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1   /*
2    * Copyright 2012 The Netty Project
3    *
4    * The Netty Project licenses this file to you under the Apache License,
5    * version 2.0 (the "License"); you may not use this file except in compliance
6    * with the 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 Random} generator used by the {@link
30   * 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   */
49  public final class ThreadLocalRandom extends Random {
50      // same constants as Random, but must be redeclared because private
51      private static final long multiplier = 0x5DEECE66DL;
52      private static final long addend = 0xBL;
53      private static final long mask = (1L << 48) - 1;
54  
55      /**
56       * The random seed. We can't use super.seed.
57       */
58      private long rnd;
59  
60      /**
61       * Initialization flag to permit the first and only allowed call
62       * to setSeed (inside Random constructor) to succeed.  We can't
63       * allow others since it would cause setting seed in one part of a
64       * program to unintentionally impact other usages by the thread.
65       */
66      private boolean initialized;
67  
68      // Padding to help avoid memory contention among seed updates in
69      // different TLRs in the common case that they are located near
70      // each other.
71      @SuppressWarnings("unused")
72      private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;
73  
74      /**
75       * The actual ThreadLocal
76       */
77      private static final ThreadLocal<ThreadLocalRandom> localRandom =
78          new ThreadLocal<ThreadLocalRandom>() {
79              @Override
80              protected ThreadLocalRandom initialValue() {
81                  return new ThreadLocalRandom();
82              }
83      };
84  
85      /**
86       * Returns the current thread's {@code ThreadLocalRandom}.
87       *
88       * @return the current thread's {@code ThreadLocalRandom}
89       */
90      public static ThreadLocalRandom current() {
91          return localRandom.get();
92      }
93  
94      /**
95       * Throws {@code UnsupportedOperationException}.  Setting seeds in
96       * this generator is not supported.
97       *
98       * @throws UnsupportedOperationException always
99       */
100     @Override
101     public void setSeed(long seed) {
102         // We rely on the fact that the superclass no-arg constructor
103         // invokes setSeed exactly once to initialize.
104         if (initialized) {
105             throw new UnsupportedOperationException();
106         }
107         initialized = true;
108         rnd = (seed ^ multiplier) & mask;
109     }
110 
111     @Override
112     protected int next(int bits) {
113         rnd = rnd * multiplier + addend & mask;
114         return (int) (rnd >>> 48 - bits);
115     }
116 
117     private static final long serialVersionUID = -5851777807851030925L;
118 }