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    1   /*
    2    * Copyright 2009 Red Hat, Inc.
    3    *
    4    * Red Hat licenses this file to you under the Apache License, version 2.0
    5    * (the "License"); you may not use this file except in compliance with the
    6    * License.  You may obtain a copy of the License at:
    7    *
    8    *    http://www.apache.org/licenses/LICENSE-2.0
    9    *
    10   * Unless required by applicable law or agreed to in writing, software
    11   * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
    12   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.  See the
    13   * License for the specific language governing permissions and limitations
    14   * under the License.
    15   */
    16
    17  /*
    18   * Written by Doug Lea with assistance from members of JCP JSR-166
    19   * Expert Group and released to the public domain, as explained at
    20   * http://creativecommons.org/licenses/publicdomain
    21   */
    22
    23  package org.jboss.netty.util.internal;
    24
    25  import java.util.Random;
    26
    27  /**
    28   * A random number generator isolated to the current thread.  Like the
    29   * global {@link java.util.Random} generator used by the {@link
    30   * java.lang.Math} class, a {@code ThreadLocalRandom} is initialized
    31   * with an internally generated seed that may not otherwise be
    32   * modified. When applicable, use of {@code ThreadLocalRandom} rather
    33   * than shared {@code Random} objects in concurrent programs will
    34   * typically encounter much less overhead and contention.  Use of
    35   * {@code ThreadLocalRandom} is particularly appropriate when multiple
    36   * tasks use random numbers in parallel in thread pools.
    37   *
    38   * <p>Usages of this class should typically be of the form:
    39   * {@code ThreadLocalRandom.current().nextX(...)} (where
    40   * {@code X} is {@code Int}, {@code Long}, etc).
    41   * When all usages are of this form, it is never possible to
    42   * accidently share a {@code ThreadLocalRandom} across multiple threads.
    43   *
    44   * <p>This class also provides additional commonly used bounded random
    45   * generation methods.
    46   *
    47   * @since 1.7
    48   * @author Doug Lea
    49   */
    50  final class ThreadLocalRandom extends Random {
    51      // same constants as Random, but must be redeclared because private
    52      private final static long multiplier = 0x5DEECE66DL;
    53      private final static long addend = 0xBL;
    54      private final static long mask = (1L << 48) - 1;
    55
    56      /**
    57       * The random seed. We can't use super.seed.
    58       */
    59      private long rnd;
    60
    61      /**
    62       * Initialization flag to permit the first and only allowed call
    63       * to setSeed (inside Random constructor) to succeed.  We can't
    64       * allow others since it would cause setting seed in one part of a
    65       * program to unintentionally impact other usages by the thread.
    66       */
    67      private boolean initialized;
    68
    69      // Padding to help avoid memory contention among seed updates in
    70      // different TLRs in the common case that they are located near
    71      // each other.
    72      @SuppressWarnings("unused")
    73      private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;
    74
    75      /**
    76       * The actual ThreadLocal
    77       */
    78      private static final ThreadLocal<ThreadLocalRandom> localRandom =
    79          new ThreadLocal<ThreadLocalRandom>() {
    80              @Override
    81              protected ThreadLocalRandom initialValue() {
    82                  return new ThreadLocalRandom();
    83              }
    84      };
    85
    86
    87      /**
    88       * Constructor called only by localRandom.initialValue.
    89       * We rely on the fact that the superclass no-arg constructor
    90       * invokes setSeed exactly once to initialize.
    91       */
    92      ThreadLocalRandom() {
    93          super();
    94      }
    95
    96      /**
    97       * Returns the current thread's {@code ThreadLocalRandom}.
    98       *
    99       * @return the current thread's {@code ThreadLocalRandom}
    100      */
    101     static ThreadLocalRandom current() {
    102         return localRandom.get();
    103     }
    104
    105     /**
    106      * Throws {@code UnsupportedOperationException}.  Setting seeds in
    107      * this generator is not supported.
    108      *
    109      * @throws UnsupportedOperationException always
    110      */
    111     @Override
    112     public void setSeed(long seed) {
    113         if (initialized) {
    114             throw new UnsupportedOperationException();
    115         }
    116         initialized = true;
    117         rnd = (seed ^ multiplier) & mask;
    118     }
    119
    120     @Override
    121     protected int next(int bits) {
    122         rnd = rnd * multiplier + addend & mask;
    123         return (int) (rnd >>> 48-bits);
    124     }
    125
    126     private static final long serialVersionUID = -5851777807851030925L;
    127 }