| A writer-preference ReadWriteLock that allows both readers and
writers to reacquire
read or write locks in the style of a ReentrantLock.
Readers are not allowed until all write locks held by
the writing thread have been released.
Among other applications, reentrancy can be useful when
write locks are held during calls or callbacks to methods that perform
reads under read locks.
Sample usage. Here is a code sketch showing how to exploit
reentrancy to perform lock downgrading after updating a cache:
class CachedData {
Object data;
volatile boolean cacheValid;
ReentrantWriterPreferenceReadWriteLock rwl = ...
void processCachedData() {
rwl.readLock().acquire();
if (!cacheValid) {
// upgrade lock:
rwl.readLock().release(); // must release first to obtain writelock
rwl.writeLock().acquire();
if (!cacheValid) { // recheck
data = ...
cacheValid = true;
}
// downgrade lock
rwl.readLock().acquire(); // reacquire read without giving up lock
rwl.writeLock().release(); // release write, still hold read
}
use(data);
rwl.readLock().release();
}
}
[ Introduction to this package. ]
See Also: ReentrantLock | 