(!file.exists()) file.mkdirs(); }} <servlet> <servlet-name>Log4jAction</servlet-name> <servlet-class>com.eias.logging.Log4jAction </servlet-class> <init-param> <param-name>log4j-properties-location</param-name> <param-value>WEB-INFroperties og4j.properties </param-value> </init-param> <load-on-startup>1</load-on-startup> </servlet> <listener> <listener-class>com.eias.logging.Log4jListener</listener-class> </listener>

45. wrapper Whenever you want to create a custom request or response object, just subclass one of the convinces request or response “wrapper” classes. A wrapper wraps the real request or response object, and passes through calls to the real thing, while still you do the extra things you need for your custom request or response. The benefit of wrapping comes from custom coding a particular wrapping object to manipulate a request or response in a way not normally done.

46. What are the core objects of servlet API ? ServletRequest ServletResponse HttpServletRequest HttpServletResponse

48. getRequest( )

49. getParameter( )

50. getParameterValues( )

51. setAttribute( )

53. Import javax.servlet.*; Public class MyRequestWrapper extends ServletRequestWrapper{ Public MyRequestWrapper (ServletRequestreq) { Super(req); } public String getParameter(String S) { String s1=super.getParameter(s); if(s1==null) s1=“ NONE ”; return s1; } }

54. Import javax.servlet.*; Public class MyWrapperFilter implements Filter { Public void init(FilterConfigfc) { } ; Public void destroy() { } ; Public void doFilter(ServletRequestreq, ServletResponse res, FilterChain chain) throws SE, IO … { MyRequestWrapper wrapperReq= new MyRequestWrapper (); Chian.doFilter(wrapperReq, res); } } Wrapped request object

55. Internationalization : It can be defined as the process of enabling a web application to present different country and region specific formats without making any changes to the code or recompiling the application. This term generally abbreviated as i18n. Localization : This is a process of of customizing a software or web apps for a specific region or language. Known as l10n

56. locale: This is a particular cultural or geographical region. It is usually referred to as a language symbol followed by a country symbol which are separated by an underscore. For example "en_US" represents English locale for US. java.util.Locale class is used while creating international java application. It is a non-abstract final class. A Locale object represents a specific geographical, language, or cultural region locale. These locale affect language choice, collation, calendar usage, date and time formats number and currency formats etc..

57. Create a Locale object using the constructors in this class: Locale(String language) Locale(String language, String country) Locale(String language, String country, String variant) Ex – Locale myLocale=new Locale(“en”,”US”); String getCountry()This method returns the country/region code in upper case for this locale in ISO 3166 2-letter format.

58. String getLanguage()This method returns the language code in lower case for this locale in ISO 639 format. String getVariant() Returns the variant code for this locale.A ResourceBundle is like a specialized form of a Hashtable that maps strings to values. the magic of ResourceBundle allows you to have different lookup tables based upon what Locale (language/country) a user is coming in from.

59. ResourceBundle class This class is used to separate localizable elements such as button labels , error messages, and headings from rest of the application. ResourceBundle "knows" how to search the hierarchy for a locale-appropriate instance, getBundle(String baseName) getBundle(String baseName, Locale locale) This is a abstract class. Two direct subclasses are : PropertyResourceBundle - commonly used. ListResourceBundle

61. Numbersrb_ge.properties

62. Numbersrb_fr.properties# Default properties in English 0=Zero: 1=One: 2=Two: 3=Three: 4=Four: 5=Five: 6=Six: 7=Seven: 8=Eight: 9=Nine: 10=Ten: # Default properties in German 0=Null: 1=Eins: 2=Zwei: 3=Drei: 4=Vier: 5=Fünf: 6=Sechs: 7=Sieben: 8=Acht: 9=Neun: 10=Zehn: # Default properties in French 0=Z: 1=Uun: 2=deux: 3=Trois: 4=Quatre: 5=cinq: 6=Six: 7=Sept: 8=Huit: 9=Neuf: 10=Dix: random=au

63. <key>=<value> in a property file having new line char as separator. Message.prop HELLO_MESSAGE= Hello, How r u ? Message_ge.prop HELLO_MESSAGE= Hallo, Wie geht es Ihnen?? Message_sp.prop HELLO_MESSAGE= Hola, ¿Cómoestás?

64. How to find out the user’s lang. preference. 2 ways allow the user to choose language. Get it as request parameter. It can use locale preferences that are sent from the client to the server using HTTP request header field Accept-Language. Use getLocale() . Implementation in application : Approach 1 Provide a version of JSPs page in each of the target locales and have the controller servlet dispatch the request to appropriate page. Choose when large amount data is available on page. Diadvantage is to create jsp pages for each locale.

65. Approach 2 Isolate the locale sensitive data on a page into resource bundles. (error message, labels, button values etc..) Can be automatically retrieved from property file display in jsp. Choose when less content. Generally used. example.jsp <%@page import=“java.util.*”%> <% ResourceBundle rb= (ResourceBundle) request.getAttribute(“resourse”); %> <%= rb.getString(“HELLO_MESSAGE”) %>

66. Public class I18NServlet extends HttpServlet { Public void service( req, response) throws SE, IO { String cc=req,getParameter(“country”) ; String lang=req,getParameter(“language”) ; Locale l=null; if(cc==null) l=new Locale(ln); else l=new Locale(ln,cc); ResourceBundle rb= ResourceBundle.getBundle(“AppsResourse”,l); req.setAttribute(“resource”,rb); req.getRequestDispatcher(“example.jsp”).forward(req,res); } }