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 }