src/org/objectweb/cjdbc/controller/scheduler/schema/TransactionExclusiveLock.java

説明を見る。

package org.objectweb.cjdbc.controller.scheduler.schema;

import java.util.ArrayList;

import org.objectweb.cjdbc.common.sql.AbstractRequest;

public class TransactionExclusiveLock
{
  private boolean   isLocked    = false;

  private long      locker;

  private ArrayList waitingList = new ArrayList();

  private class WaitingListElement
  {
    Thread thread;

    long   transactionId;

    WaitingListElement(Thread thread, long transactionId)
    {
      this.thread = thread;
      this.transactionId = transactionId;
    }

    public long getTransactionId()
    {
      return transactionId;
    }

    public Thread getThread()
    {
      return thread;
    }
  }

  public boolean acquire(AbstractRequest request)
  {
    long tid = request.getTransactionId();

    synchronized (Thread.currentThread())
    {
      WaitingListElement wle = null;
      synchronized (this)
      {
        if (!isLocked)
        { // Lock is free, take it
          locker = tid;
          isLocked = true;
          return true;
        }
        else
        {
          if ((!request.isAutoCommit()) && (locker == tid))
            return true; // We already have the lock
          else
          { // Wait for the lock
            wle = new WaitingListElement(Thread.currentThread(), tid);
            waitingList.add(wle);
          }
        }
      }
      // At this point, we have to wait for the lock.
      try
      {
        int timeout = request.getTimeout();
        if (timeout == 0)
        {
          Thread.currentThread().wait(); // No timeout
          // Note: isLocked and locker are already set.
          return true;
        }
        else
        { // Wait with timeout
          long start = System.currentTimeMillis();
          // Convert seconds to milliseconds for wait call
          long lTimeout = timeout * 1000;
          Thread.currentThread().wait(lTimeout);
          long end = System.currentTimeMillis();
          int remaining = (int) (lTimeout - (end - start));
          if (remaining > 0)
          { // Ok
            request.setTimeout(remaining);
            // Note: isLocked and locker are already set.
            return true;
          }
          else
          { // Too late, remove ourselves from the waiting list
            synchronized (this)
            {
              int idx = waitingList.indexOf(wle);
              if (idx == -1)
                // We got the lock before being able to acquire the lock on
                // this. Give the lock to the next one.
                release();
              else
                waitingList.remove(idx);
            }
            return false;
          }
        }
      }
      catch (InterruptedException ie)
      {
        synchronized (this)
        { // Something wrong happened, remove ourselves from the waiting list
          waitingList.remove(Thread.currentThread());
        }
        return false;
      }
    }
  }

  public synchronized void release()
  {
    while (!waitingList.isEmpty())
    {
      // Wake up the first waiting thread and update locker transaction id
      WaitingListElement e = (WaitingListElement) waitingList.remove(0);
      Thread thread = e.getThread();
      locker = e.getTransactionId();
      synchronized (thread)
      {
        thread.notify();
        // isLocked remains true
        return;
      }
    }
    isLocked = false;
  }

  public boolean isLocked()
  {
    return isLocked;
  }

  public long getLocker()
  {
    return locker;
  }

  public ArrayList getWaitingList()
  {
    return waitingList;
  }

  public synchronized boolean isWaiting(long transactionId)
  {
    WaitingListElement e;
    int size = waitingList.size();
    for (int i = 0; i < size; i++)
    {
      e = (WaitingListElement) waitingList.get(i);
      if (e.getTransactionId() == transactionId)
        return true;
    }
    return false;
  }
}

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