common-utils/src/main/java/org/warp/commonutils/locks/TransitLock.java

package org.warp.commonutils.locks;

import java.util.concurrent.Semaphore;
import java.util.concurrent.atomic.AtomicInteger;

public class TransitLock {

	private final FlexibleCountDownLatch unfinishedTransits = new FlexibleCountDownLatch(0);
	private final Semaphore permits = new Semaphore(Integer.MAX_VALUE);
	private final AtomicInteger allowedTransits = new AtomicInteger(1);
	private final Object synchronization = new Object();

	public TransitLock() {
	}

	public void allowTransit() {
		var allowedTransits = this.allowedTransits.incrementAndGet();
		if (allowedTransits == 1) {
			var permitsToRelease = Math.max(0, Integer.MAX_VALUE - (permits.availablePermits() + 10000));
			permits.release(permitsToRelease);
		} else if (allowedTransits > 1) {
			throw new IllegalStateException("Already allowed transit!");
		}
	}

	public void disallowTransit() {
		synchronized (synchronization) {
			var allowedTransits = this.allowedTransits.decrementAndGet();
			if (allowedTransits == 0) {
				unfinishedTransits.await();
				permits.drainPermits();
			} else if (allowedTransits > 0) {
				throw new IllegalStateException("Transits are still allowed for an unknown reason!");
			}
		}
	}

	public void transit() {
		startTransit();
		endTransit();
	}

	public void startTransit() {
		synchronized (synchronization) {
			unfinishedTransits.grow();
			permits.acquireUninterruptibly();
		}
	}

	public void endTransit() {
		unfinishedTransits.countDown();
	}
}