highlowkeygenerator.java

jGossip是一个简单而功能强大的Java论坛软件（消息板）

/*
 * $Id: HighLowKeyGenerator.java,v 1.2 2004/06/28 01:33:48 alexnet Exp $
 *
 * ***** BEGIN LICENSE BLOCK *****
 * The contents of this file are subject to the Mozilla Public License
 * Version 1.1 (the "License"); you may not use this file except in 
 * compliance with the License. You may obtain a copy of the License 
 * at http://www.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS"
 * basis, WITHOUT WARRANTY OF ANY KIND, either express or implied. See
 * the License for the specific language governing rights and 
 * limitations under the License.
 *
 * The Original Code is JGossip forum code.
 *
 * The Initial Developer of the Original Code is the JResearch, Org. 
 * Portions created by the Initial Developer are Copyright (C) 2004 
 * the Initial Developer. All Rights Reserved. 
 * 
 * Contributor(s): 
 *              Alexey Pavlov <[e-mail]>
 *        
 * ***** END LICENSE BLOCK ***** */
package org.jresearch.gossip.dao.drivers.keygen;
    
import java.sql.Connection;
import java.sql.SQLException;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;

/**
 * Obtains a key by adding 1 to the highest key in the table.
 * 
 * @author <a href="[e-mail]">A. Pavlov</a>
 * @version $version$ $Date: 2004/06/28 01:33:48 $
 */
public class HighLowKeyGenerator implements KeyGenerator
{
    /**
     * Holds a KeyKeeper for each table.  The table's name 
     * is the key and the value is a  KeyKeeper.
     */
    Map keys = Collections.synchronizedMap(new HashMap());

    /**
     * Returns a generated key.
     *
     * @param classMap provides parameters for retreiving the key
     * @param db the source of database connections for the key-values table
     * @return the generated key
     * @exception PersistenceException if an error occurs
     */
    public Object generateKey(String[] primaryKeyName, Connection connection)
 		throws SQLException
    {
		final String tableName = primaryKeyName[0];
		
		KeyKeeper keyKeeper;
		synchronized (keys) {
		    keyKeeper = (KeyKeeper)keys.get(tableName);
		    if (keyKeeper == null) {
			keyKeeper = new KeyKeeper(tableName, primaryKeyName);
			keys.put(tableName, keyKeeper);
		    }
		}	
		return keyKeeper.nextKey(connection);
    }

    /**
     * Generation of key is before insert.
     */
    public boolean isBeforeInsert() { return true; }
}