SimpleSharedUseExclusiveUseLock.java

/*
 * Distributed as part of mchange-commons-java 0.2.11
 *
 * Copyright (C) 2015 Machinery For Change, Inc.
 *
 * Author: Steve Waldman <swaldman@mchange.com>
 *
 * This library is free software; you can redistribute it and/or modify
 * it under the terms of EITHER:
 *
 *     1) The GNU Lesser General Public License (LGPL), version 2.1, as 
 *        published by the Free Software Foundation
 *
 * OR
 *
 *     2) The Eclipse Public License (EPL), version 1.0
 *
 * You may choose which license to accept if you wish to redistribute
 * or modify this work. You may offer derivatives of this work
 * under the license you have chosen, or you may provide the same
 * choice of license which you have been offered here.
 *
 * This software is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 *
 * You should have received copies of both LGPL v2.1 and EPL v1.0
 * along with this software; see the files LICENSE-EPL and LICENSE-LGPL.
 * If not, the text of these licenses are currently available at
 *
 * LGPL v2.1: http://www.gnu.org/licenses/old-licenses/lgpl-2.1.html
 *  EPL v1.0: http://www.eclipse.org/org/documents/epl-v10.php 
 * 
 */

package com.mchange.v2.lock;

/**
 * NONREENTRANT!!! 
 * also... would be more efficient if we
 * segregated readers and writers into 
 * separate wait sets.
 */
public class SimpleSharedUseExclusiveUseLock implements SharedUseExclusiveUseLock
{
    private int waiting_readers = 0;
    private int active_readers  = 0;

    private int     waiting_writers = 0;
    private boolean writer_active   = false;

    public synchronized void acquireShared() throws InterruptedException
    {
	try
	    {
		++waiting_readers;
		while (! okayToRead())
		    this.wait();
		++active_readers;
	    }
	finally
	    { 
		--waiting_readers; 
	    }
    }

    public synchronized void relinquishShared()
    { 
	--active_readers; 
	this.notifyAll();
    }

    public synchronized void acquireExclusive() throws InterruptedException
    {
	try
	    {
		++waiting_writers;
		while (! okayToWrite())
		    this.wait();
		writer_active = true;
	    }
	finally
	    {
		--waiting_writers;
	    }
    }

    public synchronized void relinquishExclusive()
    {
	writer_active = false;
	this.notifyAll();
    }

    private boolean okayToRead()
    { return (!writer_active && waiting_writers == 0); }

    private boolean okayToWrite()
    { return (active_readers == 0 && !writer_active); }
}