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  /*
   * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS HEADER.
   * 
   * Copyright 1997-2007 Sun Microsystems, Inc. All rights reserved.
   * 
   * The contents of this file are subject to the terms of either the GNU
   * General Public License Version 2 only ("GPL") or the Common Development
   * and Distribution License("CDDL") (collectively, the "License").  You
   * may not use this file except in compliance with the License. You can obtain
  * a copy of the License at https://glassfish.dev.java.net/public/CDDL+GPL.html
  * or glassfish/bootstrap/legal/LICENSE.txt.  See the License for the specific
  * language governing permissions and limitations under the License.
  * 
  * When distributing the software, include this License Header Notice in each
  * file and include the License file at glassfish/bootstrap/legal/LICENSE.txt.
  * Sun designates this particular file as subject to the "Classpath" exception
  * as provided by Sun in the GPL Version 2 section of the License file that
  * accompanied this code.  If applicable, add the following below the License
  * Header, with the fields enclosed by brackets [] replaced by your own
  * identifying information: "Portions Copyrighted [year]
  * [name of copyright owner]"
  * 
  * Contributor(s):
  * 
  * If you wish your version of this file to be governed by only the CDDL or
  * only the GPL Version 2, indicate your decision by adding "[Contributor]
  * elects to include this software in this distribution under the [CDDL or GPL
  * Version 2] license."  If you don't indicate a single choice of license, a
  * recipient has the option to distribute your version of this file under
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  * its licensees as provided above.  However, if you add GPL Version 2 code
  * and therefore, elected the GPL Version 2 license, then the option applies
  * only if the new code is made subject to such option by the copyright
  * holder.
  */
 
 /*
  * Copyright 2004-2005 Sun Microsystems, Inc.  All rights reserved.
  * Use is subject to license terms.
  */
 
 package com.sun.jts.CosTransactions;
A RWLock provides concurrency control for multiple readers single writer access patterns. This lock can provide access to multiple reader threads simultaneously as long as there are no writer threads. Once a writer thread gains access to the instance locked by a RWLock, all the reader threads wait till the writer completes accessing the instance in question.

A RWLock is extremely useful in scenarios where there are lots more readers and very few writers to a data structure. Also if the read operation by the reader thread could take significant amount of time (binary search etc.)

The usage of Lock can be see as under:


    public class MyBTree {
      private RWLock lock = new Lock();
      .....
      .....
      public Object find(Object o) {
        try {
          lock.acquireReadLock();
          ....perform complex search to get the Object ...
          return result;
        } finally {
          lock.releaseReadLock();
        }
      }

      public void insert(Object o) {
        try {
          lock.acquireWriteLock();
          ....perform complex operation to insert object ...
        } finally {
          lock.releaseWriteLock();
        }
      }
    }
 

Author(s):
Dhiru Pandey 8/7/2000
 
  
  
 public class RWLock {
 
   int currentReaders;
   int pendingReaders;
   int currentWriters;
   /*
  	 Logger to log transaction messages
   */ 
  static Logger _logger = LogDomains.getLogger(RWLock.class.);
 
  Queue writerQueue = new Queue();
  
This method is used to acquire a read lock. If there is already a writer thread accessing the object using the RWLock then the reader thread will wait until the writer completes its operation
  public synchronized void acquireReadLock() {
    if ( == 0 && .size() == 0) {
      ++;
    } else {
      ++;
      try {
        wait();
      } catch(InterruptedException ie) {
	  	.log(.,"Error in acquireReadLock",ie);
      }
    }
  }

  
This method is used to acquire a write lock. If there are already reader threads accessing the object using the RWLock, then the writer thread will wait till all the reader threads are finished with their operations.
  public void acquireWriteLock() {
    Object lock = new Object();
    synchronized(lock) {
      synchronized(this) {
        if (.size() == 0 &&  == 0 &&  == 0) {
          ++;
          // Use logging facility if you need to log this
			//_logger.log(Level.FINE," RW: incremented WriterLock count");
          return;
        }
        .enQueue(lock);
        // Use logging facility if you need to log this
			//_logger.log(Level.FINE," RW: Added WriterLock to queue");
      }
      try {
        lock.wait();
      } catch(InterruptedException ie) {
	  	.log(.,"Error in acquireWriteLock",ie);
      }
    }
  }

  
isWriteLocked returns true if the RWLock is in a write locked state.
  public boolean isWriteLocked()
  {
	  return  > 0 ;
  }
 
  
This method is used to release a read lock. It also notifies any waiting writer thread that it could now acquire a write lock.
  public synchronized void releaseReadLock() {
    if (-- == 0) 
      notifyWriters();
  }
 
  
This method is used to release a write lock. It also notifies any pending readers that they could now acquire the read lock. If there are no reader threads then it will try to notify any waiting writer thread that it could now acquire a write lock.
  public synchronized void releaseWriteLock() {
    if ( > 0) 
      notifyReaders();
    else 
      notifyWriters();
  }
  private void notifyReaders() {
     = 0;
    notifyAll();
  }
  
  private void notifyWriters() {
    if (.size() > 0) {
      Object lock = .deQueueFirst();
      ++;
      synchronized(lock) { 
        lock.notify();
      }
    }
  }
  class Queue extends LinkedList {
    public Queue() {
      super();
    }
    public void enQueue(Object o) {
      super.addLast(o);
    }
    public Object deQueueFirst() {
      return super.removeFirst();
    }
  }
}
 


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