exclusivelock.java

用Java写的面相对象的数据库管理系统

// 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) 1997-2000 by SMB GmbH. All rights reserved.
//
// $Id: ExclusiveLock.java,v 1.5 2000/10/28 16:55:16 daniela Exp $

package org.ozoneDB.core;

import java.io.*;
import org.ozoneDB.*;
import org.ozoneDB.DxLib.*;
import org.ozoneDB.util.*;


/**
 * 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: 1.5 $Date: 2000/10/28 16:55:16 $
 */
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 synchronized int tryAcquire( Transaction ta, int newLevel ) {
        if (ta.env.logWriter.hasTarget( LogWriter.DEBUG3 )) {
            ta.env.logWriter.newEntry( this, "tryAcquire(): current:" + level + " new:" + newLevel, LogWriter.DEBUG3 );
        } 
        
        int prevLevel = level( ta );
        if (locker == null || isAcquiredBy( ta )) {
            locker = ta.taID();
            level = newLevel > level ? newLevel : level;
            return prevLevel;
        } else {
            return NOT_ACQUIRED;
        } 
    } 
    
    
    public synchronized void release( Transaction ta ) {
        if (ta.env.logWriter.hasTarget( LogWriter.DEBUG3 )) {
            ta.env.logWriter.newEntry( this, "release()", LogWriter.DEBUG3 );
        } 
        
        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;
        } 
    } 
    
}