1. COMP201 Java Programming Part II: GUI Programming Topic 11: Applets Chapter 10

2. COMP201 Topic 11 / Slide 2 Introduction l Applets are java programs that run within browsers l Example: n Jmol applet –http://jmol.sourceforge.net/applet/ n NASA 3D real-time satellite tracker –http://liftoff.msfc.nasa.gov/realtime/jtrack/3d/JTrack3d.htmlhttp://liftoff.msfc.nasa.gov/realtime/jtrack/3d/JTrack3d.html Applet Client BrowserServer Applet User

3. COMP201 Topic 11 / Slide 3 An Example l Creation n Converting applications to applets l Transportation n Jar files: Move applets from servers to browsers quickly l Operation n Applet life cycle n Security restrictions n Getting resources from home n Communicating with browser Applet Client BrowserServer Applet User

4. COMP201 Topic 11 / Slide 4 An Example An applet is a Java class which extends java.applet.Applet If Swing components are used, the applet must extend from javax.swing.JApplet public class NotHelloWorldApplet extends JApplet { public void init() { JLabel label = new JLabel("Not a Hello, World applet", SwingConstants.CENTER); add(label); } } //NotHelloWorldApplet.java

5. COMP201 Topic 11 / Slide 5 l Compile and run Compile: javac NotHelloWorldApplet.java n Run: –Create a HTML file that tells the browser which file to load and how to size the applet This is an example. This text shown if browser doesn’t do Java. –View the HTML file with a browser or the command appletviewer n Note: –Textpad: Cntrl+3 – creates a simple html file and show it with appletviewer An Example

6. COMP201 Topic 11 / Slide 6 An Example l More notes n To view applet, one needs Java 2 enabled browser (recent version of IE and Netscape, e.g. IE 6, Netscape 6, Netscape 7. Netscape 4 is not Java 2 enabled) n Class files are cached by browser. –Restart browser after each modification –Alternatively, one can clear the cache from Java console, which can be accessed from Netscape or control panel on Windows

7. COMP201 Topic 11 / Slide 7 An Example

8. COMP201 Topic 11 / Slide 8 class NotHelloWorldFrame extends JFrame { public NotHelloWorldFrame() { setTitle("NotHelloWorld"); setSize(300, 200); JLabel label = new JLabel("Not a Hello, World application", SwingConstants.CENTER); add(label); }} public class NotHelloWorldApplication { public static void main(String[] args) { NotHelloWorldFrame frame = new NotHelloWorldFrame(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); }} Compare with appl.: NotHelloWorld.java

9. COMP201 Topic 11 / Slide 9 l Applets are created, run, and destroyed by web browser n Don’t set size for an applet: determined by HTML file. n Don’t set title for an applet: applets cannot have title bars. –Can have menus. No need to explicitly construct an applet. Construction code placed inside the init method. There is no main method. n An applet cannot be closed. It terminates automatically when the browser exit No need to call method setVisible(true). An applet is displayed automatically. An Example

10. COMP201 Topic 11 / Slide 10 Outline l An example l Creation n Converting applications to applets l Transportation n Jar files: Move applets from servers to browsers quickly l Operation n Applet life cycle n Security restrictions n Getting resources from home n Communicating with browser

11. COMP201 Topic 11 / Slide 11 Creating Applets from Applications l Non-IO applications for now n Pop up window for application n Embed top-level frame of application inside browser

12. COMP201 Topic 11 / Slide 12 Popup Applets from Applications l Popping up a window for application. n Assume: Separate class for creating and showing a top-level frame. (If this class also does some other things, move the other things to other classes.) class NotHelloWorldFrame extends JFrame {…} public class NotHelloWorldApplication { public static void main(String[] args) { JFrame frame = new NotHelloWorldFrame(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_ CLOSE); frame.setVisible(true); }

13. COMP201 Topic 11 / Slide 13 l Steps of conversion: n Delete the class for creating and showing the top-level frame Add an applet class whose init method contains the same instructions as main method of deleted class. n Remove code for closing window public class NHWApplet extends JApplet { public void init() { JFrame frame = new NotHelloWorldFrame(); frame.setVisible(true); } } //NHWApplet.java n The popup window coming with a warning message for security reasons, (which can be avoided for signed applets). Creating popup Applets from Applications

14. COMP201 Topic 11 / Slide 14 l Steps of conversion n Delete the class for creating and showing the top-level frame n Convert the top-level frame class into an applet –JFrame class => JApplet class; must be public –Remove setSize: set in HTML file –Remove setTitle: Applet cannot have title bar –Replace constructor with init. Creating Embedded Applets from Applications

15. COMP201 Topic 11 / Slide 15 public class PopupCalculatorApplet extends JApplet { public void init() { // create a frame with a calculator panel final JFrame frame = new JFrame(); frame.setTitle("Calculator"); frame.setSize(200, 200); frame.add(new CalculatorPanel()); // add a button that pops up or hides the calculator JButton calcButton = new JButton("Calculator"); add(calcButton); calcButton.addActionListener(new ActionListener() { public void actionPerformed(ActionEvent event) { frame.setVisible(!frame.isVisible());} }); }} Example: Embedded Calculator Applet

16. COMP201 Topic 11 / Slide 16 A Calculator Click this button to pop up a calculator. <applet align="middle" code="PopupCalculatorApplet.class" width="100" height="20"> HTML of the Embedded Calculator Applet

17. COMP201 Topic 11 / Slide 17 import java.awt.*; import javax.swing.*; public class CalculatorApplet extends JApplet { public void init() { CalculatorPanel panel = new CalculatorPanel(); add(panel); }} //CalculatorApplet.java A Calculator Here is a calculator, just in case you can't find yours. //Calculator.html Example: Popup Calculator Applet

18. COMP201 Topic 11 / Slide 18 Outline l An example l Creation n Converting applications to applets l Transportation n Jar files: Move applets from servers to browsers quickly l Operation n Applet life cycle n Security restrictions n Getting resources from home n Communicating with browser

19. COMP201 Topic 11 / Slide 19 Transportation of Applets via JAR Files l PopupCalculatorApplet involve three classes n CalculatorFrame.class, CalculatorPanel.class PopupCalculatorApplet.class l HTML file contains name of the applet class n l Class loader First fetches PopupCalculatorApplet.class In the process, it notices that some other classes are also needed. It then makes net connections to get them. n Many connections might be needed in general, especially when there are associated resources such as images and sounds.

20. COMP201 Topic 11 / Slide 20 JAR Files l Java supports an improved method for loading class files. It allows you to package all the needed class files into as single JAR file which could be downloaded with a single HTTP request to the server. l A JAR file is simply a zip file that contains classes, other files that a program may need and a manifest file, which describes special features of the archive. l The manifest file is called MANIFEST.MF and is located in a special META-INF subdirectory of the JAR file. l A manifest file can have many entries which are groouped into section. The first section is called the main section, which applies to the whole JAR file. Subsequent entries specify properties of named entities such as individual files, packages, or URLs.

21. COMP201 Topic 11 / Slide 21 JAR File l An example of manifest file: Manifest-Version:1.0 Lines describing this archive Name:Woozle.class Lines describing this file Name: com/myCompany/mypkg/ Lines describing this package l To make an new JAR file with a manifest run jar cfm MyArchive.jar mainclass.mf *class l To add items to the manifest, place the additions into a text file and use a command jar umf MyJarFile.jar manifest-additions.mf

22. COMP201 Topic 11 / Slide 22 Jar Files l Creating jar files n jar cvf PopupCalculatorAppletClasses.jar *class l In general: jar cvf myJarFile.jar *.class *.gif pack all files ending with.class or.gif

23. COMP201 Topic 11 / Slide 23 Jar Files Refer to JAR files in the APPLET tag l JAR file is downloaded via one net connection. l Class loader tries to find necessary files in JAR file before attempting to get them from the net.

24. COMP201 Topic 11 / Slide 24 Diversion/Self-Running Jar File l Just as with applets, when you ship an application, you should also package the class files and other resources required by the program in a Jar file. l You can even set it up to be Self-Running (executable) jar file. In that case you need to indicate the main class in the manifest file. 1. Create mainclass.mf with one line : Main-Class: MyApplet 2. Create the JAR file with the manifest file plus the update from the mainclass.mf 1.jar cvfm MyJarFile.jar mainclass.mf *class 3. Or, one can update the manifest files of an existing jar file jar umf mainclass.mf MyJarFile.jar l Run: n java -jar MyJarFile.jar n Or click on JAR file icon

25. COMP201 Topic 11 / Slide 25 Outline l An example l Creation n Converting applications to applets l Transportation n Jar files: Move applets from servers to browsers quickly l Operation n Applet life cycle n Security restrictions n Getting resources from home n Communicating with browser

26. COMP201 Topic 11 / Slide 26 Applet Life Cycle l An application starts from main and runs until exit l Applets are controlled by browser through 4 methods n init() –Called when loaded by browser n start() –Called right after init and when user return to page n stop() –Called when user moves off page n destroy() –Called when browser shuts down. l Overwrite the methods to control applet behavior non-existenton page off page init( ) destroy( ) stop( ) destroy( )start( )

27. COMP201 Topic 11 / Slide 27 l public void init() n One-time initialization when first loaded n Good place to process parameters and add interface components. l public void start() n Called whenever user returns to the page containing the applet after having gone off to other pages. n Can be called multiple times. n Good place to resume animation or game Applet Life Cycle

28. COMP201 Topic 11 / Slide 28 l public void stop() n Called when user moves off page (to other pages) n Good place to stop time-consuming activities such as animation and audio playing. l public void destroy() n Called when browser shuts down. n Good place to reclaim non-memory-dependent resources such as graphics contexts. n Normally, no need to worry. l Example: sound (Stop Playing when going out of page) Compare with the one of the two other versions. Applet Life Cycle

29. COMP201 Topic 11 / Slide 29 Outline l An example l Creation n Converting applications to applets l Transportation n Jar files: Move applets from servers to browsers quickly l Operation n Applet life cycle n Security restrictions n Getting resources from home n Communicating with browser

30. COMP201 Topic 11 / Slide 30 Security Restrictions l Applets are downloaded from the net and executed by a browser’s JVM immediately. l User never gets a chance to confirm or to stop an applet from running. l Consequently, applets are restricted in what they can do. The applet security manager is responsible for enforcing access rules and throws an SecurityException when an access rule is violated.

31. COMP201 Topic 11 / Slide 31 Security Restriction l By default, an applet is restricted to run “inside the sandbox”. Strictest security restrictions. l Signed applets can have more access privileges. l For now, we consider only applets playing in the sandbox.

32. l Access rights for Applets and Java Applications (JA) BR: applets running inside a browser with default applet security model AV: applets running insider Applet viewer BR Read local file N Write local file N Get file info. N Delete file N Run another program N Read theuser.name property N Connect to network port on home server Y Connect to network port on other server N Load Java library N Call exit N Create a pop-up window warning AVJA YY YY YY NY YY YY YY YY YY YY YY

33. COMP201 Topic 11 / Slide 33 Outline l An example l Creation n Converting applications to applets l Transportation n Jar files: Move applets from servers to browsers quickly l Operation n Applet life cycle n Security restrictions n Getting resources from home n Communicating with browser Applet Client BrowserServer Applet User

34. COMP201 Topic 11 / Slide 34 Resources for Applets Classes in both applets and applications often use associated data file, such as image, sound file, Text file, binary file. In Java those associated files are called resources. 1. One can provide information to applets in HTML file 2. Application/Applets can access resources at home server:

35. COMP201 Topic 11 / Slide 35 1. Passing Info to Applets via HTML File l In HTML file, use PARAM, NAME, VALUE tags …. In applet, use the getParameter method of the Applet class getParameter("title"); //returns "Diameters of the Planets “ String vString = getParameter(“values”); //returns “9” if (vString == null ) {do something} // precaution else int v=Integer.parseInt(vString); //must parse to get numbers Chart.java, Chart.html

36. COMP201 Topic 11 / Slide 36 2. Accessing Resources inside a class The class loader knows how to search for class files from the class path, or in an archive, or on a web server. You can call the getResource(filename) to get files that are not class files. x.class gives one an object of the Class class that contain information of x. Class is a special class and has method getResource. C.f. Object class x.class.getResource( resourceName ) returns URL of resource

37. COMP201 Topic 11 / Slide 37 Applets can handle images in GIF, PNG or JPEG format using: getImage(url), getImage(url, name) For example, to load a image file “about.gif”: URL url = AboutPanel.class.getRessource(“about.gif”); ImageIcon = new ImageIcon(url); Means: “locate the about.gif at the same place where you find AboutPanel.class” Accessing Images at Home Server

38. COMP201 Topic 11 / Slide 38 2. Accessing Resources at Home Server Inside a subclass of Applet getDocumentBase returns URL of the html file that calls the applet getCodeBase returns URL of the applet itself Example: Image cat = getImage(getCodeBase(), “image/cat.gif”); AudioClip meow = getAudioClip(getCodeBase(), “audio/meow.au”); or: play(getcodeBase(), “audio/meow.au”); The getImage and getAudioClip/play methods automatically search the JAR files of the applet first and load them immediately. Otherwise, the browser requests it from the web server.

39. COMP201 Topic 11 / Slide 39 l Applets can handle audio files in AU, AIFF, WAV, or MIDI format. Audio Clips (java.applet.Applet) Load: AudioClip getAudioClip(url), AudioClip getAudioClip(url, name) Then use play method of AudioClip to play and the stop method to stop Play without first loading: void play(url), void play(url, name) //SoundApplet.java Accessing Audio Files at Home Server

40. COMP201 Topic 11 / Slide 40 Accessing Text Files at Home Server Find the URL of text file and the create an InputStream using the openStream method of URL class InputStream in = url.openStream(); Or create an InputStream directly using the getResourceAsStream method of the Class class. InputStream in = x.class.getResoruceAsStream( fileName); The InputStream can be nested with other streams in the normal way (see Topic 4)

41. COMP201 Topic 11 / Slide 41 Accessing Text Files at Home Server For example: URL url = AboutPanel.class.getRessource(“about.txt”); InputStream stream = url.openStream(); or InputStream stream = AboutPanel.class.getRessourceAsStream(“about.txt”); Scanner in = Scanner.create(stream); While (in.hasNext()) textArea.append(in.nextLine() + “\n”); //resourceTest.java, resourceTest.html

42. COMP201 Topic 11 / Slide 42 Outline l An example l Creation n Converting applications to applets l Transportation n Jar files: Move applets from servers to browsers quickly l Operation n Applet life cycle n Security restrictions n Getting resources from home n Communicating with browser Applet Client BrowserServer Applet User

43. COMP201 Topic 11 / Slide 43 Communication with Browser To establish a communication channel between an applet and the ambient browser, use the getAppletContext method of the Applet class. The method returns an object of the AppletContext, which is an interface. An applet has access to two areas of the ambient browsers: The status line and the web page display area. Both use methods of the AppletContext To display a string in the status line: showStatus(“Loading data … Please wait” ) To show a different web page : showDocument(URL url ) showDocument(URL url, String target ) Ex: URL u = new URL(“http://ust.hk); getAppletContext().showDocument(u); ShowPageApplet.java

44. COMP201 Topic 11 / Slide 44 Java Web Start l A technology for simplifying deployment of Java applications n Gives you the power to launch full-featured applications with a single click from your Web browser. n You can now download and launch applications, such as a complete spreadsheet program or an Internet chat client, without going through complicated installation procedures.