Threads question

[Zoo 9000]

I've been trying to understand how wait() and notify() work
but even Sun's own explanation (here,
http://java.sun.com/products/jdk/1.2/docs/guide/misc/threadPrimitiveDeprecation.html
) isn't clear and doesn't give a working example.

So I've tried to build a simple one myself. The program (below)
compiles and runs under J2SE 1.4.1 but throws an
'IllegalMonitorStateException' (whatever that is..). If anyone can
show me where I'm going wrong I'd be grateful.

- Ken


import java.awt.*;
import java.awt.event.*;
import javax.swing.*;

public class VisualWaitNotify
extends JPanel
implements Runnable {

private static final String[] symbolList =
{ "|", "/", "-", "\\", "|", "/", "-", "\\" };

private Thread runThread;
private JTextField symbolTF;
private boolean threadSuspended = false;

public VisualWaitNotify() {
symbolTF = new JTextField();
symbolTF.setEditable(false);
symbolTF.setFont(new Font("Monospaced",
Font.BOLD, 26));
symbolTF.setHorizontalAlignment(JTextField.CENTER);
final JButton waitB = new JButton("Wait");
final JButton notifyB = new JButton("Notify");

waitB.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
waitNow();
}
});

notifyB.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
notifyNow();
}
});

JPanel innerStackP = new JPanel();
innerStackP.setLayout(new GridLayout(0, 1, 3, 3));
innerStackP.add(symbolTF);
innerStackP.add(waitB);
innerStackP.add(notifyB);

this.setLayout(new FlowLayout(FlowLayout.CENTER));
this.add(innerStackP);
}
private void waitNow() {

threadSuspended = true;

try {
synchronized (this) {
while (threadSuspended)
runThread.wait();
}
} catch (InterruptedException e){
}
}

private synchronized void notifyNow() {

threadSuspended = false;

if ( runThread != null ) {
runThread.notify();
}
}

public void run() {
try {
runThread = Thread.currentThread();
int count = 0;

while ( true ) {
// each time through, show the next symbol
symbolTF.setText(
symbolList[ count % symbolList.length ]);
Thread.sleep(200);
count++;
}
} catch ( InterruptedException x ) {
// ignore
} finally {
// The thread is about to die, make sure
// that the reference to it is also lost.
runThread = null;
}
}

public static void main(String[] args) {
VisualWaitNotify vwn = new VisualWaitNotify();
Thread t = new Thread(vwn);
t.start();

JFrame f = new JFrame("Visual Wait and Notify");
f.setContentPane(vwn);
f.setSize(320, 200);
f.setVisible(true);
f.addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
System.exit(0);
}
});
}
}

 

[Roedy Green]

I've been trying to understand how wait() and notify() work
but even Sun's own explanation (here,
http://java.sun.com/products/jdk/1.2/docs/guide/misc/threadPrimitiveDeprecation.html
) isn't clear and doesn't give a working example.

I make a stab at explaining this at
http://mindprod.com/jgloss/thread.html
and http://mindprod.com/jgloss/threadsafe.html

 

[Roedy Green]

In simple terms, wait() causes the running thread to pause until it is
notified by some other thread via notify() or notify() all that it should
wake up.

Think in terms of what happens in a barbershop with multiple customers
and multiple barbers. Customers call wait() when all barbers are
busy. And a barber who is free calls notify() to wake up one of the
waiting customers.

 

[Roedy Green]

That's not the best reference. Read Roedy's references mentioned in his post
and also look at the Java tutorial "Essential Java Classes"
http://java.sun.com/docs/books/tutorial/essential/index.html

what were you recommending there? The only thing I could see was
http://java.sun.com/docs/books/tutorial/essential/threads/index.html
which is listed already in http://mindprod.com/jgloss/threads.html

This is a complicated topic and all the material we can gather the
better.

 

[Nigel Wade]

what were you recommending there? The only thing I could see was
http://java.sun.com/docs/books/tutorial/essential/threads/index.html

That's the one.

which is listed already in http://mindprod.com/jgloss/threads.html

Sorry, didn't see that link right down at the bottom...

This is a complicated topic and all the material we can gather the
better.

It certainly is. The brain doesn't seem to be designed to think in parallel,
at last not in logical terms and with propper synchronization mechanisms.