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Internet Applications part 2

This article provides an overview of Java applets, including their execution, security restrictions, advanced features, and HTML tags for embedding in web pages.

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Internet Applications part 2

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  1. Internet Applicationspart 2 René de Vries Based on slides by M.L. Liu and Marko van Eekelen ISS - Internet Applications Part 2

  2. Overview • Applets • Servlets • Web Services • SOAP References, Exercises ISS - Internet Applications Part 2

  3. 1. Applets ISS - Internet Applications Part 2

  4. Introduction Three kinds of Java programs: • An application is a standalone program that can be invoked from the command line. • An applet is a program that runs in the context of a browser session. • A servletis a program that is invoked on demand on a server program and that runs in the context of a web server process. ISS - Internet Applications Part 2

  5. Applets, web page, client, server • Applets are programs stored on a web server, similar to web pages. • When an applet is referred to in a web page that has been fetched and processed by a browser, the browser generates a request to fetch (or download) the applet program, then executes the program in the browser’s execution context, on the client host. ISS - Internet Applications Part 2

  6. Java Applet Execution - 1 • An applet program is written as a subclass of the java.Applet class or the javax.swing.Japplet class. • There is no main method: you must override the start method. • Applet objects uses AWT for graphics. JApplet uses SWING. • An applet is a Graphics object that runs in a Thread object, so every applet can perform graphics, and runs in parallel to the browser process. ISS - Internet Applications Part 2

  7. Java Applet Execution - 2 • When the applet is loaded, these methods are automatically invoked in order: • The init( ) method is invoked by the Java Virtual Machine. • The start( ) method • The paint( ) method. • The applet is now running and rendered on the web page. • You program the start( ) method and the paint( ) method for your application, and invoke a repaint call to re-render the graphics, if necessary. • At the end of the execution, the stop( ) method is invoked, followed by the destroy( ) method to deallocate the applet’s resources. ISS - Internet Applications Part 2

  8. Applet Securityhttp://java.sun.com/docs/books/tutorial/applet/overview/ For security reasons, applets that are loaded over the network have several restrictions. • an applet cannot ordinarily read or write files on the computer that it's executing on • an applet cannot make network connections except to the host that it came from ISS - Internet Applications Part 2

  9. Advanced Applets • You can use threads in an applet. • You can make socket calls in an applet, subject to the security constraints. ISS - Internet Applications Part 2

  10. Proxy server A proxy server can be used to circumvent the security constraints. ISS - Internet Applications Part 2

  11. HTML tags for applets - 1 <APPLET specifies the beginning of the HTML applet code CODE="demoxx.class" is the actual name of the applet (usually a 'class' file) CODEBASE="demos/" is the location of the applet (relative as here, or a full URL) NAME="smily" the name you want to give to this instance of the applet on this page WIDTH="100" the physical width of the applet on your page HEIGHT="50"  the physical height of the applet on your page ALIGN="Top" where to align the applet within its page space (top, bottom, center) ISS - Internet Applications Part 2

  12. HTML tags for applets - 2 <PARAM  specifies a parameter that can be passed to the applet (applet specific) NAME=“name1" the name known internally by the applet in order to receive this parameter VALUE="000000" the value you want to pass for this parameter > end of this parameter </APPLET> specifies the end of the HTML applet code ISS - Internet Applications Part 2

  13. <HTML> <BODY> <APPLET code=hello.class width=900 height=300> </APPLET> </BODY> </HTML> // applet to display a message in a window import java.awt.*; import java.applet.*; public class hello extends Applet{ public void init( ){ setBackground(Color.yellow); } public void paint(Graphics g){ final int FONT_SIZE = 42; Font font = new Font("Serif", Font.BOLD, FONT_SIZE); // set font, and color and display message on // the screen at position 250,150 g.setFont(font); g.setColor(Color.blue); // The message in the next line is the one you will see g.drawString("Hello, world!",250,150); } } The HelloWorld Applet ISS - Internet Applications Part 2

  14. Applets Summary • An applet is a Java class • Its code is downloaded from a web server • It is run in the browser’senvironment on the client host • It is invoked by the browser when it scans a web page and encounters a class specified with the APPLET tag • For security reasons, the execution of an applet is normally subject to restrictions: • applets cannot access files in the file system on the client host • applets cannot make network connection except to the server host from which it originated ISS - Internet Applications Part 2

  15. 2. Servlets ISS - Internet Applications Part 2

  16. Introduction • Servlets are modules that extend request/response-oriented servers, such as Java-enabled web servers. • Servlets can be embedded in many different servers because the servlet API, which you use to write servlets, assumes nothing about the server's environment or protocol. • Servlets are portable. ISS - Internet Applications Part 2

  17. Java Servlet Basics • A servlet is an object of the javax.servlet class, which is not part of the JDK. • A servlet runs inside a Java Virtual Machine (JVM) on a server host. • A servlet is an object. It is loaded and runs in an object called a servlet engine, or a servlet container. ISS - Internet Applications Part 2

  18. Uses for Servletshttp://java.sun.com/docs/books/tutorial/servlets/overview/index.html • Providing the functionalities of CGI scripts with a better API and enhanced capabilities. • Allowing collaboration between people. A servlet can handle multiple requests concurrently, and can synchronize requests. This allows servlets to support systems such as on-line conferencing.   • Forwarding requests. Servlets can forward requests to other servers and servlets. Thus servlets can be used to balance load among several servers that mirror the same content, and to partition a single logical service over several servers, according to task type or organizational boundaries. ISS - Internet Applications Part 2

  19. The Servlet class • The Servlet class is not part of the Java Development Kit (JDK). There are many packages that support servlets, including • The JSWDK (Java Server Web Development Kit) – the API documentation can also be downloaded from the same site – freeware provided by Sun for demonstration of the technology, downloadable from http://java.sun.com/products/servlet/archive.htmlhttp://java.sun.com/products/servlet/download.html • The Tomcat : a free, open-source implementation of Java Servlet and JavaServer Pages technologies developed under the Jakarta project at the Apache Software Foundation. • Application servers such as WebLogic, iPlanet, WebSphere. ISS - Internet Applications Part 2

  20. The architecture for servlet support ISS - Internet Applications Part 2

  21. The Life Cycle of an HTTP Servlet • The web server loads a servlet when it is called for in a web page. • The web server invokes the init( ) method of the servlet. • The servlet handles client responses. • The server destroys the servlet (at the request of the system administrator). A servlet is normally not destroyed once it is loaded. ISS - Internet Applications Part 2

  22. The Servlet Life Cycle http://java.sun.com/docs/books/tutorial/servlets/lifecycle/index.html ISS - Internet Applications Part 2

  23. A simplified sequence diagram ISS - Internet Applications Part 2

  24. The Servlet Interface ISS - Internet Applications Part 2

  25. Generic Servlets and HTTP ServletsJava Servlet Programming, O’Reilley Press • Every servlet must implement the javax.servlet.Servlet interface • Most servlets implement the interface by extending one of these classes • javax.servlet.GenericServlet: most general • javax.servlet.http.HttpServlet: an extension of HTTP servers. • A generic servlet should override the service( ) method to process requests and generate appropriate responses. • An HTTP servlet overrides the doPost( ) and/or doGet( ) method. ISS - Internet Applications Part 2

  26. Generic and HTTP Servlets ISS - Internet Applications Part 2

  27. A simple Servlet, fromhttp://java.sun.com/docs/books/tutorial/servlets/overview/simple.html public class SimpleServlet extends HttpServlet{ /** * Handle the HTTP GET method by building a simple web page. */ public void doGet (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { PrintWriter out; String title = "Simple Servlet Output"; // set content type and other response header fields first response.setContentType("text/html"); // then write the data of the response out = response.getWriter(); out.println("<HTML><HEAD><TITLE>"); out.println(title); out.println("</TITLE></HEAD><BODY>"); out.println("<H1>" + title + "</H1>"); out.println("<P>This is output from SimpleServlet."); out.println("</BODY></HTML>"); out.close(); } //end method } // end class ISS - Internet Applications Part 2

  28. Using HTTP Servlet to process web formshttp://java.sun.com/docs/books/tutorial/servlets/client-interaction/req-res.html Requests and Responses Methods in the HttpServlet class that handle client requests take two arguments: • An HttpServletRequestobject, which encapsulates the data from the client   • An HttpServletResponse object, which encapsulates the response to the client The methods for handling HTTP requests and responses are: • public void doGet (HttpServletRequest request, HttpServletResponse response) for requests sent using the GET method. • public void doPost(HttpServletRequest request, HttpServletResponse response) for requests sent using the POST method. ISS - Internet Applications Part 2

  29. HttpServletRequest Objectshttp://java.sun.com/docs/books/tutorial/servlets/client-interaction/req-res.html An HttpServletRequest object • provides access to HTTP header data, such as any cookies found in the request and the HTTP method with which the request was made. • allows you to obtain the arguments that the client sent as part of the request. To access client data sent with an HTTP request:   • The getQueryString method returns the query string. • The getParameter method returns the value of a named parameter. • The getParameterValues method returns an array of values for the named parameter. ISS - Internet Applications Part 2

  30. HttpServletRequest Interface • public String ServletRequest.getQueryString( ); returns the query string of the request. • public String GetParameter(String name): given the name of a parameter in the query string of the request, this method returns the value. • public String[ ] GetParameterValues(String name): returns multiple values for the named parameter – use for parameters which may have multiple values, such as from checkboxes. • public Enumeration getParameterNames( ): returns an enumeration object with a list of all of the parameter names in the query string of the request. ISS - Internet Applications Part 2

  31. HttpServletResponse Objectshttp://java.sun.com/docs/books/tutorial/servlets/client-interaction/req-res.html • An HttpServletResponse object provides two ways of returning data to the user:   • The getWriter method returns a Writer • The getOutputStream method returns a ServletOutputStream • Use the getWriter method to return text data to the user, and the getOutputStream method for binary data. • Closing the Writer or ServletOutputStream after you send the response allows the server to know when the response is complete. ISS - Internet Applications Part 2

  32. HttpServletResponse Interface • public interface HttpServletResponse extends ServletResponse: “The servlet engine provides an object that implements this interface and passes it into the servlet through the service method” – “Java Server Programming” • public void setContentType(String type) : this method must be called to generate the first line of the HTTP response: • public PrintWriter getWriter( ) throws IOException: returns an object which can be used for writing the responses, one line at a time using out.println(). ISS - Internet Applications Part 2

  33. Servlets are Concurrent http://java.sun.com/docs/books/tutorial/servlets/client-interaction/req-res.html HTTP servlets are typically capable of serving multiple clients concurrently: the servlet engine uses a separate thread to invoke each method. Hence servlet methods should be thread-safe: If the methods in your servlet do work for clients by accessing a shared resource, then you must either:   • Synchronize access to that resource, or   • Ensure that the servlet can handle only one client request at a time (by using semaphores or locks).  ISS - Internet Applications Part 2

  34. Handling GET requestshttp://java.sun.com/docs/books/tutorial/servlets/client-interaction/http-methods.html public class BookDetailServlet extends HttpServlet { public void doGet (HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ... // set content-type header before accessing the Writer response.setContentType("text/html"); PrintWriter out = response.getWriter();  // then write the response out.println("<html>" + "<head><title>Book Description</title></head>" + ... );  //Get the identifier of the book to display String bookId = request.getParameter("bookId"); if (bookId != null) { // fetch the information about the book and print it ... } out.println("</body></html>"); out.close(); } ...} ISS - Internet Applications Part 2

  35. Handling POST Requestshttp://java.sun.com/docs/books/tutorial/servlets/client-interaction/http-methods.html public class ReceiptServlet extends HttpServlet { public void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { ... // set content type header before accessing the Writer response.setContentType("text/html"); PrintWriter out = response.getWriter( ); // then write the response out.println("<html>" + "<head><title> Receipt </title>" + ...); out.println("Thank you for purchasing your books from us " + request.getParameter("cardname") + ...); out.close(); } ... } ISS - Internet Applications Part 2

  36. Servlet Examples See Servlet\simple folder in code sample: • HelloWorld.java: a simple servlet • Counter.java: illustrates that a servlet is persistent • Counter2.java: illustrates the use of synchronized method with a servlet • GetForm.html, GetForm.java: illustrates the processing of data sent with an HTTP request via the GET method • PostForm.html, PostForm.java: illustrates the processing of data sent with an HTTP request via the POST method ISS - Internet Applications Part 2

  37. Session State Information • The mechanisms for state information maintenance with CGI can also be used for servlets: hidden-tag, URL suffix, file/database, cookies. • In addition, a session tracking mechanism is provided, using an HttpSession object. ISS - Internet Applications Part 2

  38. Cookies in Java http://java.sun.com/products/servlet/2.2/javadoc/index.html • A cookie has a name, a single value, and optional attributes such as a comment, path and domain qualifiers, a maximum age, and a version number. Some Web browsers have bugs in how they handle the optional attributes, so use them sparingly to improve the interoperability of your servlets. • The servlet sends cookies to the browser by using the HttpServletResponse.addCookie(javax.servelet.http.Cookie)method, which adds fields to HTTP response headers to send cookies to the browser, one at a time. The browser is expected to support 20 cookies for each Web server, 300 cookies total, and may limit cookie size to 4 KB each. • The browser returns cookies to the servlet by adding fields to HTTP request headers. Cookies can be retrieved from a request by using the HttpServletRequest.getCookies( )method. Several cookies might have the same name but different path attributes. ISS - Internet Applications Part 2

  39. Processing Cookies with JavaJava Server Programming – Wrox press • A cookie is an object of the javax.servlet.http.cookie class. • Methods to use with a cookie object: • public Cookie(String name, String value): creates a cookie with the name-value pair in the arguments. • import javax.servlet.http.* • Cookie oreo = new Cookie(“id”,”12345”); • public string getName( ) : returns the name of the cookie • public string getValue( ) : returns the value of the cookie • public void setValue(String _val) : sets the value of the cookie • public void setMaxAge(int expiry): sets the maximum age of the cookie in seconds. ISS - Internet Applications Part 2

  40. Processing Cookies with Java – 2Java Server Programming – Wrox press • public void setPath(java.lang.String uri): Specifies a path for the cookie to which the client should return the cookie. The cookie is visible to all the pages in the directory you specify, and all the pages in that directory's subdirectories. A cookie's path must include the servlet that set the cookie, for example, /catalog, which makes the cookie visible to all directories on the server under /catalog. • public java.lang.String getPath(): Returns the path on the server to which the browser returns this cookie. The cookie is visible to all subpaths on the server. • public String getDomain( ) : returns the domain of the cookie. • if orea.getDomain.equals(“.foo.com”) • … // do something related to golf • public void setDomain(String _domain): sets the cookie’s domain. ISS - Internet Applications Part 2

  41. doGet/doPost Method using cookies Public void doGet(HttpServletResponse req, HttpServletResponse res) throws ServletException, IOExceiption{ res.setContentType(“text/html”); PrintWriter out = res.getWriter( ); out.println (“<H1>Contents of your shopping cart:</H1>”); Cookie cookies[ ]; cookies = req.getCookies( ); if (cookies != null) { for ( int i = 0; i < cookies.length; i++ ) { if (cookies[i].getName( ).startWith(“Item”)) out.println( cookies[i].getName( ) + “: “ + cookies[i].getValue( )); out.close( ); } ISS - Internet Applications Part 2

  42. Servlet & Cookies Example See Servlet\cookies folder in code sample: • Cart.html: web page to allow selection of items • Cart.java: Servlet invoked by Cart.html; it instantiates a cookie object for each items selected. • Cart2.html: web page to allow viewing of items currently in cart • Cart2.java: Servlet to scan cookies received with the HTTP request and display the contents of each cookie. ISS - Internet Applications Part 2

  43. HTTP Session Objectshttp://java.sun.com/products/servlet/2.2/javadoc/index.html • The javax.servlet.http package provides apublic interface HttpSession: Provides a way to identify a user across more than one page request or visit to a Web site and to store information about that user. • The servlet container uses this interface to create a session between an HTTP client and an HTTP server. The session persists for a specified time period, across more than one connection or page request from the user. A session usually corresponds to one user, who may visit a site many times. ISS - Internet Applications Part 2

  44. HTTP Session Object - 2http://java.sun.com/products/servlet/2.2/javadoc/index.html • This interface allows servlets to • View and manipulate information about a session, such as the session identifier, creation time, and last accessed time • Bind objects to sessions, allowing user information to persist across multiple user connections • Session object allows session state information to be maintained without depending on the use of cookies (which can be disabled by a browser user.) • Session information is scoped only to the current web application (ServletContext), so information stored in one context will not be directly visible in another. ISS - Internet Applications Part 2

  45. The Session object ISS - Internet Applications Part 2

  46. Obtaining an HTTPSession Object A session object is obtained using the getSession( ) method of the HttpServletRequest object (from doPost or doGet) public HTTPSession getSession(boolean create): Returns the current HttpSession associated with this request or, if if there is no current session and create is true, returns a new session. If create is false and the request has no valid HttpSession, this method returns null. To make sure the session is properly maintained, you must call this method before the response is committed. public class ShoppingCart extends HttpServlet { public void doPost(HttpServletRequest req, HttpServletRespnse res) throws ServletException, IOException … // get session object HttpSession session = req.getSession(true) if (session != null) { … } … ISS - Internet Applications Part 2

  47. The HTTPSession Object • public java.lang.String getId( ): returns a string containing the unique identifier assigned to this session. The identifier is assigned by the servlet container and is implementation dependent. • public java.lang.Object getAttribute(java.lang.String name): returns the object bound with the specified name in this session, or null if no object is bound under the name. • public java.util.Enumeration getAttributeNames( ): returns an Enumeration of String objects containing the names of all the objects bound to this session. • public void removeAttribute(java.lang.String name): removes the object bound with the specified name from this session. If the session does not have an object bound with the specified name, this method does nothing. ISS - Internet Applications Part 2

  48. Session Object examplebased on an example in “Java Server Programming”, Wrox Press // Get item count from session object Integer itemCount = (Integer)session.getAttribute(“itemCount); // if this is the first call during the session, no attribute exists if (itemCount == null) { itemCount = new Integer(0); // process form data String[ ] itemSelected; String itemName; itemSelected = req.getParameterValues(“item”); // if user has selected items on form, add them to the session object if (itemSelected != null) { for ( int i=0; i < itemSelected.length; i ++) { itemName = itemSelected[I]; itemCount = new Integer(itemCount.intValue( ) +1); session.setAttribue(“Item” + itemCount, itemName); }// end for session.setAttribtue(“itemCount”, itemCount); } // end if ISS - Internet Applications Part 2

  49. Session object example - continued PrintWriter out = res.getWriter( ); res.setContentType(“text/html”); // … output HTML for web page heading // Output current contents of the shopping cart out.println(“<H1>Current Shopping Cart Contents</H1>”); for (int i = 1; i <= ItemCount.intValue( ); i++) ( // retrieve objects from the session object String item = (String) session.getAttribute(“”Item” + i); out.println(“item<br>”); } // … output HTML for web page footing out.close( ); ISS - Internet Applications Part 2

  50. Session Object example See Servlet\Session folder in code sample: • Cart.html: web page to allow selection of items • Cart.java: Servlet invoked by Cart.html; it instantiates a session object which contains descriptions of items selected. • Cart2.html: web page to allow viewing of items currently in cart • Cart2.java: Servlet to display items in the shopping cart, as recorded by the use a session object in the Cart servlet ISS - Internet Applications Part 2

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