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COMPS311F

COMPS311F. Li Tak Sing. Applets. An applet is any small application that performs one specific task, sometimes running in the context of a larger program, perhaps as a plug-in. A Java applet is an applet delivered to the users in the form of Java bytecode.

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COMPS311F

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  1. COMPS311F Li Tak Sing

  2. Applets • An applet is any small application that performs one specific task, sometimes running in the context of a larger program, perhaps as a plug-in. • A Java applet is an applet delivered to the users in the form of Java bytecode. • Applets are used to provide interactive features to web applications that cannot be provided by HTML alone. • They can capture mouse input and also have controls like buttons or check boxes.

  3. Applets • An applet can also be a text area only, providing, for instance, a cross platform command-line interface to some remote system. • However, applets have very little control over web page content outside the applet dedicated area, of they are less useful for improving the site appearance in general. • Applets can also play media in formats that are not natively supported by the browser.

  4. Applets • Java applets run at a speed that is comparable to (but generally slower than) other compiled languages such as C++, but many times faster than JavaScript. • Java applets can use 3D hardware acceleration that is available from Java. This makes applets well suited for non trivial computation intensive visualizations. • HTML pages may embed parameters that are passed to the applet. Hence the same applet may appear differently depending on the parameters that were passed.

  5. Applets • The same applet may appear differently depending on the parameters that were passed. • Since Java's bytecode is platform independent, Java applets can be executed by broswers for many platforms, including Microsoft Windows, Unix, Mac OC and Linux.

  6. Technical information • Java applets are executed ina sandbox by most web browsers, preventing them from accessing local data like clipboard or file system. • The code of the applet is downloaded from a web server and the browser either embeds the applet into a web page or opens a new window showing the applet's user interface. • The domain from where the applet executable has been downloaded is the only domain to which the usual (unsigned) applet is allowed to communicate. The domain can be different from the domain where the surrounding HTML document is hosted.

  7. JApplet • Java provides two classes for applets: Applet and JApplet. JApplet . • JApplet provides better look and control than Applet.

  8. Methods of JApplet • We do not use the JApple constructor to create a JApplet. It is usually created by the Web browser. • When a web page requests for an applet, it will first download the code from the server, then the default constructor of the applet will be called to create an instance of the applet. • Therefore, it does not make sense to have an applet with non-default constructor. • After the applet has been created, its init() method will be called.

  9. init() of JApplet • Where should we put the initialization code of an applet? The default constructor or init()? • You can do it both ways. However, when the constructor is called, the object may not have been created properly. For example, calling a method of the applet is limited to the current version of the method. If the applet is overridden further, then the version of the method will still be the one that is declared as the constructor.

  10. init() of JApplet • If in the constructor of A, it has invoked the meth() method, the version to be invoked is that of A, not of B. It is because at that time, it is still an A, not a B. • On the other hand, if meth() is invoked in the init() method of A and init() has not been overriddent in B, then the version of meth() invoked would be that of B. • Actually, it is always advisable to avoid invoking a member function in a constructor of a class unless it is private or final.

  11. init() • Therefore, it is better to put initialization code in init() instead of the default constructor. • So there is usually no need to define the default constructor of JApplet.

  12. main() • In all Java application, you need to have the main() method to act as the main program. In JApplet, you do not need to define the main method. The brower will know how to instantiate the applet and execute the appropriate methods of the JApplet when it is executed in a browser.

  13. A simple applet import javax.swing.*; import java.awt.*; public class MyJApplet extends JApplet { public void init() { JPanel p = new JPanel() { public void paint(Graphics g) { super.paint(g); g.drawString("Hello World!", 10, 30); } }; this.getContentPane().add(p); } }

  14. The html file for the applet • In order to execute the applet in a browser, we need an HTML file to tell the browser which applet we want to run. • The basic format is like this: <applet code="applet/MyJApplet.class" width="350" height="350"></applet>

  15. The simple applet • You can try the applet at: • http://plbpc001.ouhk.edu.hk/~mt311f/examples/mt3112010/build/classes/myjapplet.html

  16. Things to note • The MyJApplet is in the package applet. Therefore, MyJApplet.class should be placed in a directory called applet. The HTML file for the applet should be placed in the root directory of applet. • The MyJApplet.class file should be refered to as "applet/MyJApplet.class" in the HTML file. • So, if you have an applet with the full path name, abc.def.GHI, then GHI.class should be placed in a directory abc/def/ and the html file should be placed in the parent directory of abc. The class should be referred as "abc/def/GHI.class".

  17. Other attributes of the <applet> tag. • codebase • This OPTIONAL attribute specifies the base URL of the applet--the directory that contains the applet's code. If this attribute is not specified, then the document's URL is used. With this option, the .class file and the .html file can be anywhere in the system or even in different computers. • For example, assume that abc.def.GHI.class is available at http://ahost.com/xxx/abc/def/GHI.class, then, we can specify the code base as http://ahost.com/xxx.

  18. The html file with codebase <applet code="applet/MyJApplet.class" codebase="http://plbpc001.ouhk.edu.hk/~mt311f/examples/mt3112010/build/classes" width="350" height="350"></applet>

  19. Archive • This OPTIONAL attribute describes one or more achives containing classes and other resources that will be "preloaded". The archives are sparated by ",". For security reasons, the applet's class loader can read only from the same codebase from which the applet was started. This means that archives must be in the same directory as, or in a subdirectory of, the codebase. • Entries like ../a/b.jar will not work unless explicitly allowed for in the security policy file.

  20. An example html file with the archive option. <applet code="applet/MyJApplet.class" archive="dir/code.jar" width="350" height="350"></applet> • Here, we assume that in the directory that contains the html file, there is a subdirectory called dir and in that subdirectory, there is a file called code.jar. • code.jar should then contain the .class file for applet.MyJApplet .

  21. Width and Hight of an applet • This is done by specifying the width and height attributes of the <applet> tag. • The size is measured in terms of pixels.

  22. Param • We can specify some parameters to be passed to an applet. Then these parameters can be accessed through the getParameter() method of JApplet.

  23. An applet that draws a circle according to a parameter public class Circle extends JApplet { public void init() { final int x=Integer.parseInt(this.getParameter("x")); final int y=Integer.parseInt(this.getParameter("y")); final int r=Integer.parseInt(this.getParameter("r")); JPanel p=new JPanel() { public void paint(Graphics g) { super.paint(g); g.drawOval(x-r, y-r, 2*r, 2*r); } }; this.getContentPane().add(p); } }

  24. Network communication of JApplet • As stated eariler, an applet is only allowed to communicate with the host from which the applet was loaded. • We can use the getCodeBase() method of JApplet to get the code base. • Note that there is another method called getDocumentBase(). This method returns the location where the HTML file in which the applet is embeded. • The code base and document base can be different. An applet is only allowed to communicate with the computer at the codebase, not the document base.

  25. getCodeBase() method of JAplet • public URL getCodeBase() • This method return the URL from where the applet is loaded. • URL has the getHost() method that allow us to get the host of the URL.

  26. Java Web start • Java Web Start is a framework that allows users to start application software for the Java Platform directly from the Internet using a web browser. • Unlike Java applets, Web Start applications do not run inside the browser, and the sandbox in which they run need not have as many restrictions, although this can be configured. Web Start has an advantage over applets in that it overcomes many compatibility problems with browsers' Java plugins and different JVM versions. On the other hand, Web Start programs cannot communicate with the browser as easily as applets.

  27. Java Network Launching Protocol(JNLP) • JNLP consists of a set of rules defining how exactly to implement the launching mechanism. JNLP files include information such as the location of the jar package file and the name of the main class for the application, in addition to any other parameters for the program. A properly configured browser passes JNLP files to a Java Runtime Envvironment (JRE) which in turn downloads the application onto the user's machine and starts executing it.

  28. Java Network Launching Protocol(JNLP) • Important Web Start features include the ability to automatically download and install a JRE in the case where the user dose not have Java installed, and for programmers to specify which JRE version a given program needs in order to execute. • Any computer user can use JNLP by simply installing a JNLP client (most commonly Java Web Start). The installation can occur automatically such that the end user sees the client launcher downloading and installing the Java application when first executed.

  29. Java Network Launching Protocol(JNLP) • JNLP works in a similar fashion to how HTTP/HTML works for the web. For rendering a HTML webpage, after the user clicks on a weblink, the browser submits a URL to a webserver, which replies with an HTML file. The browser then requests the resources referred to by this file (images, css), and finally renders the page once it has received enough information. Page rendering usually starts before all resources have downloaded; some resources not critical to the layout of the page (such as images), can follow on afterwards — or on request if the "Load Images Automatically" browser-setting remains unset.

  30. Java Network Launching Protocol(JNLP) • JNLP mirrors this process; in the same way that a Web browser renders a webpage, a JNLP client "renders" a Java app. After the user clicks on a weblink the browser submits a URL to a webserver, which replies with a JNLP file (instead of a HTML file) for the application. The JNLP client parses this file, requests the resources specified (jar files), waits for the retrieval of all required resources, and then launches the application. The JNLP file can list resources as "lazy", which informs the JNLP client that the application does not need those resources to start, but can retrieve them later on when/if the application requests them.

  31. To create a Java Web Start application in netbeans • Create a normal Java project • Select the properties of the project • Click at Web Start in the Categories window. • Select "Enable Web Start" • In Codebase, select "Web Application Deployment" • Select Self-signed.

  32. To create a Java Web Start application in netbeans • A self-signed application is one that has a self-signed certificate. Without a certificate, the Web Start application would have limited rights like an applet. With a certificate, a Web Start application would have rights like a local application.

  33. To create a Java Web Start application in netbeans • Then, create a normal Java application. • Lets assume that we have create a Java class called WebStart.java • Then, netbeans will create the following files for you in the dist directory: • launch.html. This file is the html that that has the Web Start application. • launch.jnlp. This file is the configuration file for the Web Start application. • webstart.jar. This is the jar file that contains the required resoruces like java code.

  34. A simple Web Start Application • You can try the program at:http://plbpc001.ouhk.edu.hk/~mt311f/examples/webstart/dist/launch.html

  35. A simple Web Start Application public class WebStart extends JFrame implements ActionListener { JPanel p=new JPanel(); JTextField text=new JTextField(5); JButton button=new JButton("counter"); int count=0; public WebStart() { this.getContentPane().add(p); p.add(text); p.add(button); text.setEditable(false); pack(); setVisible(true); button.addActionListener(this); }

  36. A simple Web Start Application public void actionPerformed(ActionEvent e) { text.setText(Integer.toString(count++)); } public static void main(String st[]) { WebStart s=new WebStart(); } }

  37. Deployment of the Web Start Application • After the application has been tested locally, you need to deploy the application to a Web server. What you do is to copy all the files in the dist folder of the project to somewhere in the Web server. Then, you need to edit the .jnlp file. • You need to change the codebase to point to the web server.

  38. Deployment of the Web Start Application <?xml version="1.0" encoding="UTF-8" standalone="no"?> <jnlp codebase="http://plbpc001.ouhk.edu.hk/~tsli/abc/" href="launch.jnlp" spec="1.0+"> <information> <title>abc</title> <vendor>tsli</vendor> <homepage href=""/> <description>abc</description> <description kind="short">abc</description>

  39. Deployment of the Web Start Application </information> <update check="always"/> <security> <all-permissions/> </security> <resources> <j2se version="1.5+"/> <jar href="abc.jar" main="true"/>

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