Java AWT Reference

D

Image Loading

In this appendix:

• How Images are Loaded
• A Brief Tour of sun.awt.image

D.1 How Images are Loaded

You have seen how easy it is to display an image on screen and have probably guessed that there's more going on behind the scenes. The getImage() and drawImage() methods trigger a series of events that result in the image being available for display on the ImageObserver. The image is fetched asynchronously in another thread. The entire process^* goes as follows:

1. The call to getImage() triggers Toolkit to call createImage() for the image's InputStreamImageSource (which is a URLImageSource in this case; it would be a FileImageSource if we were loading the image from a local file).
2. The Toolkit registers the image as being “desired.” Desired just means that something will eventually want the image loaded. The system then waits until an ImageObserver registers its interest in the image.
3. The drawImage() method (use of MediaTracker or prepareImage()) registers an ImageObserver as interested.
4. Registering an ImageObserver kicks the image's ImageRepresentation into action; this is the start of the loading process, although image data isn't actually transferred until step 9. ImageRepresentation implements the ImageConsumer interface.
5. The start of production registers the image source (ImageProducer URLImageSource) with the ImageFetcher and also registers the ImageRepresentation as an ImageConsumer for the image.
6. The ImageFetcher creates a thread to get the image from its source. ...