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Note: This page assumes that you know what a thread is. If you don't, read What Is a Thread? before reading this page.Every applet can run in multiple threads. Applet drawing methods (
paint
andupdate
) are always called from the AWT drawing and event handling thread. The threads that the major milestone methods --init
,start
,stop
, anddestroy
-- 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 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.
Below are two
PrintThread
applets.PrintThread
is a modified version of SimpleApplet that prints the thread and thread group that itsinit
,start
,stop
,destroy
, andupdate
methods are called from. (Actually, due to a Netscape Navigator 2.0 for Windows 95/NT bug, the following applets don't implementupdate
. If you aren't using a PC that's running Netscape Navigator 2.0, you should be able to run the real example.) Here's the code for thehobbled example
, and for themore interesting example
. As usual, to see the output for the methods such asdestroy
that are called during unloading, you need to look at the standard output. See Displaying Diagnostics to the Standard Output and Error Streams for information about the standard output stream.
Note: If you don't see the applet running above, you need to install Java Plug-in, which happens automatically when you install the J2SE JRE or SDK. We strongly recommend that you install the latest version; at least 1.3.1 is required for all our applets. You can find more information in the Java Plug-in home page.
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 not 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.
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