No Starvation Reader Writer

NoStarveReaderWriter

public class NoStarveReaderWriterLock {
	private final Semaphore readMutex = new Semaphore(1);
	private final Semaphore writeMutex = new Semaphore(1);
	private final Semaphore noStarveMutex = new Semaphore(1);
	private final Set<Long> readerIds = Collections
			.synchronizedSet(new HashSet<Long>());
	private volatile Long writerId;
 
	public void readLock() {
		try {
		  noStarveMutex.acquire();
		  noStarveMutex.release();
			readMutex.acquire();
			readerIds.add(Thread.currentThread().getId());
			if (readerIds.size() == 1) {
				writeMutex.acquire();
			}
		} finally {
			readMutex.release();
		}
	}
 
	public void readUnlock() {
		try {
			readMutex.acquire();
			long threadId = Thread.currentThread().getId();
			if (!readerIds.contains(threadId)) {
				throw new IllegalStateException(
						String.format(
								"The current thread with id: %d never acquired a read lock before.",
								threadId));
			}
			readerIds.remove(threadId);
			if (readerIds.size() == 0) {
				writeMutex.release();
			}
		} finally {
			readMutex.release();
		}
 
	}
 
	public void writeLock() {
	  noStarveMutex.acquire();
		writeMutex.acquire();
		writerId = Thread.currentThread().getId();
	}
 
	public void writeUnlock() {
		Long threadId = Thread.currentThread().getId();
		if (writerId == null || !writerId.equals(threadId)) {
			throw new IllegalStateException(
					String.format(
							"The current thread with id: %d never acquired a write lock before.",
							threadId));
		}
    noStarveMutex.release();
		writeMutex.release();
	}
}