ch8.htm

JAVA Developing Professional JavaApplets

a greater whole. Each thread can be seen as existing as part of
a single, greater unit. For example, when the process dies, so
do all its threads.
<H2><A NAME="CreatingaThreadwiththeThreadClass"><FONT SIZE=5 COLOR=#FF0000>Creating
a Thread with the Thread Class</FONT></A></H2>
<P>
Java offers a variety of tools for writing multithreaded programs.
The most useful is the Thread class, which, as its name suggests,
is used for creating a class that runs as a thread. You need to
declare a subclass of Thread if you want to create a functional
thread. After the subclass is created, you need to override Thread's
<TT>run()</TT> method. This method
is the key to making the Thread class perform as a thread; it
defines your class's thread of execution. After the <TT>run()</TT>
method is established and your class is instantiated, you can
begin the thread with the <TT>start()</TT>
method.
<P>
A simple example shows how easy it is to create a thread in Java.
Figure 8.1 shows an applet with lines drawn randomly throughout
its workspace. These lines are drawn by a thread using the code
in Listing 8.1. The class LineThread is a subclass of Thread that
loops indefinitely, randomly drawing lines on an applet. The applet
is provided to LineThread through its constructor. Although the
LineThread class has a custom constructor, Thread classes often
do not have special constructors.
<P>
<A HREF="f8-1.gif" ><B>Figure 8.1 : </B><I>An applet with lines drawn by a thread </I></A>
<HR>
<BLOCKQUOTE>
<B>Listing 8.1. The source code for applet with lines drawn by
a thread.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>import java.awt.*;<BR>
import java.lang.*;<BR>
import java.applet.Applet;<BR>
<BR>
// This is an applet that creates a Thread that<BR>
// randomly draws lines on its workspace...<BR>
public class LineApplet extends Applet&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;Thread t;<BR>
&nbsp;&nbsp;&nbsp;// Set the size of the applet, create the thread,
<BR>
&nbsp;&nbsp;&nbsp;// and start it...<BR>
&nbsp;&nbsp;&nbsp;public void init() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;resize(300,300);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;t = new LineThread(this);
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;t.start();<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Click the mouse to kill the thread...<BR>
&nbsp;&nbsp;&nbsp;public boolean mouseDown(Event ev, int x1, int
x2) {<BR>
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;if (t != null) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;t.stop();
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;t = null;
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;return true;<BR>
&nbsp;&nbsp;&nbsp;};<BR>
}<BR>
<BR>
// Thread that randomly draws lines all over a component...<BR>
class LineThread extends Thread {<BR>
&nbsp;&nbsp;&nbsp;Applet a;&nbsp;&nbsp;// Thread needs to know
the applet...<BR>
&nbsp;&nbsp;&nbsp;// Constructor simply stores the applet to paint...
<BR>
&nbsp;&nbsp;&nbsp;public LineThread(Applet a) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;this.a = a;<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Run the thread. Lines everywhere!<BR>
&nbsp;&nbsp;&nbsp;public void run() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Get dimension data about
the applet...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;double width = (double) a.size().width;
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;double height = (double)a.size().height;
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Loop and draw lines forever...
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;while (true) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;Graphics
g = a.getGraphics();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;g.drawLine((int)(width
* Math.random()),<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(int)(height
* Math.random()),<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(int)(width
* Math.random()),<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(int)(height
* Math.random()) );<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;}<BR>
}</TT>
</BLOCKQUOTE>
<HR>
<P>
The <TT>run()</TT> method of LineThread
provides the applet's threaded activity. After getting the applet's
dimensions, the thread enters an indefinite <TT>while</TT>
loop, randomly drawing lines on each iteration. (The Component
class's <TT>getGraphics()</TT> method
is a way of getting a Graphics object to draw on a component outside
the <TT>paint()</TT> method.) The
thread will run until the applet is terminated or until the thread
is explicitly stopped.
<P>
After setting its own size and constructing a LineThread object,
the Applet class, LineApplet, begins the line drawing by calling
the <TT>start()</TT> method. This
<TT>start()</TT> method call begins
the thread and invokes the <TT>run()</TT>
method of the LineThread class. The thread will not run until
the <TT>start()</TT> method is applied.
<P>
The <TT>stop()</TT> method terminates
a thread. In a general sense, <TT>stop()</TT>
causes a thread to abruptly leave its <TT>run()</TT>
method and, therefore, end its threaded activity. In this example,
a mouse click on the applet causes the thread (and hence the line
drawing) to stop.
<H3><A NAME="EnhancingYourFirstMultithreadedApple">Enhancing Your
First Multithreaded Applet</A></H3>
<P>
A couple of things can be done to this applet to further illustrate
the Thread class. Listing 8.2 gives the source code for the enhanced
version of the applet. The threaded class, LineThread, was modified
to delay a little between the line drawings by using the <TT>sleep()</TT>
method, which causes the thread to &quot;sleep&quot; for a specific
number of milliseconds; in this case, the delay is for a tenth
of a second. Note how the <TT>sleep()</TT>
method requires catching InterruptedException objects. This is
meant for situations in which a thread has been interrupted by
another thread. However, this capability doesn't seem to be active
in the current version of the JDK. Consequently, you will see
null handlers for InterruptedException objects throughout this
book. Because it is annoying to have to have an exception handler
every time you call <TT>sleep()</TT>,
you might want to encapsulate <TT>sleep()</TT>
and the exception handling in your own method. This technique
appears in the code used elsewhere in this book.
<HR>
<BLOCKQUOTE>
<B>Listing 8.2. Enhanced version of the line-drawing applet.<BR>
</B>
</BLOCKQUOTE>
<BLOCKQUOTE>
<TT>import java.awt.*;<BR>
import java.lang.*;<BR>
import java.applet.Applet;<BR>
<BR>
// This is an applet that creates a Thread that<BR>
// randomly draws lines on its workspace...<BR>
public class LineApplet extends Applet&nbsp;&nbsp;{<BR>
&nbsp;&nbsp;&nbsp;Thread t;<BR>
&nbsp;&nbsp;&nbsp;boolean running = false;<BR>
&nbsp;&nbsp;&nbsp;// Set the size of the applet, create the thread
<BR>
&nbsp;&nbsp;&nbsp;// and start it...<BR>
&nbsp;&nbsp;&nbsp;public void init() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;resize(300,300);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;t = new LineThread(this);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;t.start();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;running = true;<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Click the mouse down to kill the thread...
<BR>
&nbsp;&nbsp;&nbsp;public boolean mouseDown(Event ev, int x1, int
x2) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;// If the thread is active, suspend the
thread<BR>
&nbsp;&nbsp;&nbsp;&nbsp;// and remove the lines...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;if (running) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp; running = false;<BR>
&nbsp;&nbsp;&nbsp;&nbsp; t.suspend();<BR>
&nbsp;&nbsp;&nbsp;&nbsp; repaint();&nbsp;&nbsp;// Removes the
lines...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;// If thread is suspended, then reactivate
it...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;else {<BR>
&nbsp;&nbsp;&nbsp;&nbsp; running = true;<BR>
&nbsp;&nbsp;&nbsp;&nbsp; t.resume();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;return true;<BR>
&nbsp;&nbsp;&nbsp;};<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Destroy the thread when the applet shuts
down...<BR>
&nbsp;&nbsp;&nbsp;public void destroy() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;// Stop the thread and wait for it to
die...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;t.stop();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;try {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; t.join();<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;catch (InterruptedException e) { }<BR>
&nbsp;&nbsp;&nbsp;}<BR>
}<BR>
<BR>
// Thread that randomly draws lines all over a component...<BR>
class LineThread extends Thread {<BR>
&nbsp;&nbsp;&nbsp;Applet a;&nbsp;&nbsp;// Thread needs to know
the applet...<BR>
&nbsp;&nbsp;&nbsp;// Constructor simply stores the applet to paint...
<BR>
&nbsp;&nbsp;&nbsp;public LineThread(Applet a) {<BR>
&nbsp;&nbsp;this.a = a;<BR>
&nbsp;&nbsp;&nbsp;}<BR>
<BR>
&nbsp;&nbsp;&nbsp;// Run the thread. Lines everywhere!<BR>
&nbsp;&nbsp;&nbsp;public void run() {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;// Get dimension data about the applet...
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;double width = (double) a.size().width;
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;double height = (double)a.size().height;
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;// Loop and draw lines forever...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;while (true) {<BR>
&nbsp;&nbsp;&nbsp;&nbsp; Graphics g = a.getGraphics();<BR>
&nbsp;&nbsp;&nbsp;&nbsp; // Randomly select a color...<BR>
&nbsp;&nbsp;&nbsp;&nbsp; Color c = new Color((int)(255.0 * Math.random()),
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(int)(255.0 * Math.random()),
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(int)(255.0 * Math.random())
);<BR>
&nbsp;&nbsp;&nbsp;&nbsp; g.setColor(c);<BR>
&nbsp;&nbsp;&nbsp;&nbsp; g.drawLine((int)(width * Math.random()),
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(int)(height * Math.random()),
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(int)(width * Math.random()),
<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;(int)(height * Math.random())
);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;// Sleep some...<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;try {<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;sleep(100);<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;catch (InterruptedException e) {
}<BR>
&nbsp;&nbsp;&nbsp;&nbsp;}<BR>
&nbsp;&nbsp;&nbsp;}<BR>
}</TT>
</BLOCKQUOTE>
<HR>
<P>
A visual enhancement also was made to LineThread. It randomly
creates a new Color object and uses it (with the Graphics class
<TT>setColor()</TT> method) in each
iteration. Figure 8.2 shows the enhancement.
<P>
<A HREF="f8-2.gif" ><B>Figure 8.2 : </B><I>Second version of the linedrawing applet </I></A>
<P>
Most of the changes occur in the Applet class LineApplet. The
method that traps the mouse clicks, <TT>mouseDown()</TT>,
is changed so it no longer stops the thread. It now toggles between
pausing and resuming the thread's execution. The <TT>suspend()</TT>
method suspends a thread's execution; when a thread is suspended,
it will not continue its stream of execution until the complementary
<TT>resume()</TT> method is invoked.
In this example, suspending a thread is accompanied by clearing
out the applet's drawing area through the <TT>repaint()</TT>
method. (This clearing occurs because the applet does not override
the <TT>paint()</TT> method.) The
<TT>running</TT> variable is used
to keep track of whether the thread is suspended; unfortunately,
there is no method in the Thread class for determining this.
<P>
The last bit of code occurs in the newly added <TT>destroy()</TT>
method, which is called when the applet shuts down. In this call,
the <TT>stop()</TT> method terminates
the thread. It is accompanied by a <TT>join()</TT>
call, which forces the calling thread to wait until the victim
thread is really &quot;dead.&quot; This is important because stopping
a thread does not cause an immediate (in the strict sense of the
word) termination of the thread. There may be some delay due to
time-slicing or some other reason. It is often dangerous not to
wait for a thread to die. In some cases, the thread may run even
after some of the objects it uses (like the applet) are no longer
valid. This will cause an exception. Furthermore, an applet that
has been stopped without waiting for its constituent threads to
terminate could cause the applet to unexpectedly &quot;hang.&quot;
Consequently, it's good practice to follow <TT>stop()</TT>
calls by a complementary call to <TT>join()</TT>
to prevent this from happening. It should be noted that the Thread
class has two additional versions of <TT>join()</TT>
that take time-out flags. Use these if your applet has some severe
time constraints and you cannot afford to wait indefinitely for
a thread to end. The time-out values (like most time-outs in Java)
are in milliseconds with an optional nanosecond precision value.