uniqueidgenerator.java

老外的在线考试

/*
 * Core - Library of useful classes that are used in many CyberDemia projects.
 * Copyright (C) 2003 CyberDemia Research and Services
 *
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Library General Public
 * License as published by the Free Software Foundation; either
 * version 2 of the License, or (at your option) any later version.
 *
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Library General Public License for more details.
 * 
 * You should have received a copy of the GNU Library General Public
 * License along with this library; if not, write to the
 * Free Software Foundation, Inc., 59 Temple Place - Suite 330,
 * Boston, MA  02111-1307, USA.
 *
 * See the COPYING file located in the top-level-directory of
 * the archive of this library for complete text of license.
 */
package com.cyberdemia.util;

import java.util.Random;
import java.net.InetAddress;

/**
 * UniqueIdGenerator is a singleton that can generate "Universal Unique IDs".
 * It is based on the algorithm described in 
 * <cite>Marinescu, F. : EJB Design Patterns (2002)</cite>.
 * It uses the following information to generate the IDs: current time,
 * computer's IP address and this generator's hash code.
 * Because it relies on the computer's IP address, it should generate 
 * unique IDs even when used in a distributed manner across different machines.
 * It does not rely on database access, so it should be quite efficient. The
 * generated ID is a 16-character String (shorter than the original algorithm
 * for faster indexing).
 * 
 * @author Alexander Yap
 * @version $Revision: 1.2 $ at $Date: 2004/02/18 08:35:17 $ by $Author: alexycyap $
 *
 */
public class UniqueIdGenerator
{
	/**
	 * Gets the single instance of UniqueIdGenerator.
	 * 
	 * @return UniqueIdGenerator
	 */
	public static UniqueIdGenerator getInstance()
	{
		return s_instance;
	}
	
	/**
	 * Creates a new unique ID.
	 * @return A 16-character String unique ID.
	 */
	public String createId()
	{
		return createId(null);
	}

	/**
	 * Creates a new unique ID that incorporates a passed in data.
	 * @param data  Additional data to construct the unique ID, or null to use a random value.
	 * @return A 16-character String unique ID.
	 */
	public String createId(Object data)
	{
		String randomHex = null;
		if (data!=null)
		{
			randomHex = getLastChars( Integer.toHexString(data.hashCode()),4);
		}
		else
		{
			randomHex = getLastChars(Integer.toHexString(m_random.nextInt()),4);
		}

		String nowHex = getLastChars(Integer.toHexString((int)System.currentTimeMillis()),8);

		return nowHex + randomHex + m_fixedPart;
	}

	private UniqueIdGenerator()
	{
		// Work out the fixed part of the ID
		int ipHash = 0;
		try
		{
			ipHash = InetAddress.getLocalHost().getHostAddress().hashCode();
		}
		catch (Exception ex)
		{	// If anything goes wrong with IP address, proceed with a random number
			ipHash = m_random.nextInt();
		}
		
		int identityHash = System.identityHashCode(this);
		// Simply combine these two hash codes into one.
		int fixedHash = (ipHash & 0xffff) + (identityHash & 0xffff);
		m_fixedPart = getLastChars(Integer.toHexString(fixedHash),4);
	}
	
	/**
	 * Gets the last n characters of str. Prepend with "0"s is str is
	 * shorter than n characters. Truncate the front of str if it is too long.
	 * @param str Original String
	 * @param n Number of characters in the result String.
	 * @return n-character String with the last portion of str.
	 */
	private static String getLastChars(String str, int n)
	{
		if (str.length()==n)
		{
			return str;
		}

		char[] outChars = new char[n];
		for (int i=str.length()-1, x=0; x<n; i--,x++)
		{
			if (i>=0)
			{
				outChars[x] = str.charAt(i); 
			}
			else
			{
				outChars[x] = '0';
			}
		}
		return new String(outChars);
	}

	private Random m_random = new Random();
	private String m_fixedPart = null;
	
	private static UniqueIdGenerator s_instance = new UniqueIdGenerator();

}