ReadPrioritaryFIFOWriteLock.java

/**
 * C-JDBC: Clustered JDBC.
 * Copyright (C) 2002-2004 French National Institute For Research In Computer
 * Science And Control (INRIA).
 * Contact: c-jdbc@objectweb.org
 * 
 * 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 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., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA.
 *
 * Initial developer(s): Emmanuel Cecchet.
 * Contributor(s): ______________________.
 */

package org.objectweb.cjdbc.common.util;

import java.util.ArrayList;

/**
 * Reader/Writer lock with write priority.
 * <p>
 * If a reader holds the lock, all incoming readers are allowed to acquire the
 * lock until a write arrives. A writer must wait for all current readers to
 * release the lock before acquiring it. <br>
 * When a writer has the lock, everybody else is blocked. <br>
 * When a writer release the lock, there is a writer priority so if another
 * writer is waiting it will have the lock even if it arrived later than
 * readers. Writers are prioritary against readers but all writers get the lock
 * in a FIFO order.
 * 
 * @author <a href="mailto:Emmanuel.Cecchet@inria.fr">Emmanuel Cecchet </a>
 * @version 1.0
 */
public class ReadPrioritaryFIFOWriteLock
{
  /** Threads executing read. */
  private int       activeReaders;

  /** Is there an active writer? */
  private boolean   activeWriter;

  /** Threads not yet in read. */
  private int       waitingReaders;

  /** Threads not yet in write. */
  private int       waitingWriters;

  private Object    readSync;
  private ArrayList writeWaitingQueue;

  /**
   * Creates a new <code>ReadPrioritaryFIFOWriteLock</code> instance.
   */
  public ReadPrioritaryFIFOWriteLock()
  {
    activeReaders = 0;
    activeWriter = false;
    waitingReaders = 0;
    waitingWriters = 0;
    readSync = new Object();
    writeWaitingQueue = new ArrayList();
  }

  /**
   * Acquires the lock for a read.
   * 
   * @throws InterruptedException if wait failed (the lock should remain in a
   *           correct state)
   */
  public void acquireRead() throws InterruptedException
  {
    synchronized (this)
    {
      if ((waitingWriters == 0) && !activeWriter)
      { // No active or pending writer
        activeReaders++;
        return;
      }
    }

    // Beyond this point, we have to wait
    synchronized (readSync)
    {
      // It becomes a little bit tricky here but a writer could have
      // released the lock between the first test at the beginning of
      // this function and this point. Therefore we have to recheck if
      // the lock is not completely idle else we'll never wake up !
      synchronized (this)
      {
        if (!activeWriter)
        { // No active or pending writer
          activeReaders++;
          return;
        }
      }

      waitingReaders++;
      try
      {
        readSync.wait();
      }
      catch (InterruptedException ie)
      {
        waitingReaders--; // roll back state
        throw ie;
      }
      waitingReaders--;
    }
    synchronized (this)
    {
      activeReaders++;
    }
  }

  /**
   * Releases a lock previously acquired for reading.
   */
  public synchronized void releaseRead()
  {
    activeReaders--;
    if ((activeReaders == 0) && (waitingWriters > 0))
    { // Wake up first waiting write if any
      Object thread = writeWaitingQueue.remove(0);
      synchronized (thread)
      {
        thread.notify();
        activeWriter = true;
        waitingWriters--;
      }
    }
  }

  /**
   * Acquires the lock for a write.
   * 
   * @throws InterruptedException if wait failed (the lock should remain in a
   *           correct state)
   */
  public void acquireWrite() throws InterruptedException
  {
    synchronized (Thread.currentThread())
    {
      synchronized (this)
      {
        if ((activeReaders == 0) && !activeWriter)
        {
          activeWriter = true;
          return;
        }
        else
        {
          waitingWriters++;
          writeWaitingQueue.add(Thread.currentThread());
        }
      }
      try
      {
        Thread.currentThread().wait();
      }
      catch (InterruptedException ie)
      {
        releaseWrite();
        throw ie;
      }
    }
  }

  /**
   * Releases a lock previously acquired for writing.
   */
  public synchronized void releaseWrite()
  {
    activeWriter = false;

    // Writer priority
    if (waitingWriters > 0)
    { // Wake up first waiting write if any
      Object thread = writeWaitingQueue.remove(0);
      synchronized (thread)
      {
        thread.notify();
        activeWriter = true;
        waitingWriters--;
      }
    }

    // Wake up readers
    else
      synchronized (readSync)
      {
        if (waitingReaders > 0)
          readSync.notifyAll();
      }
  }

  /**
   * Tests if the lock is currently held by at least one reader.
   * 
   * @return <code>true</code> if the lock is held by a reader
   */
  public final synchronized boolean isReadLocked()
  {
    return activeReaders > 0;
  }

  /**
   * Tests if the lock is currently held by a writer.
   * 
   * @return <code>true</code> if the lock is held by a writer
   */
  public final synchronized boolean isWriteLocked()
  {
    return activeWriter;
  }
}

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