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Java Servlets

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Short presentation about Java Servlets.

Short presentation about Java Servlets.

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  • ## * * 07/16/96
  • ## * * 07/16/96
  • ## * * 07/16/96 Example of HTTP GET: Google search Example of HTTP POST: Login page
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  • ## * * 07/16/96
  • Note: As of Servlet 2.2, the getValue() method is now deprecated. Use getAttribute() instead.
  • Note: As of Servlet 2.2, the getValue() method is now deprecated. Use getAttribute() instead.
  • Transcript

    • 1. Java Servlets Svetlin Nakov Borislava Spasova
    • 2. Contents
      • Java Servlets Technology Overview
        • What is a Java Servlet?
        • Servlet Services
        • Why Use Servlets?
        • Time Servlet – Example
        • Deploying Servlets on Eclipse IDE
      • Servlets Architecture
        • Servlets API
        • Servlets Life-Cycle
    • 3. Contents (2)
      • Servlet Examples
        • Processing Parameters – Hello Servlet
        • Image Counter Servlet
      • Using Sessions
        • What is a Session?
        • The Sessions API
        • Session Timeout
      • Session Examples
        • Login / Logout Application
        • The Browser's Cache Problems
    • 4. Java Servlets Technology Overview
    • 5. What is a Java Servlet?
      • Java Servlets are:
        • Technology for generating dynamic Web pages (like PHP, ASP, ASP.NET, ...)
        • P rotocol and platform-independent server side components, written in Java , which extend the standard Web servers
        • Java programs that serve HTTP requests
      • The HttpServlet class
        • Provides d ynamic Web content generation (HTML, XML, …)
    • 6. What is a Java Servlet? (2)
      • Servlets
        • P rovide a general framework for services built on the request-response paradigm
        • Portable to any Java application server
        • Have access to the entire family of Java and Java EE APIs
          • JDBC, Persistence, EJB, JMS, JAX-WS, JTA, JTS, RMI, JNDI, JAXP, ...
        • Fundamental part of all Java Web application technologies (JSP, JSF, ...)
    • 7. Servlet Services
      • Java Servlets provide many useful services
        • Provides low-level API for building Internet services
        • Serves as foundation to JavaServer Pages (JSP) and JavaServer Faces (JSF) technologies
        • Can deliver multiple types of data to any client
          • XML, HTML, WML, GIF, etc...
        • Can serve as “Controller” of JSP/Servlet application
    • 8. Why Use Servlets?
      • Portability
        • Write once, serve everywhere
      • Power
        • Can take advantage of all Java APIs
      • Elegance
        • Simplicity due to abstraction
      • Efficiency & Endurance
        • Highly scalable
    • 9. Why Use Servlets? (2)
      • Safety
        • Strong type-checking
        • Memory management
      • Integration
        • Servlets tightly coupled with server
      • Extensibility & Flexibility
        • Servlets designed to be easily extensible, though currently optimized for HTTP uses
        • Flexible invocation of servlet (SSI, servlet-chaining, filters, etc.)
    • 10. Time Servlet – Example
      • import java.io.*;
      • import javax.servlet.*;
      • import javax.servlet.http.*;
      • public class Time Servlet extends HttpServlet {
      • public void doGet(HttpServletRequest aRequest,
      • HttpServletResponse aResponse)
      • throws ServletException, IOException {
      • PrintWriter out = aResponse.getWriter();
      • out.println(&quot;<HTML>&quot;);
      • out.println(&quot;The time is: &quot; +
      • new java.util.Date());
      • out.println(&quot;</HTML>&quot;);
      • }
      • }
    • 11. Deploying Servlets on Eclipse IDE
      • First create new Web application
    • 12. Deploying Servlets on Eclipse IDE (2)
      • Add new servlet to the Web application
    • 13. Deploying Servlets on Eclipse IDE (3)
      • Run the servlet
    • 14. Deploying Servlets on Eclipse IDE (4)
      • The servlet in action
    • 15. Java Servlets Technical Architecture
    • 16. Servlets Architecture
      • The HttpServlet class
        • Serves client's HTTP requests
      • For each of the HTTP methods, GET, POST, and others , there is c orresponding method :
        • doGet ( … ) – serves HTTP GET requests
        • doPost ( … ) – serves HTTP POST requests
        • doPut ( … ) , doHead ( … ) , doDelete ( … ) , doTrace ( … ) , doOptions ( … )
      • The Servlet usually must implement one of the first two methods or the service( … ) method
    • 17. Servlets Architecture (2)
      • The HttpServletRequest object
        • Contains the request data from the client
          • HTTP request headers
          • Form data and query parameters
          • Other client data (cookies, path, etc.)
      • The HttpServletResponse object
        • Encapsulates data sent back to client
          • HTTP response headers (content type, cookies, etc.)
          • Response body (as OutputStream )
    • 18. Servlets Architecture (3)
      • The HTTP GET method is used when:
        • The processing of the request does not change the state of the server
        • The amount of form data is small
        • You want to allow the request to be bookmarked
      • The HTTP POST method is used when:
        • The processing of the request changes the state of the server, e.g. storing data in a DB
        • The amount of form data is large
        • The contents of the data should not be visible in the URL (for example, passwords)
    • 19. Servlets API
      • The most important servlet functionality:
        • Retrieve the HTML form parameters from the request (both GET and POST parameters)
        • Retrieve a servlet initialization parameter
        • Retrieve HTTP request header information
      HttpServletRequest.getParameter( String ) ServletConfig.getInitParameter () HttpServletRequest.getHeader( String )
    • 20. Servlets API (2)
        • Set an HTTP response header / content type
        • Acquire a text stream for the response
        • Acquire a binary stream for the response
        • Redirect an HTTP request to another URL
      HttpServletResponse.setHeader (<name>, <value>) / HttpServletResponse.setContentType( String ) HttpServletResponse.getWriter() HttpServletResponse . getOutputStream() HttpServletResponse.sendRedirect()
    • 21. Servlets Life-Cycle
      • You can provide an implementation of these methods in HttpServlet descendent classes to manipulate the servlet instance and the resources it depends on
      • The Web container manages the life cycle of servlet instances
      • The life-cycle methods should not be called by your code
      init() ...() service() doGet() doPost() doDelete() destroy() doPut() New Destroyed Running
    • 22. The init() Method
      • Called by the Web container when the servlet instance is first created
      • The Servlets specification guarantees that no requests will be processed by this servlet until the init method has completed
      • Override the init() method when:
        • You need to create or open any servlet-specific resources that you need for processing user requests
        • You need to initialize the state of the servlet
    • 23. The service() Method
      • Called by the Web container to process a user request
      • Dispatches the HTTP requests to doGet( … ) , doPost( … ) , etc. depending on the HTTP request method (GET, POST, and so on)
        • Sends the result as HTTP response
      • Usually we do not need to override this method
    • 24. The destroy() Method
      • Called by the Web container when the servlet instance is being eliminated
      • The Servlet specification guarantees that all requests will be completely processed before this method is called
      • Override the destroy method when:
        • You need to release any servlet-specific resources that you had opened in the init() method
        • You need to persist the state of the servlet
    • 25. Java Servlets Examples
    • 26. Processing Parameters – Hello Servlet
      • We want to create a servlet that takes an user name as a parameter and says &quot;Hello, <user_name>&quot;
      • We need HTML form with a text field
      • The servlet can later retrieve the value entered in the form field
      <form method=&quot; GET or POST &quot; action=&quot; the servlet &quot;> <input type=&quot;text&quot; name=&quot; user_name &quot;> </form> String n ame = r equest.getParameter(&quot;user_name&quot;);
    • 27. Hello Servlet – Example
      • <html><body>
      • <form method=&quot;GET&quot; action=&quot;HelloServlet&quot;>
      • Please enter your name:
      • <input type=&quot;text&quot; name=&quot; user_name &quot;>
      • <input type=&quot;submit&quot; value=&quot;OK&quot;>
      • </form>
      • </body></html>
      HelloForm.html import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloServlet extends HttpServlet { Hello Servlet . java
    • 28. Hello Servlet – Example public void doGet(HttpServletRequest r equest, HttpServletResponse r esponse) throws ServletException, IOException { r esponse.setContentType(&quot;text/html&quot;); ServletOutputStream out = r esponse.getOutputStream(); String userName = r equest.getParameter(&quot;user_name&quot;); out.println(&quot;<html> <head> &quot;); out.println(&quot;t<title>Hello Servlet</title>&quot;); out.println(&quot;</head><body>&quot;); out.println(&quot;t<h1>Hello, &quot; + userName + &quot;</h1>&quot;); out.println(&quot;</body></html>&quot;); } Hello Servlet . java
    • 29. Creating The Form in Eclipse IDE
      • Create new HTML form
    • 30. Creating New Servlet in Eclipse IDE
      • Create new Servlet
    • 31. Hello Servlet in Action
    • 32. Hello Servlet – HTTP Request
      • What happens when the user enters his name?
        • Internet Explorer (IE) sends the following HTTP request to Tomcat
      GET /FirstWebApp/HelloServlet?user_name=Nakov HTTP/1.1 Accept: image/gif, image/x-xbitmap, image/jpeg,image/pjpeg, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, application/x-shockwave-flash, */* Accept-Language: bg Accept-Encoding: gzip, deflate User-Agent: Mozilla/4.0 (compatible; MSIE 6.0; Windows NT 5.1; Q312461) Host: nakov:808 4 Connection: Keep-Alive
    • 33. Hello Servlet – HTTP Response
      • What happens when Tomcat receive and process the HTTP request
        • Tomcat sends the following HTTP response to Internet Explorer
      HTTP/1.1 200 OK Content-Length: 100 Date: Fri, 26 Mar 200 6 10:06:28 GMT Server: Apache-Coyote/1.1 <html><head> <title>Hello Servlet</title> </head><body> <h1>Hello, Nakov</h1> </body></html>
    • 34. Image Counter Servlet
      • We want to create a servlet that displays an image counter (as JPEG image)
      • The servlet should maintain an internal counter
        • Can be initialized in the init() method and incremented in the doGet () method
      • It should produce binary output (the JPEG) image
        • The content type should be set to &quot;image/jpeg&quot;
    • 35. Image Counter Servlet (2) import javax.servlet.*; import javax.servlet.http.*; ... public class ImageCounterServlet extends HttpServlet { private String mStartDate; private int mVisitCounter; public void init() { mStartDate = (new Date()).toString(); mVisitCounter = 0; } public BufferedImage createImage(String msg) { ... }
    • 36. Image Counter Servlet (3) public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, ServletException { String msg; synchronized(this) { mVisitCounter++; msg = &quot;&quot; + mVisitCounter + &quot; visits since &quot; + mStartDate; } BufferedImage image = createImage(msg); response.setContentType(&quot;image/jpeg&quot;); OutputStream out = response.getOutputStream(); // Encode the image in JPEG format and // write the image to the output stream } }
    • 37. Image Counter Servlet in Action
    • 38. Using Sessions
    • 39. What is a Session?
      • A session is a state associated with particular user that is maintained at the server side
        • Sessions persist between the HTTP requests
      • Sessions e nable creating applications that depend on individual user data. For example:
        • Login / logout functionality
        • Wizard pages
        • Shopping c arts
        • Personalization s ervices
        • Maintaining state about the user’s preferences
    • 40. Sessions in Servlets
      • Servlets include a built-in Session s API
        • Sessions are maintained automatically, with no additional coding
        • The Web container associates an unique HttpSession object to each different client
        • Different clients have different session objects at the server
        • Requests from the same client have the same session object
        • Sessions can store various data
    • 41. The Sessions API
      • The sessions API allows
        • To get the HttpSession object from the HTTPServletRequest object
        • Extract data from the user’s session object
        • Append data to the user’s session object
        • Extract meta-information about the session object, e.g. when was the session created
    • 42. Getting The Session Object
      • To get the session object use the method HttpServletRequest.getSession()
      • Example:
      • If the user already has a session, the existing session is returned
      • If no session still exists, a new one is created and returned
      • If you want to know if this is a new session, call the isNew() method
      HttpSession session = request.getSession();
    • 43. Behind T he Scenes
      • When you call getSession () each user is automatically assigned a unique Session ID
      • How does this Session ID get to the user?
        • Option 1: If the browser supports cookies, the servlet will automatically create a session cookie, and store the session ID within the cookie
          • In Tomcat, the cookie is called JSESSIONID
        • Option 2: If the browser does not support cookies, the servlet will try to extract the session ID from the URL
    • 44. Extracting Data From The Session
      • The s ession object works like a HashMap
        • E nables storing any type of Java object
        • Objects are stored by key (like in hash tables)
      • Extracting existing object:
      • Getting a list of all “keys” associated with the session
      Integer accessCount = (Integer) session.getAttribute(&quot;accessCount&quot;); Enumeration attributes = request.getAttributeNames();
    • 45. Storing Data In The Session
      • We can store data in the session object for using it later
      • Objects in the session can be removed when not needed more
      HttpSession session = request.getSession(); session.setAttribute(&quot;name&quot;, &quot;Svetlin Nakov&quot;); session. remove Attribute(&quot;name&quot;);
    • 46. Getting Additional Session Information
      • Getting the unique session ID associated with this user, e.g. gj9xswvw9p
      • Checking if the session was just created
      • Checking when the session was first created
      • Checking when the session was last active
      public boolean isNew(); public String getId(); public long getLastAccessedTime(); public long getCreationTime();
    • 47. Session Timeout
      • We can get the maximal session validity interval ( in seconds )
        • After such interval of inactivity the session is automatically invalidated
      • We can modify the maximal inactivity interval
        • A negative value specifies that the session should never time out
      public int getMaxInactiveInterval(); public void setMaxInactiveInterval (int seconds) ;
    • 48. Terminating Sessions
      • To terminate session manually use the method:
        • Typically done during the &quot;user logout&quot;
      • The session can become invalid not only manually
        • Sessions can expire automatically due to inactivity
      public void invalidate() ;
    • 49. Login / Logout – Example
      • We want to create a simple Web application that restricts the access by login form
        • We will use sessions to store information about the authenticated users
          • We will use the key &quot;username&quot;
          • When it present, there is a logged in user
        • During the login we will add the user name in the session
        • Logout will invalidate the session
        • The main servlet will check the current user
    • 50. Login Form <html> <head><title>Login</title></head> <body> <form method=&quot;POST&quot; action=&quot;LoginServlet&quot;> Please login:<br> Username: <input type=&quot;text&quot; name=&quot;username&quot;><br> Password: <input type=&quot;password&quot; name=&quot;password&quot;><br> <input type=&quot;submit&quot; value=&quot;Login&quot;> </form> </body> </html> LoginForm.html
    • 51. Login Servlet public class LoginServlet extends HttpServlet { public void doPost(HttpServletRequest req, HttpServletResponse resp) throws IOException, ServletException { String username = req.getParameter(&quot;username&quot;); String password = req.getParameter(&quot;password&quot;); PrintWriter out = resp.getWriter(); if (isLoginValid(username, password)) { HttpSession session = req.getSession(); session.setAttribute(&quot;USER&quot;, username); response.sendRedirect(&quot;MainServlet&quot;); } else { response.sendRedirect(&quot;InvalidLogin.html&quot;); } } } Login Servlet . java
    • 52. Main Servlet public class MainServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse resp) throws ServletException, IOException { HttpSession session = request.getSession(); String userName = (String) s ession.getAttribute(&quot;USER&quot;); if (userName != null) { response.setContentType(&quot;text/html&quot;); ServletOutputStream out = resp.getOutputStream(); out.println(&quot;<html> <body><h1> &quot;); out.println(&quot;Hello, &quot; + userName + &quot;! &quot;); out.println(&quot; </h1> </body></html>&quot;); } else { response.sendRedirect(&quot;LoginForm.html&quot;); } } } MainServlet.java
    • 53. Logout Servlet public class LogoutServlet extends HttpServlet { protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { HttpSession session = request.getSession(); session.invalidate(); response.setContentType(&quot;text/html&quot;); ServletOutputStream out = response.getOutputStream(); out.println(&quot;<html><head>&quot;); out.println(&quot;<title>Logout</title></head>&quot;); out.println(&quot;<body>&quot;); out.println(&quot;<h1>Logout successfull.</h1>&quot;); out.println(&quot;</body></html>&quot;); } } LogoutServlet.java
    • 54. Invalid Login Page <html> <head> <title>Error</title> </head> <body> <h1>Invalid login!</h1> Please <a href=&quot;LoginForm.html&quot;>try again</a>. </body> </html> InvalidLogin.html
    • 55. The Browser's Cache Problems
      • Most Web browsers use caching of the displayed pages and images
        • This can cause the user to see old state of the pages
          • Seems like a bug in the application
      • To prevent showing the old state we need to disable the browser cache:
      response.setHeader(&quot;Pragma&quot;, &quot;No-cache&quot;); response.setDateHeader(&quot;Expires&quot;, 0); response.setHeader(&quot;Cache-Control&quot;, &quot;no-cache&quot;);
    • 56. Problems
      • Create a servlet that prints in a table the numbers from 1 to 1000 and their square root.
      • Create a servlet that takes as parameters two integer numbers and calculates their sum.
      • Create a HTML form that invokes the servlet. Try to use GET and POST methods.
      • Implement a servlet that plays the &quot;Number guess game&quot;. When the client first invoke the servlet it generates a random number in the range [1..100]. The user is asked to guess this number. At each guess the servlet says only &quot;greater&quot; or &quot;smaller&quot;. The game ends when the user tell the number.
    • 57. Homework
      • Create a servlet that takes as a parameter a number and displays it as image that is hard to be recognized by OCR software. The image should have intentionally inserted defects.
      • Create an HTML form and a servlet for performing conversions of distances from one metric to another. The metrics that should be supported are: meter, centimeter, kilometer, foot, inch, yard, mile.
      • 1 cm = 0.01 meters 1 km = 1000 meters
      • 1 foot = 0.3048 meters 1 inch = 0.0254 meters
      • 1 yard = 0.9144 meters 1 mile = 1609.344 meters
    • 58. Homework (2)
      • Create a sequence of HTML forms and servlets that allow entering information about a student. The information is entered in 3 steps in 3 separate forms:
      • Step 1: First name, last name, age
      • Step 2: Address (country, town, street)
      • Step 3: University, faculty, specialty
      • The data entered in the 3 steps should be stored in the session and finally displayed.
      • Create a servlet that reads an image (from WEB-INFimglogo.gif ) and returns it.

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