UniqueLockMonitor.java

/**
 * A stateful implementation of an auto-closable lock/monitor 
 * for the specified lockwith an ability to unlock it preliminarily.
 * The class philosophy and namings are borrowed from C++ std::unique_lock<>.
 *
 * @apiNote this class isn't reentrant and isn't supposed to be shared across threads 
 * or to be used as a field.
 * The instances of this class should be used as 
 * stack/method local variables with try-with-resources.
 */
public final class UniqueLockMonitor implements AutoCloseable {
	private boolean locked;
	private final Lock lock;

	/**
	 * Private constructor to initialise a monitor
	 *
	 * @param lock   underlying lock
	 * @param locked the current lock status
	 */
	private UniqueLockMonitor(final Lock lock, final boolean locked) {
		assert lock != null;
		this.lock = lock;
		this.locked = locked;
	}

	/**
	 * Lock and create a monitor on the specified lock
	 *
	 * @param lock a lock
	 * @return a monitor instance on the specified lock
	 */
	public static UniqueLockMonitor lockAndMonitor(final Lock lock) {
		final UniqueLockMonitor result = new UniqueLockMonitor(lock, false);
		result.lock();
		return result;
	}

	/**
	 * Lock manually.
	 *
	 * @apiNote Consequent multiple locks aren't allowed 
	 * to avoid mistakes and sharing monitor instances.
	 */
	void lock() {
		assert !this.locked;

		lock.lock();
		this.locked = true;
	}

	/**
	 * Unlock manually.
	 *
	 * @apiNote Consequent multiple unlocks aren't allowed 
	 * to avoid mistakes and sharing monitor instances.
	 */
	void unlock() {
		assert this.locked;

		lock.unlock();
		this.locked = false;
	}

	@Override
	public void close() {
		if (this.locked) {
			unlock();
		}
	}
}