AJP. CHAPTER 5: SERVLET -20 marks

Transcription

1 1) Draw and explain the life cycle of servlet. (Explanation 3 Marks, Diagram -1 Marks) AJP CHAPTER 5: SERVLET -20 marks Ans : Three methods are central to the life cycle of a servlet. These are init( ), service( ), and destroy( ). They are implemented by every servlet and are invoked at specific times by the server. Let us consider a typical user scenario to understand when these methods are called. First, assume that a user enters a Uniform Resource Locator (URL) to a web browser. The browser then generates an HTTP request for this URL. This request is then sent to the appropriate server. Second, this HTTP request is received by the web server. The server maps this request to a particular servlet. The servlet is dynamically retrieved and loaded into the address space of the server. Third, the server invokes the init( ) method of the servlet. This method is invoked only when the servlet is first loaded into memory. It is possible to pass initialization parameters to the servlet so it may configure itself. Fourth, the server invokes the service( ) method of the servlet. This method is called to process the HTTP request. You will see that it is possible for the servlet to read data that has been provided in the HTTP request. It may also formulate an HTTP response for the client. The servlet remains in the server s address space and is available to process any other HTTP requests received from clients. The service( ) method is called for each HTTP request. Finally, the server may decide to unload the servlet from its memory. The algorithms by which this determination is made are specific to each server. The server calls the destroy( ) method to relinquish any resources such as file handles that are allocated for the servlet. Important data may be saved to a persistent store. The memory allocated for the servlet and its objects can then be garbage collected. A servlet life cycle can be defined as the entire process from its creation till the destruction. The following are the paths followed by a servlet The servlet is initialized by calling the init () method.

2 The servlet calls service() method to process a client's request. The servlet is terminated by calling the destroy() method. Finally, servlet is garbage collected by the garbage collector of the JVM.

3 2)List four interfaces in javax.servlet package. Ans :The javax.servlet Package The javax.servlet package contains a number of interfaces and classes that establish the framework in which servlets operate. Interface Description Servlet Declares life cycle methods for a servlet. ServletConfig Allows servlets to get initialization parameters. ServletContext Enables servlets to log events and access information about their environment. ServletRequest Used to read data from a client request. ServletResponse Used to write data to a client response. SingleThreadModel Indicates that the servlet is thread safe. 1. The Servlet Interface : All servlets must implement the Servlet interface. It declares the init( ), service( ), and destroy( ) methods that are called by the server during the life cycle of a servlet. 2. The ServletConfig Interface : The ServletConfig interface is implemented by the server. It allows a servlet to obtain configuration data when it is loaded. 3. The ServletContext Interface: The ServletContext interface is implemented by the server. It enables servlets to obtain information about their environment. 4. The ServletRequest Interface: The ServletRequest interface is implemented by the server. It enables a servlet to obtain information about a client request. 5. The ServletResponse Interface: The ServletResponse interface is implemented by the server. It enables a servlet to formulate a response for a client 6. The SingleThreadModel Interface: This interface is used to indicate that only a single thread will execute the service( ) method of a servlet at a given time. It defines no constants and declares no methods. If a servlet implements this interface, the server has two options. First, it can create several instances of the servlet. When a client request arrives, it is sent to an available instance of the servlet. Second, it can synchronize access to the servlet.

4 3)List four Class in javax.servlet package. Ans:

5 4) List four interface in javax.servlet.http.package. ans: 5) List four Class in javax.servlet.http.package.

6 6) Give steps to create simple servlet with example. 4 (Steps- 2 Marks, Any one Example 2 Marks) import java.io.*; import javax.servlet.*; public class WelcomeServletDemo extends GenericServlet public void service(servletrequest request,servletresponse response) throws ServletException,IOException response.setcontenttype("text/html"); PrintWriter pw = response.getwriter(); pw.println("<b>welcome to Servlet"); pw.close();

7 7) Explain the techniques of session tracking /management. (Any four techniques with explanation- 1 Mark each) Ans: Session tracking Techniques: 1. User Authorization Users can be authorized to use the web application in different ways. Basic concept is that the user will provide username and password to login to the application. Based on that the user can be identified and the session can be maintained. 2. Hidden Fields <INPUT TYPE= hidden NAME= technology VALUE= servlet > Hidden fields like the above can be inserted in the web pages and information can be sent to the server for session tracking. These fields are not visible directly to the user, but can be viewed using view source option from the browsers. This type doesn t need any special configuration from the browser of server and by default available to use for session tracking. This cannot be used for session tracking when the conversation included static resources lik html pages. 3. URL Rewriting Original URL: rewritten URL: When a request is made, additional parameter is appended with the url. In general added additional parameter will be sessionid or sometimes the userid. It will suffice to track the session. This type of session tracking doesn t need any special support from the browser. Disadvantage is, implementing this type of session tracking is tedious. We need to keep track of the parameter as a chain link until the conversation completes and also should make sure that, the parameter doesn t clash with other application parameters.

8 4. Cookies Cookies are the mostly used technology for session tracking. Cookie is a key value pair of information, sent by the server to the browser. This should be saved by the browser in its space in the client computer. Whenever the browser sends a request to that server it sends the cookie along with it. Then the server can identify the client using the cookie. In java, following is the source code snippet to create a cookie: Cookie cookie = new Cookie ( userid, 7456 ); res.addcookie(cookie); 4. Session tracking API Session tracking API is built on top of the first four methods. This is inorder to help the developer to minimize the overhead of session tracking. This type of session tracking is provided by the underlying technology. Lets take the java servlet example. Then, the servlet container manages the session tracking task and the user need not do it explicitly using the java servlets. This is the best of all methods, because all the management and errors related to session tracking will be taken care of by the container itself.

9 8) Explain the use of cookies with example. (Explanation - 2 Marks, Example - 2 Marks any one) Cookie is a small piece of information that is passed back and forth in the HTTP request and response. Even though a cookie can be created on the client side using some scripting language such as JavaScript, it is usually created by the server resource, such as a Servlet. The Cookies sent by the server when the client requests another page from the same application. In Servlet programming a cookie is represented by a Cookie class in the javax.servlet.http package. You can create a cookie by calling the Cookie class constructor and passing two string objects: the name and value of the cookie. For instance, the following code creates a cookie object called c1. The cookie has the name mycookie and value of secret : Cookie c1=new Cookie( mycookie, secret ); You then can add the cookie to the http response using addcookie method of the HTTPServletResponse interface: Response.addCookie(c1); Program for Add cookies import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class AddCookieServlet extends HttpServlet public void dopost(httpservletrequest request, HttpServletResponse response) throws ServletException, IOException // Get parameter from HTTP request. String data = request.getparameter("data"); // Create cookie. Cookie cookie = new Cookie("MyCookie", data); // Add cookie to HTTP response. response.addcookie(cookie); // Write output to browser. response.setcontenttype("text/html"); PrintWriter pw = response.getwriter(); pw.println("<b>mycookie has been set to"); pw.println(data); pw.close();

10 9) Write a program to create a servlet that handle HTTP GET Request 1) Program to retrieve user s data : Contents Of ColorGet.html File : <html> <body> <center> <form name="form1" action=" <B>Color:</B> <select name="color" size="1"> <option value="red">red</option> <option value="green">green</option> <option value="blue">blue</option> </select> <br><br> <input type=submit value="submit"> </form> </body> </html> Contents Of ColorGetServlet.java File : import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class ColorGetServlet extends HttpServlet public void doget(httpservletrequest request, HttpServletResponse response) throws ServletException, IOException String color = request.getparameter("color"); response.setcontenttype("text/html"); PrintWriter pw = response.getwriter(); pw.println("<b>the selected color is: "); pw.println(color); pw.close();

11 Contents Of web.xml File : <web-app> <servlet> <servlet-name>colorgetservlet</servlet-name> <servlet-class>colorgetservlet</servlet-class> </servlet> <servlet-mapping> <servlet-name>colorgetservlet</servlet-name> <url-pattern>/colorgetservlet</url-pattern> </servlet-mapping> </web-app> Opening : ColorGet.html File After clicking on Submit button :

12 10) Write servlet which shows how many times user has visited the page in session. Program to display the number of times a page has been accessed. Contents Of SessionTracker.java File : import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class SessionTracker extends HttpServlet public void doget(httpservletrequest req, HttpServletResponse res) throws ServletException, IOException res.setcontenttype("text/html"); PrintWriter pw = res.getwriter(); HttpSession session = req.getsession(); Integer count = (Integer)session.getValue("tracker.count"); if(count == null) count = new Integer(1); else count = new Integer(count.intValue() + 1); session.putvalue("tracker.count", count); pw.println("<html><body><h2>session Tracking Demo</H2>"); pw.println("you have visited this page " + count + " times."); pw.println("</body></html>");

13 Contents of web.xml File : <web-app> <servlet> <servlet-name>sessiontracker</servlet-name> <servlet-class>sessiontracker</servlet-class> </servlet> <servlet-mapping> <servlet-name>sessiontracker</servlet-name> <url-pattern>/sessiontracker</url-pattern> </servlet-mapping> </web-app> Opening : After refreshing this page 7 times :

14 11)Write a servlet to store username and password accepted from client and display it. Ans: Contents of Exp15.html File : <html> <body> <form name="form1" method="post" action=" <table> <tr> <td><b>username</td> <td><input type=textbox name="username" size="25" value=""></td> </tr> <tr> <td><b>password</td> <td><input type=password name="password" size="25" value=""></td> </tr> </table> <input type=submit value="submit"> </body> </html> contents Of Experiment15.java file : import java.io.*; import java.util.*; import javax.servlet.*; public class Experiment15 extends GenericServlet public void service(servletrequest request, ServletResponse response) throws ServletException, IOException // Get print writer. PrintWriter pw = response.getwriter(); // Get enumeration of parameter names. Enumeration e = request.getparameternames(); // Display parameter names and values. while(e.hasmoreelements()) String pname = (String)e.nextElement(); pw.print(pname + " : "); String pvalue = request.getparameter(pname); pw.println(pvalue + "<br>"); pw.close();

15 Contents Of web.xml file : <servlet> <servlet-name>experiment15</servlet-name> <servlet-class>experiment15</servlet-class> </servlet> <servlet-mapping> <servlet-name>experiment15</servlet-name> <url-pattern>/experiment15</url-pattern> </servlet-mapping> </web-app> Execution of Exp15.html file. After clicking on Submit button :