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Jun 02 Cover Story

Volume Number: 18 (2002)
Issue Number: 06
Column Tag: Java Programming

by Andrew S. Downs

Dock Tile Imaging

Changing a Java application’s dock tile at runtime


The Dock Manager API allows a programmer to alter the tile for an application at runtime. Using the Java Native Interface, a Java application can change its tile dynamically as well. This involves a combination of Java and native code.

Two approaches are illustrated in this article. The first captures the pixels from a Java image and passes them to a native library function, which uses the CoreGraphics (Quartz) API to replace the application’s tile (see Figure 1).

Figure 1. A modified Dock tile.

The second example uses QuickDraw to paint a progress bar over the tile, as shown in Figure 2. The progress data is sent from Java to a native library function, and the imaging is done in the library.

Figure 2. The progress bar at the bottom of the tile, showing 60% complete.

If you want to code along with the examples, my recommendation is to first download the JNISample project from Apple’s developer site (see the URLs at the end of this article). That project served as the structural basis for the code in this article. All the build settings are already in place, making it an easy-to-use learning tool. (There are targets for compiling both the Java and native code, generating the javah file, building a library, etc.) I kept the filenames the same, but replaced the content of the various files. In the listings below you will see the filenames as they exist in that project.


One class (DockTiler) provides most of the functionality for this example. It relies on two other classes for getting and drawing (offscreen) an image from a local file. You can also load an image via a non-local URL, which would allow the app to change the tile in response to outside conditions. For example, an application that retrieves weather data can change the tile to reflect current conditions or the forecast.

The JNISample class contains main(), the entry point for the Java application. It is used simply to instantiate DockTiler and invoke one of its instance methods.

Listing 1:

The classes contained here include:
   JNISample: creates and calls a DockTiler instance.
   DockTiler: loads the image and sends it to the native drawing code.
   LocalFiler: allows the user to select a local file for display in the tile.
   PictureFrame: an offscreen Canvas which draws the image, allowing DockTiler
    to retrieve the image pixels.

// The image support comes from the AWT and the image package. 
import java.awt.*;
import java.awt.image.*;
import java.util.*;

public class JNISample {
   public JNISample() {}

   // Test code.
   public static void main (String args[]) {
      DockTiler dock = new DockTiler();

      System.out.println( “Finished.” );

class DockTiler {
   // If we have trouble loading an image, the values of its width and height will 
   // remain at -1. See loadImage().
   int mWidth = -1, mHeight = -1;
   int mPixels[];

   static {
      // Load the library when this class gets loaded.
      System.loadLibrary( “Example” );

   public DockTiler() {
   // These are the two functions in the shared library that we will call.
   // Note the declaration as native.
   native void setDockTile( int[] pixels, int width, 
      int height );

   native void updateProgressBar( int currPercent );
   // The primary test driver.
   public void test() {
      setDockTile( mPixels, mWidth, mHeight );


   // Read in the image and retrieve its pixels.
   protected void loadImage() {
      LocalFile lf = new LocalFile();
      String filename = lf.getFilePath();
      Image image = 
         Toolkit.getDefaultToolkit().getImage( filename );

      // Send the image to the Canvas, where it will be rendered.
      PictureFrame pf = new PictureFrame( image );
      Frame f = new Frame( “Image” );

      // Setup offscreen.
      f.setBounds( -250, -250, 200, 200 );

      f.setLayout( new BorderLayout() );
      f.add( “Center”, pf );
      pf.setSize( 128, 128 );

      // An invisible window won’t render an image.
      f.setVisible( true );

      // Use the Canvas as the observer during the loading process.
      int width = image.getWidth( pf );
      int height = image.getHeight( pf );
      // Allocate storage for the image data.
      mPixels = new int[ width * height ];
      // Create an object to copy the image pixel data into our array.
      PixelGrabber pg = new PixelGrabber( image, 0, 0, 
         width, height, mPixels, 0, width );
      // Copy the pixels to the array.
      try {
         // Check for error using bit values in the ImageObserver class.
         if ( ( pg.getStatus() & ImageObserver.ABORT ) != 0 )
         // If successful, set instance attributes to legitimate values (not –1).
         mWidth = width;
         mHeight = height;
      catch ( InterruptedException e ) {

   // For a task that may take some time, it helps to wrap it in a separate method or
   // even class, and spin it off as a thread. Here, simply get the current progress
   // and display it.
   protected void performTask() {
      int percentComplete = 0;
      boolean taskComplete = false;
      while ( !taskComplete ) {
         // Call the native method that draws the progress bar.
         updateProgressBar( percentComplete );
         percentComplete = updateTask();
         if ( percentComplete >= 100 )
            taskComplete = true;

   int mCount = 0;
   // Lengthy tasks will use a sophisticated approach to determining completion.
   // This example uses a simple loop so we can watch the bar move.
   protected int updateTask() {
      return mCount++;

class LocalFile {
   FileDialog mFileDialog = null;
   // Display a dialog asking the user to choose a file.
   public LocalFile() {
      if ( mFileDialog == null ) {
         mFileDialog = new FileDialog( new Frame(), 
            “Select an image file”, FileDialog.LOAD );
   // Build and return the path to the file (if selected).
   public String getFilePath() {
      mFileDialog.setVisible( true );
      String retval = “”;
      if ( mFileDialog.getFile() != null && 
         mFileDialog.getFile().length() > 0 ) {
         retval = mFileDialog.getDirectory();
         if ( !retval.endsWith( 
            System.getProperty( “file.separator” ) ) )
            retval += System.getProperty( “file.separator” );
         retval += mFileDialog.getFile();
      return retval;

// A subclass of java.awt.Canvas that draws an Image object.
class PictureFrame extends Canvas {
   Image mImage;
   PictureFrame( Image img ) {
      mImage = img;
      setBackground( Color.white );
   public void update( Graphics g ) {
      paint( g );
   // Draw the image.
   public void paint( Graphics g ) {
      g.drawImage( mImage, 0, 0, this );

Java Native Interface Code

JNI code is C code that bridges the Java and native worlds, handling the conversion between Java data types and their native counterparts. The JNIEnv pointer in each function indirectly points to a function table containing JNI functions, and the jobject references the instance of the class making the call. Any additional arguments are the values passed from the Java code to the native code.

Listing 2: ExampleJNILib.c

The JNI glue for converting arguments prior to calling through to the native code.

#include “JNISample.h”
#include “ExampleDylib.h”

JNIEXPORT void JNICALL Java_DockTiler_setDockTile( 
   JNIEnv *env, jobject this, jintArray pixels, jint width, 
   jint height ) {
   // Obtain a pointer to the array to pass to the native function.
   jint *theArray = (*env)->GetIntArrayElements( 
      env, pixels, NULL );

   if ( theArray != NULL ) {
      // Call the library function.
      // Note that no adjustments are made to the primitive values.
      setDockTile( theArray, width, height );
      // Tell the VM we are no longer interested in the array.
      (*env)->ReleaseIntArrayElements( env, pixels, theArray, 
         0 );

JNIEXPORT void JNICALL Java_DockTiler_updateProgressBar( 
   JNIEnv *env, jobject this, jint currPercent ) {
   // Call the library function.
   // No additional translation is needed on primitive values.
   updateProgressBar( currPercent );

The Library

The native library contains the actual tile drawing code. One function creates an image and replaces the existing tile with the new one. The second function uses a completion percentage value to determine how much of the progress bar to paint. It draws over the bottom of the existing tile.

Listing 3: ExampleDylib.c

Perform drawing in the Dock tile.


// Args include the array of pixel RGBA values, and the actual image width and height.
extern void setDockTile( int * imagePixels, int width, 
   int height ) {
   // How many bytes in each pixel? Java uses 4-byte ints.
   int kNumComponents = 4;
   OSStatus   theError;

   // Several CoreGraphics variables.
   CGContextRef theContext;
   CGDataProviderRef theProvider;
   CGColorSpaceRef theColorspace;
   CGImageRef theImage;

   // How many bytes in each row?
   size_t bytesPerRow = width * kNumComponents;

   // Obtain graphics context in which to render.
   theContext = BeginCGContextForApplicationDockTile();

   if ( theContext != NULL ) {   
      // Use the pixels passed in as the image source.
      theProvider = CGDataProviderCreateWithData( 
         NULL, imagePixels, ( bytesPerRow * height ), NULL );
      theColorspace = CGColorSpaceCreateDeviceRGB();
      // Create the image. This is similar to creating a PixMap. 
      // - The width and height were passed as arguments. 
      // - The next two values (8 and 32) are the bits per pixel component and 
      //    total bits per pixel, respectively. 
      // - bytesPerRow was calculated above. 
      // - Use the colorspace ref obtained previously. 
      // - The alpha or transparency data is in the first byte of each pixel. 
      // - Use the data source created a few lines above.
      // - The remaining parameters are typical defaults. Consult the API docs for 
      //    more info.
      theImage = CGImageCreate( width, height, 8, 32, 
         bytesPerRow, theColorspace, kCGImageAlphaFirst, 
         theProvider, NULL, 0, kCGRenderingIntentDefault );
      CGDataProviderRelease( theProvider );
      CGColorSpaceRelease( theColorspace );
      // Set the created image as the tile.
      theError = SetApplicationDockTileImage( theImage );

      CGContextFlush( theContext );
      CGImageRelease( theImage );

      EndCGContextForApplicationDockTile( theContext );

extern void updateProgressBar( const int currPercent ) {
   CgrafPtr thePort;
   Rect   theRect;
   float   right = 0;
   // Obtain graphics context.
   thePort = BeginQDContextForApplicationDockTile();

   if ( thePort != NULL ) {
      // Good ol’ QuickDraw.
      GetPortBounds( thePort, &theRect );
      // Initially, draw the background of the bar and frame it.
      if ( currPercent == 0 ) {
         SetRect( &theRect, theRect.left, theRect.bottom - 10, 
            theRect.right, theRect.bottom );
         ForeColor( redColor );
         PaintRect( &theRect );
         ForeColor( blackColor );
         FrameRect( &theRect );
      // Calculate right-edge of progress bar.
      if ( currPercent >= 100 )
         right = ( float )theRect.right;
         right = ( ( ( float )theRect.right – 
            ( float )theRect.left ) / 
            ( float )100 ) * ( float )currPercent;

      // Draw the entire progress bar up until this point.
      ForeColor( greenColor );

      // Inset the progress rectangle on our own.
      SetRect( &theRect, theRect.left + 1, 
         theRect.bottom - 9, ( int )right, 
         theRect.bottom - 1 );

      PaintRect( &theRect );

      QDFlushPortBuffer( thePort, NULL );

      EndQDContextForApplicationDockTile( thePort );

The image creation using the CoreGraphics API is the trickiest part. This example uses the fact that a Java int is 4 bytes, and that alpha (transparency) data, if included, is stored in the most significant byte. After some trial and error, I found that the settings shown here work for the images I tested against.

Useful URLs

I used quite a few outside sources (primarily Apple) in preparing this article. Though some of these URLs may change, I want to at least point you in the right direction.

Andrew has been a Java fan since his first encounter with the language at the WebEdge III conference in 1996. You can reach him at


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