001    /*
002     * Copyright 2014 UnboundID Corp.
003     * All Rights Reserved.
004     */
005    /*
006     * Copyright (C) 2014 UnboundID Corp.
007     *
008     * This program is free software; you can redistribute it and/or modify
009     * it under the terms of the GNU General Public License (GPLv2 only)
010     * or the terms of the GNU Lesser General Public License (LGPLv2.1 only)
011     * as published by the Free Software Foundation.
012     *
013     * This program is distributed in the hope that it will be useful,
014     * but WITHOUT ANY WARRANTY; without even the implied warranty of
015     * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
016     * GNU General Public License for more details.
017     *
018     * You should have received a copy of the GNU General Public License
019     * along with this program; if not, see <http://www.gnu.org/licenses>.
020     */
021    package com.unboundid.util;
022    
023    
024    
025    import java.util.Random;
026    
027    
028    
029    /**
030     * This class provides a means of obtaining a thread-local random number
031     * generator that can be used by the current thread without the need for
032     * synchronization.
033     */
034    @ThreadSafety(level=ThreadSafetyLevel.COMPLETELY_THREADSAFE)
035    public final class ThreadLocalRandom
036    {
037      /**
038       * The random number generator that will be used to seed per-thread instances.
039       */
040      private static final Random SEED_RANDOM = new Random();
041    
042    
043    
044      /**
045       * The thread-local instances that have been created.
046       */
047      private static final ThreadLocal<Random> INSTANCES =
048           new ThreadLocal<Random>();
049    
050    
051    
052      /**
053       * Prevents this class from being instantiated.
054       */
055      private ThreadLocalRandom()
056      {
057        // No implementation required.
058      }
059    
060    
061    
062      /**
063       * Gets a thread-local random number generator instance.
064       *
065       * @return  A thread-local random number generator instance.
066       */
067      public static Random get()
068      {
069        Random r = INSTANCES.get();
070        if (r == null)
071        {
072          final long seed;
073          synchronized (SEED_RANDOM)
074          {
075            seed = SEED_RANDOM.nextLong();
076          }
077    
078          r = new Random(seed);
079          INSTANCES.set(r);
080        }
081    
082        return r;
083      }
084    }