Re: Locking objects in an array
Philipp wrote:
Hello,
I've come accross a threading problem for which I can't find a nice
solution. (SSCCP at end of post)
I have a bidimensional array of objects (view it as a 2D lattice). I
want to make atomic operations on random square 2x2 regions of the
lattice. Thus I want to lock the 4 objects of the region, then perform
the change, then unlock those objects. Several threads should be
allowed to work on the array at the same time, if each thread is
accessing a 2x2 region which does not overlap with that of another,
they should be capable of doing the work in a parallel way.
How should I design the code to achieve this?
My solution (which may be misguided) so far is that, each object of
the lattice contains a Lock and has a lock() and unlock() method which
delegate to the lock.
So the approach is basically:
1. call lock() on each of the 4 objects (always in the same order)
2. make change
3. call unlock() on each of the 4 objects
What I don't like about this, is that
a) lock and unlock really have nothing to do in the API of the objects
in the lattice. Would it be possible to achieve the same result
without using an explicit Lock (thus no lock() method), but using the
Java synchronized() mechanism?
b) if an exception is thrown anywhere, eg. where only a part of the
lattice region has been locked, the cleanup is ugly because you can't
test if a lock is presently locked. You have to rely on unlock() to
throw.
c) An exception may leave the lattice in an inconsistent state.
Here is the design I would consider:
Note, this is *completely* untested. It is also just a first iteration.
Replacing "boolean[][] locks" with a BitSet may be advantageous,
depending on the size of your data set.
If you really want to ensure exceptions don't leave the lattice in bad
shape, then you might make the objects themselves immutable, and have
Opeartion.operate return new objects, and have those be replaced into
the array upon success.
package latice;
import java.util.Arrays;
/**
* @author Daniel Pitts
*/
public class Latice<T> {
public static interface Operator<T> {
void operate(T[][] data);
}
private final T[][] data;
private final boolean[][] locks;
public Latice(int width, int height) {
data = allocate(width, height);
locks = new boolean[height][width];
}
public boolean operateOnRegion(int x, int y, int width, int height,
Operator<T> operator, long timeout) throws InterruptedException {
if (!lockRegion(x, y, width, height, timeout)) {
return false;
}
try {
operator.operate(getRegion(x, y, width, height));
return true;
} finally {
unlockRegion(x, y, width, height);
}
}
private void unlockRegion(int x, int y, int width, int height) {
synchronized (locks) {
setLockValue(x, y, width, height, false);
locks.notifyAll();
}
}
private void setLockValue(int x, int y, int width, int height,
boolean lockValue) {
for (int i = 0; i < height; ++i) {
Arrays.fill(locks[y + i], x, x+width, lockValue);
}
}
private T[][] getRegion(int x, int y, int width, int height) {
T[][] region = allocate(width, height);
for (int i = 0; i < height; ++i) {
System.arraycopy(data[i+y], x, region[i], 0, width);
}
return region;
}
private static <T> T[][] allocate(int width, int height) {
@SuppressWarnings({"unchecked", "UnnecessaryLocalVariable"})
final T[][] genericsBroken = (T[][]) new Object[height][width];
return genericsBroken;
}
private boolean lockRegion(int x, int y, int width, int height,
long timeout) throws InterruptedException {
final long endTime = System.currentTimeMillis() + timeout;
synchronized (locks) {
do {
final long timeNow = System.currentTimeMillis();
if (checkLocks(x, y, width, height)) {
break;
}
if (timeout == 0) {
locks.wait();
} else if (timeNow < endTime) {
locks.wait(endTime - timeNow);
} else {
return false;
}
} while (true);
setLockValue(x, y, width, height, true);
}
return true;
}
private boolean checkLocks(int x, int y, int width, int height) {
for (int j = 0; j < height; ++j) {
final boolean[] row = locks[y + j];
for (int i = 0; i < width; ++i) {
if (row[x+i]) {
return false;
}
}
}
return true;
}
}
--
Daniel Pitts' Tech Blog: <http://virtualinfinity.net/wordpress/>