Advance java session 17
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The document discusses implementing filters in Java servlets. Filters can intercept requests and responses and are used for tasks like authentication, compression, encryption, and logging. The key points are that filters implement the Filter interface, are configured in web.xml, can access initialization parameters, and can modify requests and responses by calling the next filter in the chain or omitting that call. Common filter uses include authentication, compression, and transforming content.
Advance java session 17
- 2. Agenda • Exploring the need of filter • Exploring filter API • Creating a Filter • Configuring a Filter • Using initialization parameter in Filters
- 3. The Need for Filter • Servlet Filters are Java classes that can be used in Servlet Programming for the following purposes: - To intercept requests from a client before they access a resource at back end. - To manipulate responses from server before they are sent back to the client.
- 4. Types of Filter suggested • Authentication Filters. • Data compression Filters • Encryption Filters . • Filters that trigger resource access events. • Image Conversion Filters . • Logging and Auditing Filters. • MIME-TYPE Chain Filters. • Tokenizing Filters . • XSL/T Filters That Transform XML Content
- 5. Exploring the Filter API • Defined in javax.servlet.* package - Filter - FilterChain - FilterConfig • We define the filter by implementing the Filter interface • FilterChain is passed to a filter by Container for invoking the series of Filter • FilterConfig contains initialization data if required
- 6. Exploring the Filter interface • Filter interface contains doFilter method which is the heart of filter • It usually performs the following action - Examines the request header - Customizes the request Object (like header,data etc) - Customizes the response object(like header, data etc)
- 7. Creating a Filter • Basic format public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException { … chain.doFilter(request,response); } • Note on first two arguments - They are of type ServletRequest and ServletResponse, not HttpServletRequest and HttpServletResponse. • Do a typecast if you need HTTP-specific capabilities
- 8. Simple Report Filter public class ReportFilter implements Filter { public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws ServletException, IOException { HttpServletRequest req = (HttpServletRequest)request; System.out.println(req.getRemoteHost() +" tried to access " + req.getRequestURL() + " on "+new Date() + "."); chain.doFilter(request,response); } public void init(FilterConfig config) throws ServletException { } public void destroy() {} }
- 9. Registering the Filter in web.xml <web-app> <!-- Register the name "Reporter” for ReportFilter. --> <filter> <filter-name>Reporter</filter-name> <filter-class>moreservlets.filters.ReportFilte</filter-class> </filter> Important note Servers load filters into memory when the Web app first comes up. So, if that filter is not found, your entire Web app is disabled.
- 10. Associating Reporting with given URL’s <!-- Apply Reporter filter to home page. --> <filter-mapping> <filter-name>Reporter</filter-name> <url-pattern>/index.jsp</url-pattern> </filter-mapping> <!-- Also apply Reporter filter to servlet named "TodaysSpecial". --> <filter-mapping> <filter-name>Reporter</filter-name> <servlet-name>TodaysSpecial</servlet-name> </filter-mapping> </web-app>
- 11. Using initialization Parameter for Filter • Filter can be configured to use initialization parameters like a normal servlet like - <init-param> - <context-param> • <init-param> - Use FilterConfig inside init method to read the parameters • <context-param> - Using FilterConfig need to obtain ServletContext. - Read the parameter from the ServletContext thus obtained
- 12. Advantages of Filter • Encapsulate common behavior. - Have 30 different servlets or JSP pages that need to compress their content to decrease download time? Make 1 compression filter and apply it to all 30 resources. • Separate high-level access decisions from presentation code. - Want to block access from certain sites without modifying the individual pages to which these access restrictions apply? Create an access restriction filter and apply it to as many pages as you like. • Apply wholesale changes to many different resources. - Have a bunch of existing resources that should remain unchanged except that the company name should be changed? Make a string replacement filter and apply it wherever appropriate.
- 13. Summary • Implement the Filter interface • Override doFilter, init, and destroy - init and destroy are often empty • Declare filter in web.xml - Give it a name and designate URLs to which it applies • Accessing the servlet context - Look it up in init, store it in a field • Filters have init parameters • Blocking resources - Simply omit call to FilterChain.doFilter • Modifying response - Pass a wrapper to resource, invoke resource, extract output from wrapper, modify it, pass it to client..