Special Effects and Game Development in Java(TM) -Improve the image loading with MediaTracker 

by Anibal Wainstein

4.0.2 Improve the image loading with MediaTracker 

Did you notice that you could practically see the image being loaded in the last section's example? This was because the paint() method is called several times during the image download! The reason that Sun chose to make the system this way, is that they wanted to keep the effect of seeing something while waiting for a page to download. This was usual in older web browsers (new web browsers such as IE4.0, will only display the image when it is fully downloaded, unless it does not happen to be an interlaced GIF image). The paint() method will be called arbitrary by the web browser when it has managed to download a new piece of the image. This could cause problems and can look bad in some applets. But there is a solution, there is a class named MediaTracker, which makes sure that the image is fully downloaded before you use it.
Let us rewrite the init() method in the last section to:

public void init()
{
    logo=getImage(getDocumentBase(),"logo.jpg");
    //The following line creates a tracker.
    MediaTracker tracker=new MediaTracker(this);
    //We give the image we want to track the
    //identification number 0.
    tracker.addImage(logo,0);
   //With the waitForAll() method the image can
    //be loaded. It will throw an InterrupterException
    //if something happens.
try
    {
        tracker.waitForAll();
    }
catch(InterruptedException e)
    {
System.out.println("Something happened while reading image...");
    }
}

A tracker object requires a reference to the applet when it is created (you get that with "this"). With the addImage() method we can give the image the ID number 0, in the methods internal "download list". Please note that you cannot give several images the same ID number, each must have a unique number. Finally, we can use the waitForAll() method that makes sure to load all the images. This method throws an exception that you must "catch".

Click here to see an improved version of the last section's applet. In this example the difference is minimal and maybe not even noticeable, but in more complicated graphics applets it is a necessity.

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