Threads in Applets

Note: This page assumes that you know what a thread is. If you don't, read What Are Threads?before reading this page.
Every applet can run in multiple threads. Applet drawing methods (paint() and update()) are always called from the AWT drawing and event handling thread. The threads that the major milestone methods -- init(), start(), stop(), and destroy() -- are called from depends on the application that's running the applet. But no application ever calls them from the AWT drawing and event handling thread.
Many browsers, such as Netscape Navigator 2.0 for Solaris, allocate a thread for each applet on a page, using that thread for all calls to the applet's major milestone methods. Some browsers allocate a thread group for each applet, so that it's easy to kill all the threads that belong to a particular applet. In any case, you're guaranteed that every thread that any of an applet's major milestone methods creates belongs to the same thread group. [CHECK; any more guarantees?] [CHECK: Can an applet create a thread in paint()? I'd think not.]
Below are two PrintThread applets. PrintThread is a modified version of SimpleApplet that prints the thread and thread group that init(), start(), stop(), destroy(), paint(), and update() are called from. (Here's the code.) PrintThread calls repaint() unnecessarily every once in a while, so that you'll be able to see how its update() and paint() get called. As usual, to see the output for the methods such as destroy() that are called during unloading, you need to look at the standard output. [LINK]

You can't run applets. Here's what you'd see if you could:

So why would an applet need to create and use its own threads? Imagine an applet that performs some time-consuming initialization -- loading images, for example -- in its init() method. The thread that invokes init() can't do anything else until init() returns. In some browsers, this might mean that the browser can't display the applet or anything after it until the applet has finished initializing itself. So if the applet is at the top of the page, for example, then nothing would appear on the page until the applet has finished initializing itself.
Even in browsers that create a separate thread for each applet, it makes sense to put any time-consuming tasks into an applet-created thread, so that the applet can perform other tasks while it waits for the time-consuming ones to be completed.

Rule of Thumb: If an applet performs a time-consuming task, it should create and use its own thread to perform that task.

Applets typically perform two kinds of time-consuming tasks: tasks that they perform once, and tasks that they perform repeatedly. The next page gives an example of both.

Overview of Applets