Write and read locks in Java

Here's a crude but very transparent way of doing the locking. Note the below code assumes that the implementer will ensure by design that there are NO concurrent writers but concurrent readers i.e., only a single thread will write but multiple threads could read.

int _readers = 0;
boolean _writeWaiting = false;

writeMethod() {

_writeWaiting = true;
boolean canWrite = false;

while (!canWrite) {

synchronized (lock) {

canWrite = _readers > 0 ? false : true;
if (canWrite) {
_writeWaiting = false;
writeToFile;
}
}
}
}

readMethod() {

while (_writeWaiting) {
sleep;
}

synchronized (lock) {
_readers++;
}

readFile;

synchronized (lock) {
_readers--;
}
}

The following code uses Java 1.5 APIs and is a lot more efficient than the above pseudocode. But, it is very opaque. If your JVM implementation has a bug ( and JVMs do have bugs ), the below code could be very tricky to validate and debug.

static ReentrantReadWriteLock _rwl = new ReentrantReadWriteLock();

writeMethod() {

rwl.writeLock().lock();
writeToFile();
rwl.writeLock().unlock();

}

readMethod() {

rwl.readLock().lock();
readFromFile();
rwl.readLock().unlock();

}

Update to post: Edit made on Jan 1, 2011 to clarify the beginning paragraph of this article.