How can I make a browser execute a part of code before loading a jar? I want to make a splashscreen for a big applet that is contained in a jar, but the jars are loaded before the splashimage is displayed.

The example shows a tiny "loader" applet (initial.java) which serves to purposes:

1. it displays the splash screen
2. it references the rest of the applet (by string), causing the remaining applet code to load.

    import java.awt.*;
    import java.applet.Applet;

    public class initial extends Applet implements Runnable {
        private String classToLoad;

        private void fetchClass(String className) {
            classToLoad = className;
            Thread t = new Thread(this);
            t.start();
        }
        
        public void run() {
            try {
                Class mainClass = Class.forName(classToLoad);
                Component mainInstance = 
                   (Component)mainClass.newInstance();
                add(mainInstance);
             }
             catch (Exception e) {
                 e.printStackTrace();
             }
        
             invalidate();
             validate();
         }
        
         /**
          *  Called when the applet is initialised -
          *  here we start the asynchronous load of the rest of
          *  the applet.
          */
         public void init() {                
             setLayout(new BorderLayout());
             fetchClass("second");
         }
        
         /**
          *  Display a simple message 
          */
         public void paint(Graphics g) {
             g.drawString("loading...",30,30);
         }
    }

And here's second.java, the source for our simple example second-phase:

  import java.awt.*;

  public class second extends Canvas {
     public void paint(Graphics g){
       Dimension d = getSize();
       g.setColor(Color.green);
       g.fillRect(0,0,d.width, d.height);
     }
  }

You'll see from initial that it loads the subsequent code in a new thread - this is because Class.forName() blocks - if we didn't spawn a new thread then init() wouldn't terminate until second was loaded - and initial can't paint until its init() method exits.

Here's the HTML code for loading the applet:

  <h1>multi-stage applet<h1>
  <applet code=initial.class width=300 height=300>
  </applet>

Notice that CODE points just to initial.class, not to subsequent classes like second.class - but the applet classloader will still need to be able to load second.class later, so it must be accessible, i.e. it must be in a place specified by the CODEBASE parameter.

Now, ideally we'd have many classes in the second part, and the logical place to keep them would be a JAR file, but we can't name that JAR file in the applet's ARCHIVE parameter, as the applet environment always loads all the JARs mentioned in the ARCHIVE parameter, even if they don't appear to be referenced initially. Thus we're stuck loading individual classfiles across the network.