src/org/objectweb/cjdbc/common/util/ReadPrioritaryFIFOWriteLock.java

説明を見る。

package org.objectweb.cjdbc.common.util;

import java.util.ArrayList;

public class ReadPrioritaryFIFOWriteLock
{
  private int activeReaders;

  private boolean activeWriter;

  private int waitingReaders;

  private int waitingWriters;

  private Object readSync;
  private ArrayList writeWaitingQueue;

  public ReadPrioritaryFIFOWriteLock()
  {
    activeReaders = 0;
    activeWriter = false;
    waitingReaders = 0;
    waitingWriters = 0;
    readSync = new Object();
    writeWaitingQueue = new ArrayList();
  }

  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++;
    }
  }

  public synchronized void releaseRead()
  {
    activeReaders--;
    if ((activeReaders == 0) && (waitingWriters > 0))
    { // Wake up first waiting write if any
      Object ww = writeWaitingQueue.remove(0);
      synchronized (ww)
      {
        ww.notify();
        activeWriter = true;
        waitingWriters--;
      }
    }
  }

  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;
      }
    }
  }

  public synchronized void releaseWrite()
  {
    activeWriter = false;

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

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

  public final synchronized boolean isReadLocked()
  {
    return activeReaders > 0;
  }

  public final synchronized boolean isWriteLocked()
  {
    return activeWriter;
  }
}

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