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[1] goes as follows:

[1] This summary covers Sun's implementation (  JDK). Implementations that don't derive from the JDK may behave completely differently.

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.

7. The ImageFetcher reads data and passes it along to the InputStreamImageSource, which is a URLImageSource.

8. The URLImageSource determines that JPEGImageDecoder is the proper ImageDecoder for converting the input stream into an Image. (Other ImageDecoders are used for other image types, like GIF.)

9. The ImageProducer starts reading the image data from the source; it calls the ImageConsumer (i.e., the ImageRepresentation) as it processes the image. The most important method in the ImageConsumer interface is setPixels(), which delivers pixel data to the consumer for rendering onscreen.

10. As the ImageConsumer (i.e., the ImageRepresentation) gets additional information, it notifies the ImageObserver via imageUpdate() calls.

11. When the image is fully acquired across the network, the thread started by the ImageFetcher stops.

As you see, there are a lot of unfamiliar moving pieces. Many of them are from the java.awt.image package and are discussed in Chapter 12, Image Processing. Others are from the sun.awt.image package; they are hidden in that you don't need to know anything about them to do image processing in Java. However, if you're curious, we'll briefly summarize these classes in the next section.