Special Effects and Game Development
in Java(TM) - The theory behind program threads
by Anibal Wainstein
3.0 The theory behind program
threads
Multiple program threads were introduced to PC programming
with the release of Windows 95. Operating systems such as
UNIX have had this for a long time. A thread works like a
"little worker" working with the program code. If
you have several programs running in your system, then there
is at least one thread for each program. Some programs may
have several threads, for instance a web browser that supports
Java. To be able to have animated Java applets we are forced
to use two threads. First there is the program or main thread,
which is the Java Virtual Machines own thread. This
is used for the so-called event system, which handles user
input such as mouse pointer movements and keyboard events.
In the last chapter we had only worked with the main thread.
The problem with animations is that they often requires that
you have a thread that is constantly working with the animation
code. We cannot use the main thread because then the applet's
other functions will stop working, the applet hangs.
A program that hangs, usually does that because the main thread
gets stuck in an program loop and cannot get free, or that
it collides with another thread and stops. Assume that two
web designers are working with a homepage. If they do not
decide who is going to do what, then they will probably end
up writing each other work. This leads to aggression, the cooperation
breaks down and with that also the project. In the same way
you must be careful when using threads.
3.0.1 An applet's start()
and stop() methods (The Runnable interface)
Applets have no natural support for threads. You add thread
support by adding the Runnable interface :
import java.applet.*;
import java.awt.*;
public class threadtest extends Applet implements Runnable {
}
In our empty threadtest applet we have added "implements
Runnable". With the declaration "implements"
you can add several new functions to a class. It is used to
increase the class' abilities. "Runnable" will give access
to three new methods: start(), stop() and run(). In an applet
that supports threads the methods init() and paint() are first
called by the Java Virtual Machine's main thread, the
third method called by this thread is the start() method.
In the start() method you can initialize and start your animation
thread.
public Thread programthread = null;
public void start()
{
if (animationthread == null)
{
animationthread = new Thread(this,"animationthread");
animationthread.start();
}
}
The variable "animationthread" is a global variable within the
applet and you must declare it yourself. What the start() method
does first is that it checks that the thread is not already
created (sometimes some web browsers can execute the start()
method several times). The next step is to create the thread
itself. It demands a reference to your applet (this) and a name
that could be anything (we call it "animationthread"
for the sake of simplicity). After this you start the thread
with its own start() method.
When the homepage visitor changes the page or shut down the
web browser, then the stop() methods is immediately called by
the applet's main thread:
public void stop()
{
if ((animationthread != null) && animationthread.isAlive())
animationthread.stop();
animationthread = null;
}
For security reasons you should make sure that the thread is
created and that it is running with the method isAlive(). If
it is then the thread is stopped with it's own stop() method
and we set the variable "animationthread" to the value
null.
You will probably always just cut and paste these two methods
into your programs in the future. They are general methods
that all the special effects or game applets have.
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