RWNode.java Source code

Introduction

Here is the source code for RWNode.java

Source

/*
 *
 * Copyright (c) 1997-1999 Scott Oaks and Henry Wong. All Rights Reserved.
 *
 * Permission to use, copy, modify, and distribute this software
 * and its documentation for NON-COMMERCIAL purposes and
 * without fee is hereby granted.
 *
 * This sample source code is provided for example only,
 * on an unsupported, as-is basis. 
 *
 * AUTHOR MAKES NO REPRESENTATIONS OR WARRANTIES ABOUT THE SUITABILITY OF
 * THE SOFTWARE, EITHER EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
 * TO THE IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A
 * PARTICULAR PURPOSE, OR NON-INFRINGEMENT. AUTHOR SHALL NOT BE LIABLE FOR
 * ANY DAMAGES SUFFERED BY LICENSEE AS A RESULT OF USING, MODIFYING OR
 * DISTRIBUTING THIS SOFTWARE OR ITS DERIVATIVES.
 *
 * THIS SOFTWARE IS NOT DESIGNED OR INTENDED FOR USE OR RESALE AS ON-LINE
 * CONTROL EQUIPMENT IN HAZARDOUS ENVIRONMENTS REQUIRING FAIL-SAFE
 * PERFORMANCE, SUCH AS IN THE OPERATION OF NUCLEAR FACILITIES, AIRCRAFT
 * NAVIGATION OR COMMUNICATION SYSTEMS, AIR TRAFFIC CONTROL, DIRECT LIFE
 * SUPPORT MACHINES, OR WEAPONS SYSTEMS, IN WHICH THE FAILURE OF THE
 * SOFTWARE COULD LEAD DIRECTLY TO DEATH, PERSONAL INJURY, OR SEVERE
 * PHYSICAL OR ENVIRONMENTAL DAMAGE ("HIGH RISK ACTIVITIES").  AUTHOR
 * SPECIFICALLY DISCLAIMS ANY EXPRESS OR IMPLIED WARRANTY OF FITNESS FOR
 * HIGH RISK ACTIVITIES.
 */

import java.util.*;

class RWNode {
    static final int READER = 0;
    static final int WRITER = 1;
    Thread t;
    int state;
    int nAcquires;

    RWNode(Thread t, int state) {
        this.t = t;
        this.state = state;
        nAcquires = 0;
    }
}

public class RWLock {
    private Vector waiters;

    private int firstWriter() {
        Enumeration e;
        int index;
        for (index = 0, e = waiters.elements(); e.hasMoreElements(); index++) {
            RWNode node = (RWNode) e.nextElement();
            if (node.state == RWNode.WRITER)
                return index;
        }
        return Integer.MAX_VALUE;
    }

    private int getIndex(Thread t) {
        Enumeration e;
        int index;
        for (index = 0, e = waiters.elements(); e.hasMoreElements(); index++) {
            RWNode node = (RWNode) e.nextElement();
            if (node.t == t)
                return index;
        }
        return -1;
    }

    public RWLock() {
        waiters = new Vector();
    }

    public synchronized void lockRead() {
        RWNode node;
        Thread me = Thread.currentThread();
        int index = getIndex(me);
        if (index == -1) {
            node = new RWNode(me, RWNode.READER);
            waiters.addElement(node);
        } else
            node = (RWNode) waiters.elementAt(index);
        while (getIndex(me) > firstWriter()) {
            try {
                wait();
            } catch (Exception e) {
            }
        }
        node.nAcquires++;
    }

    public synchronized void lockWrite() {
        RWNode node;
        Thread me = Thread.currentThread();
        int index = getIndex(me);
        if (index == -1) {
            node = new RWNode(me, RWNode.WRITER);
            waiters.addElement(node);
        } else {
            node = (RWNode) waiters.elementAt(index);
            if (node.state == RWNode.READER)
                throw new IllegalArgumentException("Upgrade lock");
            node.state = RWNode.WRITER;
        }
        while (getIndex(me) != 0) {
            try {
                wait();
            } catch (Exception e) {
            }
        }
        node.nAcquires++;
    }

    public synchronized void unlock() {
        RWNode node;
        Thread me = Thread.currentThread();
        int index;
        index = getIndex(me);
        if (index > firstWriter())
            throw new IllegalArgumentException("Lock not held");
        node = (RWNode) waiters.elementAt(index);
        node.nAcquires--;
        if (node.nAcquires == 0) {
            waiters.removeElementAt(index);
            notifyAll();
        }
    }
}