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Java Servlets

Java Servlets. What Are Servlets?. Basically, a java program that runs on the server Creates dynamic web pages. What Do They Do?. Handle data/requests sent by users (clients) Create and format results Send results back to user. Who Uses Servlets?.

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Java Servlets

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  1. Java Servlets

  2. What Are Servlets? • Basically, a java program that runs on the server • Creates dynamic web pages

  3. What Do They Do? • Handle data/requests sent by users (clients) • Create and format results • Send results back to user

  4. Who Uses Servlets? • Servlets are useful in many business oriented websites • … and MANY others

  5. History • Dynamic websites were often created with CGI • CGI: Common Gateway Interface • Poor solution to today’s needs • A better solution was needed

  6. Servlets vs. CGI CGI • Servlet Advantages • Efficient • Single lightweight java thread handles multiple requests • Optimizations such as computation caching and keeping connections to databases open • Convenient • Many programmers today already know java • Powerful • Can talk directly to the web server • Share data with other servlets • Maintain data from request to request • Portable • Java is supported by every major web browser (through plugins) • Inexpensive • Adding servlet support to a server is cheap or free

  7. Servlets vs. CGI • CGI Advantages • CGI scripts can be written in any language • Does not depend on servlet-enabled server

  8. What Servlets Need • JavaServer Web Development Kit (JSWDK) • Servlet capable server • Java Server Pages (JSP) • Servlet code

  9. Java Server Web Development Kit • JSWDK • Small, stand-alone server for testing servlets and JSP pages • The J2EE SDK • Includes Java Servlets 2.4

  10. Servlet capable server Apache • Popular, open-source server • Tomcat • A “servlet container” used with Apache Other servers are available

  11. Java Server Pages • Lets you mix regular, static HTML pages with dynamically-generated HTML • Does not extend functionality of Servlets • Allows you to separate “look” of the site from the dynamic “content” • Webpage designers create the HTML • Servlet programmers create the dynamic content • Changes in HTML don’t effect servlets

  12. <head> </head> <body> <% // jsp sample code out.println(" JSP, ASP, CF, PHP - you name it, we support it!"); %> </body> </html> <html> <head> </head> <body> <b> JSP, ASP, CF, PHP - you name it, we support it! </b> </body> </html> </font>

  13. <head> </head> <body> <% // jsp sample code out.println(" JSP, ASP, CF, PHP - you name it, we support it!"); %> </body> </html> <html> <head> </head> <body> <b> JSP, ASP, CF, PHP - you name it, we support it! </b> </body> </html> </font>

  14. <head> </head> <body> <% // jsp sample code out.println(" JSP, ASP, CF, PHP - you name it, we support it!"); %> </body> </html> <html> <head> </head> <body> <b> JSP, ASP, CF, PHP - you name it, we support it! </b> </body> </html> </font>

  15. <head> </head> <body> <% // jsp sample code out.println(" JSP, ASP, CF, PHP - you name it, we support it!"); %> </body> </html> <html> <head> </head> <body> <b> JSP, ASP, CF, PHP - you name it, we support it! </b> </body> </html> </font>

  16. Servlet Code • Written in standard Java • Implement the javax.servlet.Servlet interface

  17. package servlet_tutorials.PhoneBook;import javax.servlet.*;import javax.servlet.http.*;import java.io.*;import java.util.*; import java.sql.*; import java.net.*;public class SearchPhoneBookServlet extends HttpServlet { public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { String query = null; String where = null; String firstname = null; String lastname = null; ResultSet rs = null;res.setContentType("text/html"); PrintWriter out = res.getWriter();// check which if any fields in the submitted form are empty if (req.getParameter("FirstName").length() > 0) firstname = req.getParameter("FirstName"); else firstname = null;}

  18. Main Concepts of Servlet Programming • Life Cycle • Client Interaction • Saving State • Servlet Communication • Calling Servlets • Request Attributes and Resources • Multithreading

  19. Life Cycle • Initialize • Service • Destroy

  20. Life Cycle: Initialize • Servlet is created when servlet container receives a request from the client • Init() method is called only once

  21. Life Cycle: Service • Any requests will be forwarded to the service() method • doGet() • doPost() • doDelete() • doOptions() • doPut() • doTrace()

  22. Life Cycle: Destroy • destroy() method is called only once • Occurs when • Application is stopped • Servlet container shuts down • Allows resources to be freed

  23. Client Interaction • Request • Client (browser) sends a request containing • Request line (method type, URL, protocol) • Header variables (optional) • Message body (optional) • Response • Sent by server to client • response line (server protocol and status code) • header variables (server and response information) • message body (response, such as HTML)

  24. Servlets • Thin clients (minimize download) • Java all “server side” Client Server

  25. Saving State • Session Tracking • A mechanism that servlets use to maintain state about a series of requests from the same user (browser) across some period of time. • Cookies • A mechanism that a servlet uses to have clients hold a small amount of state-information associated with the user.

  26. Servlet Communication • To satisfy client requests, servlets sometimes need to access network resources: other servlets, HTML pages, objects shared among servlets at the same server, and so on.

  27. Calling Servlets • Typing a servlet URL into a browser window • Servlets can be called directly by typing their URL into a browser's location window. • Calling a servlet from within an HTML page • Servlet URLs can be used in HTML tags, where a URL for a CGI-bin script or file URL might be found.

  28. Request Attributes and Resources • Request Attributes • getAttribute • getAttributeNames • setAttribute • Request Resources - gives you access to external resources • getResource • getResourceAsStream

  29. Multithreading • Concurrent requests for a servlet are handled by separate threads executing the corresponding request processing method (e.g. doGet or doPost). It's therefore important that these methods are thread safe. • The easiest way to guarantee that the code is thread safe is to avoid instance variables altogether and instead use synchronized blocks.

  30. Simple Counter Example import java.io.*; import javax.servlet.*; import javax.servlet.http.*; • public class SimpleCounter extends HttpServlet { • int count = 0; • public void doGet(HttpServletRequest req, HttpServletResponse res) throws ServletException, IOException { • res.setContentType("text/plain"); • PrintWriter out = res.getWriter(); • count++; • out.println("This servlet has been accessed " + count + " times since loading"); • } • }

  31. MultiThread Problems • Problem - Synchronization between threads count++; // by thread1 count++; // by thread2 out.println.. // by thread1 out.println.. // by thread2 Two Requests will get the same value of counter • Solution - Use Synchronized Block! • Synchronized Block • Lock(Monitor)

  32. Better Approach • The approach would be to synchronize only the section of code that needs to be executed automically: PrintWriter out = res.getWriter(); synchronized(this) {    count++;     out.println("This servlet has been accessed " +              count + "times since loading");}This reduces the amount of time the servlet spends in its synchronized block, and still maintains a consistent count.

  33. Example: On-line Phone Book • Design

  34. Example: On-line Phone Book

  35. Search Form Java server Page Search_phone_book.jsp <html><head> <title>Search Phonebook</title> </head> <body bgcolor="#FFFFFF"> <p><b>Search Company Phone Book</b></p> <form name="form1" method="get" action="servlet/servlet_tutorials.PhoneBook.SearchPhoneBookServlet"> <table border="0" cellspacing="0" cellpadding="6"> <tr> <td >Search by</td> <td></td> </tr> <tr> <td><b>First Name </b></td> <td> <input type="text" name="FirstName"> AND/OR</td> </tr> <tr> <td ><b>Last Name</b></td> <td ><input type="text" name="LastName"></td> </tr> <tr> <td ></td> <td ><input type="submit" name="Submit" value="Submit"></td> </tr> </table></form></body></html>

  36. Example: On-line Phone Book

  37. Display Results Java Server Page Display_search_results.jsp <html><%@page import ="java.sql.*" %><jsp:useBean id="phone" class="servlet_tutorials.PhoneBook.PhoneBookBean"/> <%@ page buffer=35 %><%@ page errorPage="error.jsp" %><html><head><title>Phone Book Search Results</title><meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1"> </head><body bgcolor="#FFFFFF"> <b>Search Results</b> <p> <% String q = request.getParameter("query"); ResultSet rs = phone.getResultSet(q); %><% if (rs.wasNull()) { %> "NO RESULTS FOUND" <% } else %><table> <tr> <td> <div align="center">First Name</b></div> </td> <td> <div align="center">Last Name</font></b></div> </td> <td> <div align="center">Phone Number</font></b></div> </td> <td> <div align="center">Email</font></b></div> </td> </tr><% while(rs.next()) { %> <tr> <td><%= rs.getString("first_name") %></td> <td><%= rs.getString("last_name") %></td> <td><%= rs.getString("phone_number") %></td> <td><%= rs.getString("e_mail") %></td> </tr> <% } %></table>

  38. Servlet

  39. Java Bean & Database linked

  40. Conclusion

  41. Questions? Comments?

  42. References • http://www.apl.jhu.edu/~hall/java/Servlet-Tutorial/Servlet-Tutorial-Overview.html • www.cis.upenn.edu/~matuszek/ cit597-2004/Lectures/21-servlets.ppt • http://learning.unl.ac.uk/im269/lectures/week6servletsp1.ppt • http://java.sun.com/docs/books/tutorialNB/download/tut-servlets.zip • http://www.webdevelopersjournal.com/articles/intro_to_servlets.html

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