Problems using wait() and notifyAll() with locks

[cuffJ]

Hi,

I'm trying to create a merge sort program that animates a merge sort using threads, with the ability to pause when a button is clicked in the overlying JFrame. The merge sort animation works when the button is not used, but when it is pressed I get an IllegalMonitorStateException, and the program crashes. I understand that this occurs when the lock is not owned, however all my calls to wait() or notifyAll() occur within a lock. If someone could help me out, I know there's a lot of code, but most of it is just managing the sort, and I just need help with wait() problem. I think it has something to do with the fact that I'm calling wait() in so many different methods, but I'm not sure. pauseActions triggers waits, resumeActions calls notifyAll();

Thanks!

/**
* A JPanel performing a merge sort using Threads.
* @param currElement1 One of the 2 current elements being inspected.
* @param currElement2 One of the 2 current elements being inspected.
* @param f Current range being.
* @param t Current range end.
* @param lineSpacing Spacing between Lines
* @param lines Array of heights
* @param sortStateLock Lock to control thread synchronization.
* @param ARRAY_LENGTH Length of Array.
* @param DELAY Pause for each element inspected.
* @author Owner
*
*/
public class MergePanel extends JPanel
{
public MergePanel()
{
sortStateLock=new ReentrantLock();
}
/**
* Initiates random arrays and begins threads.
*/
public void initiate()
{
isPaused=false;
f=0;
t=0;
currElement1=0;
currElement2=0;
lineSpacing=this.getWidth()/ARRAY_LENGTH;
//Fills array Randomly
Random generator=new Random();
lines=new int[ARRAY_LENGTH];
for (int i=0; i<lines.length; i++)
{
lines[i]=generator.nextInt(100);
}
try
{
MyRunnable r=new MyRunnable(0,lines.length-1);
Thread t=new Thread(r);
t.start();
t.join();
}
catch(InterruptedException e)
{
e.printStackTrace();
}
System.out.println(Arrays.toString(lines));
}
/**
* Class used to create spawned threads.
* @author Owner
*
*/
class MyRunnable implements Runnable
{
private int from;
private int to;
MyRunnable(int f, int t)
{
from=f;
to=t;
}
public void run()
{
try
{
mergeSort(from,to);
}
catch(InterruptedException e)
{
e.printStackTrace();
}
}
}
/**
* Divides array up continuously until the array size is 1, spawning new thread for each division.
* At the end, merge is called, merging spawned arrays after they have died (join command).
* @param from Begin of array range being inspected
* @param to End of array range being inspected.
* @throws InterruptedException
*/
public void mergeSort(int from, int to) throws InterruptedException
{
if (to==from) return;
int mid=0;
sortStateLock.lock();
try
{
while (isPaused)
sortStateLock.wait();
//System.out.println(from+" "+to);
mid=(from+to)/2;
}
finally
{
sortStateLock.unlock();
}
if (!isPaused)
{
MyRunnable r1=new MyRunnable(from,mid);
MyRunnable r2=new MyRunnable(mid+1,to);
Thread t1=new Thread(r1);
Thread t2=new Thread(r2);
t1.start();
t2.start();
t1.join();
t2.join();
merge(from, mid, to);
}

}
/**
* Merges array sections from to mid and mid+1 to to, pausing after each element inspected in merge
* operation to repaint.
* @param from Beginning of first range.
* @param mid End of first range and beginning of second range.
* @param to End of second range.
* @throws InterruptedException
*/
public void merge(int from, int mid, int to) throws InterruptedException
{
while (i<first.length && j<second.length)
{
sortStateLock.lock();
try
{
while (isPaused)
sortStateLock.wait();
//Methods for merging now shown
}
finally
{
sortStateLock.unlock();
}
pause(2);
}
repaint();
}
public void paint(Graphics g)
{
sortStateLock.lock();
try
{
try
{
while (isPaused)
sortStateLock.wait();
}
catch(InterruptedException e)
{
System.out.println("Problem in painting");
e.printStackTrace();
}
//Methods for painting lines not shown
}
finally
{
sortStateLock.unlock();
}
}
/**
* Repaints and pauses to see efficiency of algorithm when compared with others.
* @param steps Number of steps to pause for.
* @throws InterruptedException
*/
public void pause(int steps) throws InterruptedException
{
repaint();
Thread.sleep(DELAY*steps);
}
public boolean isPaused()
{
return isPaused;
}
public void pauseActions()
{
sortStateLock.lock();
try
{
isPaused=true;
}
finally
{
sortStateLock.unlock();
}
}
public void resumeActions()
{
sortStateLock.lock();
try
{
sortStateLock.notifyAll();
isPaused=false;
}
finally
{
sortStateLock.unlock();
}
}
private boolean isPaused;
private int f;
private int t;
private int currElement1;
private int currElement2;
private int lineSpacing;
private static final int ARRAY_LENGTH=100;
private static final int DELAY=100;
private Lock sortStateLock;
private int[] lines;
}

[Norm]

Please edit you posted code and add code tags to preserve indentations.

Have you tried debugging your code by adding println() to show where/what the code is doing and how variables are changing?


Your code doesn't compile:

Running: D:\Java\jdk1.6.0_02\bin\javac.exe -Xlint -g -deprecation -classpath D:\JavaDevelopment\;.;..\..\..\..\. MergePanel.java

MergePanel.java:141: cannot find symbol
symbol  : variable i
location: class MergePanel
while (i<first.length && j<second.length)
       ^
MergePanel.java:141: cannot find symbol
symbol  : variable first
location: class MergePanel
while (i<first.length && j<second.length)
         ^
MergePanel.java:141: cannot find symbol
symbol  : variable j
location: class MergePanel
while (i<first.length && j<second.length)
                         ^
MergePanel.java:141: cannot find symbol
symbol  : variable second
location: class MergePanel
while (i<first.length && j<second.length)
                           ^
MergePanel.java:35: warning: [serial] serializable class MergePanel has no definition of serialVersionUID
public class MergePanel extends JPanel {
       ^
4 errors
1 warning

3 error(s)

[Maxideon]

If you are going to use Lock objects for your synchronization purposes, you should use Condition objects for your wait() and notifyAll() purposes. As it is, you are calling wait() directly on your ReentrantLock. That's not how it works.

[Eranga]

And also please use code tags when you are posting again. Unformated codes are really hard to read.

[Eranga]

Please edit you posted code and add code tags to preserve indentations.

Have you tried debugging your code by adding println() to show where/what the code is doing and how variables are changing?

Your code doesn't compile:

yeah, seems to me this code is not written by the OP or may be this is a part of an assignment. :rolleyes:

[JosAH]

Maxideon is right: the OP should read the API documentation for ReentrantLock objects and Condition variables.

kind regards,

Jos

[cuffJ]

Ok, thanks everyone. Really sorry about the indentations; i'm new to these forums. The condition tip helped alot. Thanks again.