content: /* ========================================================================
   * JCommon : a free general purpose class library for the Java(tm) platform
   * ========================================================================
   *
   * (C) Copyright 2000-2005, by Object Refinery Limited and Contributors.
   * 
   * Project Info:  http://www.jfree.org/jcommon/index.html
   *
   * This library is free software; you can redistribute it and/or modify it 
   * under the terms of the GNU Lesser General Public License as published by 
   * the Free Software Foundation; either version 2.1 of the License, or 
   * (at your option) any later version.
   *
   * This library 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. See the GNU Lesser General Public 
   * License for more details.
   *
   * You should have received a copy of the GNU Lesser General Public
   * License along with this library; if not, write to the Free Software
   * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301, 
   * USA.  
   *
   * [Java is a trademark or registered trademark of Sun Microsystems, Inc. 
   * in the United States and other countries.]
   * 
   * ---------------------
   * ReaderWriterLock.java
   * ---------------------
   *
   * $Id: ReaderWriterLock.java,v 1.3 2005/10/18 13:18:34 mungady Exp $
   *
   * Changes
   * -------
   * 29-Jan-2003 : Added standard header (DG);
   *
   */
  
  package org.jfree.threads;
  
  import java.util.ArrayList;
  import java.util.Iterator;
  
  /**
   * A reader-writer lock from "Java Threads" by Scott Oak and Henry Wong.
   *
   * @author Scott Oak and Henry Wong
   */
  public class ReaderWriterLock {
  
      /**
       * A node for the waiting list.
       *
       * @author Scott Oak and Henry Wong
       */
      private static class ReaderWriterNode {
  
          /** A reader. */
          protected static final int READER = 0;
  
          /** A writer. */
          protected static final int WRITER = 1;
  
          /** The thread. */
          protected Thread t;
  
          /** The state. */
          protected int state;
  
          /** The number of acquires.*/
          protected int nAcquires;
  
          /**
           * Creates a new node.
           *
           * @param t  the thread.
           * @param state  the state.
           */
          private ReaderWriterNode(final Thread t, final int state) {
              this.t = t;
              this.state = state;
              this.nAcquires = 0;
          }
  
      }
  
      /** The waiting threads. */
      private ArrayList waiters;
  
      /**
       * Default constructor.
       */
      public ReaderWriterLock() {
          this.waiters = new ArrayList();
      }
  
      /**
       * Grab the read lock.
       */
      public synchronized void lockRead() {
          final ReaderWriterNode node;
          final Thread me = Thread.currentThread();
          final int index = getIndex(me);
          if (index == -1) {
              node = new ReaderWriterNode(me, ReaderWriterNode.READER);
              this.waiters.add(node);
          }
          else {
              node = (ReaderWriterNode) this.waiters.get(index);
          }
          while (getIndex(me) > firstWriter()) {
              try {
                  wait();
              }
              catch (Exception e) {
                  System.err.println("ReaderWriterLock.lockRead(): exception.");
                  System.err.print(e.getMessage());
              }
          }
          node.nAcquires++;
      }
  
      /**
       * Grab the write lock.
       */
      public synchronized void lockWrite() {
          final ReaderWriterNode node;
          final Thread me = Thread.currentThread();
          final int index = getIndex(me);
          if (index == -1) {
              node = new ReaderWriterNode(me, ReaderWriterNode.WRITER);
              this.waiters.add(node);
          }
          else {
              node = (ReaderWriterNode) this.waiters.get(index);
              if (node.state == ReaderWriterNode.READER) {
                  throw new IllegalArgumentException("Upgrade lock");
              }
              node.state = ReaderWriterNode.WRITER;
          }
          while (getIndex(me) != 0) {
              try {
                  wait();
              }
              catch (Exception e) {
                  System.err.println("ReaderWriterLock.lockWrite(): exception.");
                  System.err.print(e.getMessage());
              }
          }
          node.nAcquires++;
      }
  
      /**
       * Unlock.
       */
      public synchronized void unlock() {
  
          final ReaderWriterNode node;
          final Thread me = Thread.currentThread();
          final int index = getIndex(me);
          if (index > firstWriter()) {
              throw new IllegalArgumentException("Lock not held");
          }
          node = (ReaderWriterNode) this.waiters.get(index);
          node.nAcquires--;
          if (node.nAcquires == 0) {
              this.waiters.remove(index);
          }
          notifyAll();
      }
  
      /**
       * Returns the index of the first waiting writer.
       *
       * @return The index.
       */
      private int firstWriter() {
          final Iterator e = this.waiters.iterator();
          int index = 0;
          while (e.hasNext()) {
              final ReaderWriterNode node = (ReaderWriterNode) e.next();
              if (node.state == ReaderWriterNode.WRITER) {
                  return index;
              }
              index += 1;
          }
          return Integer.MAX_VALUE;
      }
  
      /**
       * Returns the index of a thread.
       *
       * @param t  the thread.
       *
       * @return The index.
       */
      private int getIndex(final Thread t) {
          final Iterator e = this.waiters.iterator();
          int index = 0;
          while (e.hasNext()) {
              final ReaderWriterNode node = (ReaderWriterNode) e.next();
              if (node.t == t) {
                  return index;
              }
              index += 1;
          }
          return -1;
      }
  
  }
  