monetsavepoint.java

这个是内存数据库的客户端

/*
 * The contents of this file are subject to the MonetDB 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://monetdb.cwi.nl/Legal/MonetDBLicense-1.1.html
 *
 * 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 the MonetDB Database System.
 *
 * The Initial Developer of the Original Code is CWI.
 * Portions created by CWI are Copyright (C) 1997-2007 CWI.
 * All Rights Reserved.
 */

package nl.cwi.monetdb.jdbc;

import java.sql.*;

/**
 * The representation of a savepoint, which is a point within the current
 * transaction that can be referenced from the Connection.rollback method.
 * When a transaction is rolled back to a savepoint all changes made after
 * that savepoint are undone.
 * Savepoints can be either named or unnamed. Unnamed savepoints are
 * identified by an ID generated by the underlying data source.
 *
 * This little class is nothing more than a container for a name and/or
 * an id. Each instance of this class always has an id, which is used for
 * internal representation of the save point.
 *
 * Because the IDs which get generated are a logical sequence, application
 * wide, two concurrent transactions are guaranteed to not to have the same
 * save point identifiers. In this implementation the validaty of save points
 * is determined by the server, which makes this a light implementation.
 *
 * @author Fabian Groffen <[e-mail]>
 * @version 1.0
 */
public class MonetSavepoint implements Savepoint {
	/** The id of the last created Savepoint */
	private static int highestId = 0;

	/** The name of this Savepoint */
	private final String name;
	/** The id of this Savepoint */
	private final int id;

	public MonetSavepoint(String name) throws IllegalArgumentException {
		if (name == null) throw new IllegalArgumentException("null not allowed");

		this.id = getNextId();
		this.name = name;
	}

	public MonetSavepoint() {
		this.id = getNextId();
		this.name = null;
	}


	/**
	 * Retrieves the generated ID for the savepoint that this Savepoint object
	 * represents.
	 *
	 * @return the numeric ID of this savepoint
	 * @throws SQLException if this is a named savepoint
	 */
	public int getSavepointId() throws SQLException {
		if (name != null) throw
			new SQLException("Access to ID denied in named savepoint.");

		return(getId());
	}

	/**
	 * Retrieves the name of the savepoint that this Savepoint object
	 * represents.
	 *
	 * @return the name of this savepoint
	 * @throws SQLException if this is an un-named savepoint
	 */
	public String getSavepointName() throws SQLException {
		if (name == null) throw
			new SQLException("Unable to retrieve name of unnamed savepoint");

		return(name);
	}

	//== end of methods from Savepoint interface

	/**
	 * Retrieves the savepoint id, like the getSavepointId method with the only
	 * difference that this method will always return the id, regardless of
	 * whether it is named or not.
	 *
	 * @return the numeric ID of this savepoint
	 */
	int getId() {
		return(id);
	}

	/**
	 * Returns the name to use when referencing this save point to the MonetDB
	 * database. The returned value is guaranteed to be unique and consists of
	 * a prefix string and a sequence number.
	 *
	 * @return the unique savepoint name
	 */
	String getName() {
		return("MonetDBSP" + id);
	}


	/**
	 * Returns the next id, which is larger than the last returned id. This
	 * method is synchronized to prevent race conditions. Since this is static
	 * code, this method is shared by requests from multiple Connection objects.
	 * Therefore two successive calls to this method need not to have a
	 * difference of 1.
	 *
	 * @return the next int which is guaranteed to be higher than the one
	 *         at the last call to this method.
	 */
	private static synchronized int getNextId() {
		return(highestId++);
	}

	/**
	 * Returns the highest id returned by getNextId(). This method is also
	 * synchronized to prevent race conditions and thus guaranteed to be
	 * thread-safe.
	 *
	 * @return the highest id returned by a call to getNextId()
	 */
	private static synchronized int getHighestId() {
		return(highestId);
	}
}