sharedlock.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: SharedLock.java,v 1.4 2000/10/28 16:55:17 daniela Exp $

package org.ozoneDB.core;

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


/**
 * This class implements a non-exclusive lock policy. Multiple transactions may
 * hold this lock.
 * 
 * 
 * @author <a href="http://www.softwarebuero.de/">SMB</a>
 * @version $Revision: 1.4 $Date: 2000/10/28 16:55:17 $
 */
public final class SharedLock extends AbstractLock {
    
    protected final static long serialVersionUID = 1;
    protected final static byte subSerialVersionUID = 1;
    
    private int level = LEVEL_NONE;
    
    private transient int prevLevel = LEVEL_NONE;
    
    /**
     * All transactions that hold this lock. Holds TransactionIDs.
     */
    public DxSet lockers;
    
    
    public SharedLock() {
        reset();
    }
    
    
    public synchronized void reset() {
        level = prevLevel = LEVEL_NONE;
        lockers = new DxHashSet( 8 );
    } 
    
    
    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 );
        lockers.add( ta.taID() );
        
        if (!lockers.contains( ta.taID() )) {
            throw new RuntimeException( "tryAcquire(): unable to add ta to lockers." );
        //ta.env.logWriter.newEntry (this, "tryAcquire(): unable to add ta to lockers.", LogWriter.WARN);
        } 
        
        level = newLevel > level ? newLevel : level;
        return prevLevel;
    } 
    
    
    public synchronized void release( Transaction ta ) {
        if (ta.env.logWriter.hasTarget( LogWriter.DEBUG3 )) {
            ta.env.logWriter.newEntry( this, "release()", LogWriter.DEBUG3 );
        } 
        
        if (!lockers.remove( ta.taID )) {
            throw new RuntimeException( "release(): transaction does not hold the lock." );
        } 
        
        if (lockers.isEmpty()) {
            level = LEVEL_NONE;
        } 
        
        ta.env.logWriter.newEntry( this, "release(): count=" + lockers.count() + ", level=" + level, LogWriter.DEBUG2 );
    } 
    
    
    public boolean isAcquiredBy( Transaction ta ) {
        return lockers != null ? lockers.contains( ta.taID() ) : false;
    } 
    
    
    public DxCollection lockerIDs() {
        return lockers != null ? lockers : new DxHashSet();
    } 
    
    
    public int level( Transaction ta ) {
        if (ta != null) {
            return isAcquiredBy( ta ) ? level : LEVEL_NONE;
        } else {
            return level;
        } 
    } 
    
    
    public int previousLevel() {
        return prevLevel;
    } 
    
    
    public boolean hasChanged() {
        return level > prevLevel;
    } 
}