Swing Hacks

Stuff Stuff in JARs #85

Chapter 11, Native Integration and Packaging

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HACK

An Obvious Secret

JAR files—the acronym is short for Java ARchive—must be the best-known,

least-used feature in Java. Many developers throw a JAR in their classpath to

pick up some standard extension API or third-party library, but how many

actually distribute their software that way?

And JARs aren’t just about code. It’s really easy to put the files your pro-

gram needs into a JAR. This has the added advantage of hiding your images,

sounds, default settings, and so forth from end users.

But to load these items, you need to make a change in how you load stuff in

your code. Instead of specifying a known path or URL, you ask a

ClassLoader to find these resources along the classpath. By doing this, you

can get your resources from flat files while you’re developing, and then eas-

ily switch to getting them from inside a JAR when the code is deployed in

the field.

The key is the

ClassLoader’s getResource( ) and getResources( ) methods,

which take a path relative to the loaded class and return a

URL and an array

of

URLs respectively. A getResourceAsStream( ) method converts the URL to an

InputStream as a convenience.

To clarify the relative path: say you have a directory that includes your com-

piled classes in a path like com/mycompany/mypackage/…, an images direc-

tory, and a sounds directory. A relative path would be one of the form

images/something.png, sounds/something.aiff, etc. By using a resource on the

classpath, there is no difference (to the user) between files in a JAR and files

in sub-directories on a filesystem. Either way, you get a

URL that you can use

by passing it to methods that take

URL arguments, by opening an

InputStream from it, etc.

Showing Off

Example 11-7 shows an example of this technique. It uses the getResource( )

method to load an image and put it in an ImageIcon, which in turn is used to

create a

JButton. It also loads in a sound, which is played via JavaSound

when you click the button.

Example 11-7. Loading image and sound as resources along the classpath

import java.awt.*;

import java.awt.event.*;

import javax.swing.*;

import javax.sound.sampled.*;

import java.net.*;

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Chapter 11, Native Integration and Packaging

#85 Stuff Stuff in JARs

HACK

public class JarResourceLoading extends JFrame

implements ActionListener {

JButton button;

ImageIcon buttonIcon;

Clip buhClip;

public final static String SOUND_PATH = "sounds/buhbuhbuh.aiff";

public final static String IMAGE_PATH = "images/keagan-buh.jpeg";

public JarResourceLoading ( ) {

super ("Resources from .jar");

// get image and make button

URL imageURL = getClass().getClassLoader( ).getResource (IMAGE_PATH);

System.out.println ("found image at " + imageURL);

buttonIcon = new ImageIcon (imageURL);

button = new JButton ("Click to Buh!", buttonIcon);

button.setHorizontalTextPosition (SwingConstants.CENTER);

button.setVerticalTextPosition (SwingConstants.BOTTOM);

button.addActionListener (this);

getContentPane( ).add (button);

// load sound into Clip

try {

URL soundURL = getClass().getClassLoader( ).getResource (SOUND_PATH);

System.out.println ("found sound at " + soundURL);

Line.Info linfo = new Line.Info (Clip.class);

Line line = AudioSystem.getLine (linfo);

buhClip = (Clip) line;

AudioInputStream ais = AudioSystem.getAudioInputStream(soundURL);

buhClip.open(ais);

} catch (Exception e) {

e.printStackTrace( );

}

}

public void actionPerformed (ActionEvent e) {

System.out.println ("click!");

if (buhClip != null) {

buhClip.setFramePosition (0);

buhClip.start( );

}

else

JOptionPane.showMessageDialog (this,

"Couldn't load sound",

"Error",

JOptionPane.ERROR_MESSAGE);

}

public static final void main (String[] args) {

JFrame frame = new JarResourceLoading( );

Example 11-7. Loading image and sound as resources along the classpath (continued)

Stuff Stuff in JARs #85

Chapter 11, Native Integration and Packaging

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431

HACK

Notice how the paths to the sound and the image are relative:

public final static String SOUND_PATH = "sounds/buhbuhbuh.aiff";

These paths specify the path from a given starting point. As

getResource( )

checks each entry in its classpath—whether they’re directories or JAR files

(ZIP files are also allowed)—the class loader looks for a directory called

sounds and, if that’s found, for an entry inside it called buhbuhbuh.aiff. It’s

also worth noting that you use the Unix-style forward slashes, regardless of

what operating system this is run on.

When you compile and run this code from the source directory, the result

looks like Figure 11-10.

Along with showing this simple GUI, it also prints to standard out the URLs

returned by

getResource( ). Running from the source directory, the output

looks like this:

[tonberry:] cadamson% java JarResourceLoading

found image at file:/Users/cadamson/Documents/O'Reilly/books/swing%20hacks/

HacksBook/PackagingInstalling/97/images/keagan-buh.jpeg

found sound at file:/Users/cadamson/Documents/O'Reilly/books/swing%20hacks/

HacksBook/PackagingInstalling/97/sounds/buhbuhbuh.aiff

frame.pack( );

frame.setVisible(true);

}

}

Figure 11-10. Application using image and sound loaded with getResource( )

Example 11-7. Loading image and sound as resources along the classpath (continued)