public class

MediaTracker

extends Object
implements Serializable
java.lang.Object
   ↳ java.awt.MediaTracker

Class Overview

The MediaTracker class is a utility class to track the status of a number of media objects. Media objects could include audio clips as well as images, though currently only images are supported.

To use a media tracker, create an instance of MediaTracker and call its addImage method for each image to be tracked. In addition, each image can be assigned a unique identifier. This identifier controls the priority order in which the images are fetched. It can also be used to identify unique subsets of the images that can be waited on independently. Images with a lower ID are loaded in preference to those with a higher ID number.

Tracking an animated image might not always be useful due to the multi-part nature of animated image loading and painting, but it is supported. MediaTracker treats an animated image as completely loaded when the first frame is completely loaded. At that point, the MediaTracker signals any waiters that the image is completely loaded. If no ImageObservers are observing the image when the first frame has finished loading, the image might flush itself to conserve resources (see flush()).

Here is an example of using MediaTracker: 

 import java.applet.Applet;
 import java.awt.Color;
 import java.awt.Image;
 import java.awt.Graphics;
 import java.awt.MediaTracker;

 public class ImageBlaster extends Applet implements Runnable {
      MediaTracker tracker;
      Image bg;
      Image anim[] = new Image[5];
      int index;
      Thread animator;

      // Get the images for the background (id == 0)
      // and the animation frames (id == 1)
      // and add them to the MediaTracker
      public void init() {
          tracker = new MediaTracker(this);
          bg = getImage(getDocumentBase(),
                  "images/background.gif");
          tracker.addImage(bg, 0);
          for (int i = 0; i < 5; i++) {
              anim[i] = getImage(getDocumentBase(),
                      "images/anim"+i+".gif");
              tracker.addImage(anim[i], 1);
          }
      }

      // Start the animation thread.
      public void start() {
          animator = new Thread(this);
          animator.start();
      }

      // Stop the animation thread.
      public void stop() {
          animator = null;
      }

      // Run the animation thread.
      // First wait for the background image to fully load
      // and paint.  Then wait for all of the animation
      // frames to finish loading. Finally, loop and
      // increment the animation frame index.
      public void run() {
          try {
              tracker.waitForID(0);
              tracker.waitForID(1);
          } catch (InterruptedException e) {
              return;
          }
          Thread me = Thread.currentThread();
          while (animator == me) {
              try {
                  Thread.sleep(100);
              } catch (InterruptedException e) {
                  break;
              }
              synchronized (this) {
                  index++;
                  if (index >= anim.length) {
                      index = 0;
                  }
              }
              repaint();
          }
      }

      // The background image fills the frame so we
      // don't need to clear the applet on repaints.
      // Just call the paint method.
      public void update(Graphics g) {
          paint(g);
      }

      // Paint a large red rectangle if there are any errors
      // loading the images.  Otherwise always paint the
      // background so that it appears incrementally as it
      // is loading.  Finally, only paint the current animation
      // frame if all of the frames (id == 1) are done loading,
      // so that we don't get partial animations.
      public void paint(Graphics g) {
          if ((tracker.statusAll(false) & MediaTracker.ERRORED) != 0) {
              g.setColor(Color.red);
              g.fillRect(0, 0, size().width, size().height);
              return;
          }
          g.drawImage(bg, 0, 0, this);
          if (tracker.statusID(1, false) == MediaTracker.COMPLETE) {
              g.drawImage(anim[index], 10, 10, this);
          }
      }
 }

Summary

Constants
int ABORTED Flag indicating that the downloading of media was aborted.
int COMPLETE Flag indicating that the downloading of media was completed successfully.
int ERRORED Flag indicating that the downloading of media encountered an error.
int LOADING Flag indicating that media is currently being loaded.
Public Constructors
MediaTracker(Component comp)
Creates a media tracker to track images for a given component.
Public Methods
void addImage(Image image, int id)
Adds an image to the list of images being tracked by this media tracker.
synchronized void addImage(Image image, int id, int w, int h)
Adds a scaled image to the list of images being tracked by this media tracker.
boolean checkAll(boolean load)
Checks to see if all images being tracked by this media tracker have finished loading.
boolean checkAll()
Checks to see if all images being tracked by this media tracker have finished loading.
boolean checkID(int id, boolean load)
Checks to see if all images tracked by this media tracker that are tagged with the specified identifier have finished loading.
boolean checkID(int id)
Checks to see if all images tracked by this media tracker that are tagged with the specified identifier have finished loading.
synchronized Object[] getErrorsAny()
Returns a list of all media that have encountered an error.
synchronized Object[] getErrorsID(int id)
Returns a list of media with the specified ID that have encountered an error.
synchronized boolean isErrorAny()
Checks the error status of all of the images.
synchronized boolean isErrorID(int id)
Checks the error status of all of the images tracked by this media tracker with the specified identifier.
synchronized void removeImage(Image image, int id)
Removes the specified image from the specified tracking ID of this media tracker.
synchronized void removeImage(Image image)
Removes the specified image from this media tracker.
synchronized void removeImage(Image image, int id, int width, int height)
Removes the specified image with the specified width, height, and ID from this media tracker.
int statusAll(boolean load)
Calculates and returns the bitwise inclusive OR of the status of all media that are tracked by this media tracker.
int statusID(int id, boolean load)
Calculates and returns the bitwise inclusive OR of the status of all media with the specified identifier that are tracked by this media tracker.
void waitForAll()
Starts loading all images tracked by this media tracker.
synchronized boolean waitForAll(long ms)
Starts loading all images tracked by this media tracker.
void waitForID(int id)
Starts loading all images tracked by this media tracker with the specified identifier.
synchronized boolean waitForID(int id, long ms)
Starts loading all images tracked by this media tracker with the specified identifier.
[Expand]
Inherited Methods
From class java.lang.Object

Constants

public static final int ABORTED

Flag indicating that the downloading of media was aborted.

See Also
Constant Value: 2 (0x00000002)

public static final int COMPLETE

Flag indicating that the downloading of media was completed successfully.

See Also
Constant Value: 8 (0x00000008)

public static final int ERRORED

Flag indicating that the downloading of media encountered an error.

See Also
Constant Value: 4 (0x00000004)

public static final int LOADING

Flag indicating that media is currently being loaded.

See Also
Constant Value: 1 (0x00000001)

Public Constructors

public MediaTracker (Component comp)

Creates a media tracker to track images for a given component.

Parameters
comp the component on which the images will eventually be drawn

Public Methods

public void addImage (Image image, int id)

Adds an image to the list of images being tracked by this media tracker. The image will eventually be rendered at its default (unscaled) size.

Parameters
image the image to be tracked
id an identifier used to track this image

public synchronized void addImage (Image image, int id, int w, int h)

Adds a scaled image to the list of images being tracked by this media tracker. The image will eventually be rendered at the indicated width and height.

Parameters
image the image to be tracked
id an identifier that can be used to track this image
w the width at which the image is rendered
h the height at which the image is rendered

public boolean checkAll (boolean load)

Checks to see if all images being tracked by this media tracker have finished loading.

If the value of the load flag is true, then this method starts loading any images that are not yet being loaded.

If there is an error while loading or scaling an image, that image is considered to have finished loading. Use the isErrorAny and isErrorID methods to check for errors.

Parameters
load if true, start loading any images that are not yet being loaded
Returns
  • true if all images have finished loading, have been aborted, or have encountered an error; false otherwise
See Also

public boolean checkAll ()

Checks to see if all images being tracked by this media tracker have finished loading.

This method does not start loading the images if they are not already loading.

If there is an error while loading or scaling an image, then that image is considered to have finished loading. Use the isErrorAny or isErrorID methods to check for errors.

Returns
  • true if all images have finished loading, have been aborted, or have encountered an error; false otherwise
See Also

public boolean checkID (int id, boolean load)

Checks to see if all images tracked by this media tracker that are tagged with the specified identifier have finished loading.

If the value of the load flag is true, then this method starts loading any images that are not yet being loaded.

If there is an error while loading or scaling an image, then that image is considered to have finished loading. Use the isErrorAny or isErrorID methods to check for errors.

Parameters
id the identifier of the images to check
load if true, start loading any images that are not yet being loaded
Returns
  • true if all images have finished loading, have been aborted, or have encountered an error; false otherwise
See Also

public boolean checkID (int id)

Checks to see if all images tracked by this media tracker that are tagged with the specified identifier have finished loading.

This method does not start loading the images if they are not already loading.

If there is an error while loading or scaling an image, then that image is considered to have finished loading. Use the isErrorAny or isErrorID methods to check for errors.

Parameters
id the identifier of the images to check
Returns
  • true if all images have finished loading, have been aborted, or have encountered an error; false otherwise
See Also

public synchronized Object[] getErrorsAny ()

Returns a list of all media that have encountered an error.

Returns
  • an array of media objects tracked by this media tracker that have encountered an error, or null if there are none with errors
See Also

public synchronized Object[] getErrorsID (int id)

Returns a list of media with the specified ID that have encountered an error.

Parameters
id the identifier of the images to check
Returns
  • an array of media objects tracked by this media tracker with the specified identifier that have encountered an error, or null if there are none with errors
See Also

public synchronized boolean isErrorAny ()

Checks the error status of all of the images.

Returns
  • true if any of the images tracked by this media tracker had an error during loading; false otherwise
See Also

public synchronized boolean isErrorID (int id)

Checks the error status of all of the images tracked by this media tracker with the specified identifier.

Parameters
id the identifier of the images to check
Returns
  • true if any of the images with the specified identifier had an error during loading; false otherwise
See Also

public synchronized void removeImage (Image image, int id)

Removes the specified image from the specified tracking ID of this media tracker. All instances of Image being tracked under the specified ID are removed regardless of scale.

Parameters
image the image to be removed
id the tracking ID frrom which to remove the image
See Also

public synchronized void removeImage (Image image)

Removes the specified image from this media tracker. All instances of the specified image are removed, regardless of scale or ID.

Parameters
image the image to be removed
See Also

public synchronized void removeImage (Image image, int id, int width, int height)

Removes the specified image with the specified width, height, and ID from this media tracker. Only the specified instance (with any duplicates) is removed.

Parameters
image the image to be removed
id the tracking ID from which to remove the image
width the width to remove (-1 for unscaled)
height the height to remove (-1 for unscaled)
See Also

public int statusAll (boolean load)

Calculates and returns the bitwise inclusive OR of the status of all media that are tracked by this media tracker.

Possible flags defined by the MediaTracker class are LOADING, ABORTED, ERRORED, and COMPLETE. An image that hasn't started loading has zero as its status.

If the value of load is true, then this method starts loading any images that are not yet being loaded.

Parameters
load if true, start loading any images that are not yet being loaded
Returns
  • the bitwise inclusive OR of the status of all of the media being tracked
See Also

public int statusID (int id, boolean load)

Calculates and returns the bitwise inclusive OR of the status of all media with the specified identifier that are tracked by this media tracker.

Possible flags defined by the MediaTracker class are LOADING, ABORTED, ERRORED, and COMPLETE. An image that hasn't started loading has zero as its status.

If the value of load is true, then this method starts loading any images that are not yet being loaded.

Parameters
id the identifier of the images to check
load if true, start loading any images that are not yet being loaded
Returns
  • the bitwise inclusive OR of the status of all of the media with the specified identifier that are being tracked
See Also

public void waitForAll ()

Starts loading all images tracked by this media tracker. This method waits until all the images being tracked have finished loading.

If there is an error while loading or scaling an image, then that image is considered to have finished loading. Use the isErrorAny or isErrorID methods to check for errors.

Throws
InterruptedException if any thread has interrupted this thread
See Also

public synchronized boolean waitForAll (long ms)

Starts loading all images tracked by this media tracker. This method waits until all the images being tracked have finished loading, or until the length of time specified in milliseconds by the ms argument has passed.

If there is an error while loading or scaling an image, then that image is considered to have finished loading. Use the isErrorAny or isErrorID methods to check for errors.

Parameters
ms the number of milliseconds to wait for the loading to complete
Returns
  • true if all images were successfully loaded; false otherwise
Throws
InterruptedException if any thread has interrupted this thread.
See Also

public void waitForID (int id)

Starts loading all images tracked by this media tracker with the specified identifier. This method waits until all the images with the specified identifier have finished loading.

If there is an error while loading or scaling an image, then that image is considered to have finished loading. Use the isErrorAny and isErrorID methods to check for errors.

Parameters
id the identifier of the images to check
Throws
InterruptedException if any thread has interrupted this thread.
See Also

public synchronized boolean waitForID (int id, long ms)

Starts loading all images tracked by this media tracker with the specified identifier. This method waits until all the images with the specified identifier have finished loading, or until the length of time specified in milliseconds by the ms argument has passed.

If there is an error while loading or scaling an image, then that image is considered to have finished loading. Use the statusID, isErrorID, and isErrorAny methods to check for errors.

Parameters
id the identifier of the images to check
ms the length of time, in milliseconds, to wait for the loading to complete
Throws
InterruptedException if any thread has interrupted this thread.
See Also