Core Java® Volume I–Fundamentals

(Continues)

31

2.4 Running a Graphical Application

From the Library of Hristo Dimov Hristov

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Listing 2.2

 (Continued)

 8 

  * @author Cay Horstmann

 9 

  */

10 

 public class ImageViewer

11 

 {

12 

    public static void main(String[] args)

13 

    {

14 

       EventQueue.invokeLater(() -> {

15 

          JFrame frame = new ImageViewerFrame();

16 

          frame.setTitle("ImageViewer");

17 

          frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

18 

          frame.setVisible(true);

19 

       });

20 

    }

21 

 }

22 

23 

 /**

24 

  * A frame with a label to show an image.

25 

  */

26 

 class ImageViewerFrame extends JFrame

27 

 {

28 

    private JLabel label;

29 

    private JFileChooser chooser;

30 

    private static final int DEFAULT_WIDTH = 300;

31 

    private static final int DEFAULT_HEIGHT = 400;

32 

33 

    public ImageViewerFrame()

34 

    {

35 

       setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);

36 

37 

       // use a label to display the images

38 

       label = new JLabel();

39 

       add(label);

40 

41 

       // set up the file chooser

42 

       chooser = new JFileChooser();

43 

       chooser.setCurrentDirectory(new File("."));

44 

45 

       // set up the menu bar

46 

       JMenuBar menuBar = new JMenuBar();

47 

       setJMenuBar(menuBar);

48 

49 

       JMenu menu = new JMenu("File");

50 

       menuBar.add(menu);

51 

52 

       JMenuItem openItem = new JMenuItem("Open");

53 

       menu.add(openItem);

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From the Library of Hristo Dimov Hristov

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54 

       openItem.addActionListener(event -> {

55 

          // show file chooser dialog

56 

             int result = chooser.showOpenDialog(null);

57 

58 

             // if file selected, set it as icon of the label

59 

             if (result == JFileChooser.APPROVE_OPTION)

60 

             {

61 

                String name = chooser.getSelectedFile().getPath();

62 

                label.setIcon(new ImageIcon(name));

63 

             }

64 

          });

65 

66 

       JMenuItem exitItem = new JMenuItem("Exit");

67 

       menu.add(exitItem);

68 

       exitItem.addActionListener(event -> System.exit(0));

69 

    }

70 

 }

2.5 Building and Running Applets

The first two programs presented in this book are Java applications—stand-alone

programs like any native programs. On the other hand, as mentioned in the pre-

vious chapter, most of the early hype about Java came from its ability to run applets

inside a web browser.

If you are interested in experiencing a “blast from the past,” follow along to see

how to build and run an applet and how to display it in a web browser; if you

aren’t interested, by all means, skip this example and move on to Chapter 3.

Open a terminal window and go to the directory 

corejava/v1ch02/RoadApplet

, then enter

the following commands:

javac RoadApplet.java

jar cvfm RoadApplet.jar RoadApplet.mf *.class

appletviewer RoadApplet.html

Figure 2.9 shows what you see in the applet viewer window. This applet visualizes

how traffic jams can be caused by drivers who randomly slow down. In 1996,

applets were a great tool for creating such visualizations.

The first command is the now-familiar command to invoke the Java compiler.

This compiles the 

RoadApplet.java

 source into the bytecode file 

RoadApplet.class

.

This time, however, you do not run the 

java

 program. First, you bundle the class

files into a “JAR file,” using the 

jar

 utility. Then you invoke the 

appletviewer

 program,

a tool included with the JDK that lets you quickly test an applet. You need to give

this program an HTML file name, rather than the name of a Java class file. The

contents of the 

RoadApplet.html

 file are shown at the end of this section in Listing 2.3.

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2.5 Building and Running Applets

From the Library of Hristo Dimov Hristov

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Figure 2.9

The RoadApplet as viewed by the applet viewer

If you are familiar with HTML, you will notice standard HTML markup and the

applet

 tag, telling the applet viewer to load the applet whose code is stored in

RoadApplet.jar

. The applet viewer ignores all HTML tags except for the 

applet

 tag.

Of course, applets are meant to be viewed in a browser. Unfortunately, nowadays,

many browsers do not have Java support, or make it difficult to enable it. Your

best bet is to use Firefox.

If you use Windows or Mac OS X, Firefox should automatically pick up the Java

installation on your computer. Under Linux, you need to enable the plug-in with

the following commands:

mkdir -p ~/.mozilla/plugins

cd ~/.mozilla/plugins

ln -s 

jdk

/jre/lib/amd64/libnpjp2.so

To double-check, type 

about:plugins

 into the address bar and look for the Java Plug-in.

Make sure it uses the Java SE 8 version of the plug-in—look for a MIME type of

application/x-java-applet;version=1.8

.

Next, turn your browser to 

http://horstmann.com/applets/RoadApplet/RoadApplet.html

, agree to

all the scary security prompts, and make sure the applet appears.

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From the Library of Hristo Dimov Hristov

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Unfortunately, that is not enough to test the applet that you just compiled. The

applet on the 

horstmann.com

 server is digitally signed. I had to expend some effort,

getting a certificate issuer that is trusted by the Java virtual machine to trust me

and sell me a certificate, which I used to sign the JAR file. The browser plug-in

will no longer run untrusted applets. This is a big change from the past, when a

simple applet that draws pixels on the screen would have been confined to the

“sandbox” and would work without being signed. Sadly, not even Oracle has

faith in the security of the sandbox any more.

To overcome this problem, you can temporarily configure Java to trust applets

from the local file system. First, open the Java control panel.

•

In Windows, look inside the Programs section of the control panel.

•

On a Mac, open System Preferences.

•

On Linux, run 

jcontrol

.

Then click the Security tab and the Edit Site List button. Click Add and type in

file:///

. Click OK, accept another security prompt, and click OK again (see

Figure 2.10).

Figure 2.10

Configuring Java to trust local applets

Now you should be able to load the file 

corejava/v1ch02/RoadApplet/RoadApplet.html

 into

your browser and have the applet appear, together with the surrounding text. It

will look something like Figure 2.11.

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2.5 Building and Running Applets

From the Library of Hristo Dimov Hristov

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Figure 2.11

Running the RoadApplet in a browser

The code for the applet class is shown in Listing 2.4. At this point, do not give it

more than a glance. We will come back to writing applets in Chapter 13.

Listing 2.3

RoadApplet/RoadApplet.html

 1 

 

 2 

    

 3 

        

 4