exclusivelock.java

Java的面向对象数据库系统的源代码

// You can redistribute this software and/or modify it under the terms of
// the Ozone Core License version 1 published by ozone-db.org.
//
// The original code and portions created by SMB are
// Copyright (C) 1997-@year@ by SMB GmbH. All rights reserved.
//
// $Id: ExclusiveLock.java,v 1.4 2002/09/18 06:54:14 per_nyfelt Exp $

package org.ozoneDB.core;

import org.ozoneDB.DxLib.*;
import org.ozoneDB.util.LogWriter;


/**
 * This class implements an exclusive lock policy. Only one transaction can hold
 * this lock at one time.
 *
 *
 * @author <a href="http://www.softwarebuero.de/">SMB</a>
 * @version $Revision: 1.4 $Date: 2002/09/18 06:54:14 $
 */
public final class ExclusiveLock extends AbstractLock {

    protected final static long serialVersionUID = 1;
    protected final static byte subSerialVersionUID = 1;

    private int level = LEVEL_NONE;

    /**
     * ID of the Transaction that holds this lock or null.
     */
    public TransactionID locker;


    public ExclusiveLock() {
        reset();
    }


    public synchronized void reset() {
        level = LEVEL_NONE;
        locker = null;
    }


    public int tryAcquire( Transaction ta, int newLevel ) {
        if (false&&ta.env.logWriter.hasTarget( LogWriter.DEBUG3 )) {
            ta.env.logWriter.newEntry( this, "tryAcquire(): current:" + level + " new:" + newLevel, LogWriter.DEBUG3 );
        }
        synchronized (this) {
            int prevLevel = level( ta );
            if (locker == null || isAcquiredBy( ta )) {
                locker = ta.taID();
                level = newLevel > level ? newLevel : level;
                return prevLevel;
            } else {
                return NOT_ACQUIRED;
            }
        }
    }


    public void release( Transaction ta ) {
        if (false&&ta.env.logWriter.hasTarget( LogWriter.DEBUG3 )) {
            ta.env.logWriter.newEntry( this, "release()", LogWriter.DEBUG3 );
        }
        synchronized (this) {
            if (!isAcquiredBy( ta )) {
                ta.env.logWriter.newEntry( this, "release(): specified transaction has not aquired lock.", LogWriter.WARN );
            }
            locker = null;
            level = LEVEL_NONE;
        }
    }


    public boolean isAcquiredBy( Transaction ta ) {
        return locker != null ? locker.equals( ta.taID() ) : false;
    }


    public DxCollection lockerIDs() {
        DxBag result = new DxArrayBag();
        if (locker != null) {
            result.add( locker );
        }
        return result;
    }


    public int level( Transaction ta ) {
        if (ta != null) {
            return isAcquiredBy( ta ) ? level : LEVEL_NONE;
        } else {
            return level;
        }
    }

    public TransactionID getLocker() {
        return locker;
    }
}