* When the lock is released, those threads attempting to aquire a write lock
* will take priority over those trying to get a read lock.
*
* @author Leo Sutic
*/
public class ReadWriteLock
{
/**
* The number of read locks currently held.
*/
private int m_numReadLocksHeld = 0;
/**
* The number of threads waiting for a write lock.
*/
private int m_numWaitingForWrite = 0;
/**
* Synchronization primitive.
*/
private Object m_lock = new Object ();
/**
* Default constructor.
*/
public ReadWriteLock ()
{
}
/**
* Attempts to aquire a read lock. If no lock could be aquired
* the thread will wait until it can be obtained.
*
* @throws InterruptedException if the thread is interrupted while waiting for
* a lock.
*/
public void aquireRead ()
throws InterruptedException
{
synchronized ( m_lock )
{
while ( !(m_numReadLocksHeld != -1 && m_numWaitingForWrite == 0) )
{
m_lock.wait();
}
m_numReadLocksHeld++;
}
}
/**
* Attempts to aquire a write lock. If no lock could be aquired
* the thread will wait until it can be obtained.
*
* @throws InterruptedException if the thread is interrupted while waiting for
* a lock.
*/
public void aquireWrite ()
throws InterruptedException
{
synchronized ( m_lock )
{
m_numWaitingForWrite++;
while ( m_numReadLocksHeld != 0 )
{
m_lock.wait();
}
m_numReadLocksHeld = -1;
m_numWaitingForWrite--;
}
}
/**
* Releases a lock. This method will release both types of locks.
*
* @throws IllegalStateException when an attempt is made to release
* an unlocked lock.
*/
public void release ()
{
synchronized ( m_lock )
{
if ( m_numReadLocksHeld == 0 )
{
throw new IllegalStateException ("Attempted to release an unlocked ReadWriteLock.");
}
if ( m_numReadLocksHeld == -1 )
{
m_numReadLocksHeld = 0;
}
else
{
m_numReadLocksHeld--;
}
m_lock.notifyAll();
}
}
/**
* Attempts to aquire a read lock. This method returns immediately.
*
* @return true iff the lock was successfully obtained.
*/
public boolean tryAquireRead ()
{
synchronized ( m_lock )
{
if ( m_numReadLocksHeld != -1 && m_numWaitingForWrite == 0 )
{
m_numReadLocksHeld++;
return true;
}
else
{
return false;
}
}
}
/**
* Attempts to aquire a write lock. This method returns immediately.
*
* @return true iff the lock was successfully obtained.
*/
public boolean tryAquireWrite ()
{
synchronized ( m_lock )
{
if ( m_numReadLocksHeld == 0 )
{
m_numReadLocksHeld = -1;
return true;
}
else
{
return false;
}
}
}
}