Generates random UUIDs : UUID « Development « Java Tutorial

1. Java Tutorial
2. Development
3. UUID

/* 
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (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.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing,
 * software distributed under the License is distributed on an
 * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
 * KIND, either express or implied.  See the License for the
 * specific language governing permissions and limitations
 * under the License.
 *
 */

import java.security.SecureRandom;

/**
 *
 * RandomUuidFactory is a {@linkplain UuidFactory UuidFactory} that generates 
 * random UUIDs. This implementation uses the JDK's java.security.SecureRandom 
 * to generate sufficiently random values for the UUIDs.
 * <br><br>
 * This class is a singleton, so it must be constructed through the static 
 * getInstance() method.
 *
 * @author Dan Jemiolo (danj)
 *
 */

public class RandomUuidFactory 
{
    private static final RandomUuidFactory _SINGLETON = new RandomUuidFactory();
    
    private static final char[] _HEX_VALUES = new char[] {
            '0', '1', '2', '3', '4', '5', '6', '7', '8', '9', 
            'a', 'b', 'c', 'd', 'e', 'f'
    };
    
    //
    // Used to create all random numbers - guarantees unique strings 
    // during the process lifetime
    //
    private static final SecureRandom _RNG = new SecureRandom();
    
    /**
     * 
     * The default constructor is explicit so we can make it private and 
     * require use of getInstance() for instantiation.
     * 
     * @see #getInstance()
     *
     */
    private RandomUuidFactory()
    {
        //
        // this is just to prevent instantiation
        //
    }
    
    /**
     * 
     * @return A unique UUID of the form <em>uuid:<b>X</b></em>, where 
     *        <b>X</b> is the generated value.
     *
     */
    public String createUUID()
    {
        //
        // first get 16 random bytes...
        //
        byte[] bytes = new byte[16];
        _RNG.nextBytes(bytes);
        
        StringBuffer uuid = new StringBuffer(41);
        uuid.append("uuid:");
        
        //
        // ...then we need to shift so they're valid hex (0-9, a-f)
        //
        for (int n = 0; n < 16; ++n) 
        {
            //
            // (there's really no elegant way to add the dashes...)
            //
            if (n == 4 || n == 6 || n == 8 || n == 10)
                uuid.append('-');
            
            //
            // shift the bits so they are proper hex
            //
            int hex = bytes[n] & 255;
            uuid.append(_HEX_VALUES[hex >> 4]);
            uuid.append(_HEX_VALUES[hex & 15]);
        }
        
        return uuid.toString();
    }
    
    /**
     * 
     * @return The singleton instance of this class.
     *
     */
    public static RandomUuidFactory getInstance()
    {
        return _SINGLETON;
    }
}