Read Write Lock
/*
* Copyright 2002 (C) TJDO.
* All rights reserved.
*
* This software is distributed under the terms of the TJDO License version 1.0.
* See the terms of the TJDO License in the documentation provided with this software.
*
* $Id: ReadWriteLock.java,v 1.5 2006/08/02 22:41:25 jackknifebarber Exp $
*/
/**
* A simple read-write lock implementation. Multiple threads may lock using
* readLock(), only one can lock using writeLock(). The caller is responsible
* for coding a try-finally that ensures unlock() is called for every readLock()
* and writeLock() call.
*
* <p>A ReadWriteLock is recursive; with one exception, a thread can re-lock an
* object it already has locked. Multiple read locks can be acquired by the
* same thread, as can multiple write locks. The exception however is that a
* write lock cannot be acquired when a read lock is already held (to allow
* this would cause deadlocks).
*
* <p>Successive lock calls from the same thread must be matched by an
* equal number of unlock() calls.
*
* @author <a href="mailto:mmartin5@austin.rr.com">Mike Martin</a>
* @version $Revision: 1.5 $
*/
public class ReadWriteLock
{
private static final int WAIT_LOG_INTERVAL = 5000;
/** A count for each thread indicating the number of read locks it holds. */
private ThreadLocal readLocksByThread;
/** The number of read locks held across all threads. */
private int readLocks;
/** The number of write locks held (by writeLockedBy). */
private int writeLocks;
/** The thread holding the write lock(s), if any. */
private Thread writeLockedBy;
/**
* An object holding a per-thread read-lock count.
*/
private static class Count
{
public int value = 0;
}
/**
* Constructs read-write lock.
*/
public ReadWriteLock()
{
readLocksByThread = new ThreadLocal()
{
public Object initialValue()
{
return new Count();
}
};
readLocks = 0;
writeLocks = 0;
writeLockedBy = null;
}
/**
* Acquire a read lock. The calling thread will be suspended until no other
* thread holds a write lock.
*
* <p>If the calling thread already owns a write lock for the object a read
* lock is immediately acquired.
*
* @exception InterruptedException
* If the thread is interrupted while attempting to acquire the lock.
*/
public synchronized void readLock() throws InterruptedException
{
Thread me = Thread.currentThread();
Count myReadLocks = (Count)readLocksByThread.get();
if (writeLockedBy != me)
{
while (writeLocks > 0)
{
wait(WAIT_LOG_INTERVAL);
if (writeLocks > 0)
System.out.println("Still waiting for read lock on ");
}
}
++readLocks;
++myReadLocks.value;
}
/**
* Acquire a write lock. The calling thread will be suspended until no
* other thread holds a read or write lock.
*
* <p>This method cannot be called if the thread already owns a read lock on
* the same ReadWriteLock object, otherwise an
* <code>IllegalStateException</code> is thrown.
*
* @exception IllegalStateException
* If the thread already holds a read lock on the same object.
* @exception InterruptedException
* If the thread is interrupted while attempting to acquire the lock.
*/
public synchronized void writeLock() throws InterruptedException
{
Thread me = Thread.currentThread();
Count myReadLocks = (Count)readLocksByThread.get();
if (myReadLocks.value > 0)
throw new IllegalStateException("Thread already holds a read lock");
if (writeLockedBy != me)
{
while (writeLocks > 0 || readLocks > 0)
{
wait(WAIT_LOG_INTERVAL);
if (writeLocks > 0 || readLocks > 0)
System.out.println("Still waiting for write lock on ");
}
writeLockedBy = me;
}
++writeLocks;
}
/**
* Release a read or write lock. Must be called in a finally block after
* acquiring a lock.
*/
public synchronized void unlock()
{
Thread me = Thread.currentThread();
Count myReadLocks = (Count)readLocksByThread.get();
if (myReadLocks.value > 0)
{
--myReadLocks.value;
--readLocks;
}
else if (writeLockedBy == me)
{
if (writeLocks > 0)
{
if (--writeLocks == 0)
writeLockedBy = null;
}
}
notifyAll();
}
public String toString()
{
StringBuffer s = new StringBuffer(super.toString());
s.append(": readLocks = ").append(readLocks)
.append(", writeLocks = ").append(writeLocks)
.append(", writeLockedBy = ").append(writeLockedBy);
return s.toString();
}
}
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