util/concurrent/ThreadLocalRandom.java

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

package java.util.concurrent;

import java.util.Random;

/**
 * A random number generator isolated to the current thread.  Like the
 * global {@link java.util.Random} generator used by the {@link
 * java.lang.Math} class, a {@code ThreadLocalRandom} is initialized
 * with an internally generated seed that may not otherwise be
 * modified. When applicable, use of {@code ThreadLocalRandom} rather
 * than shared {@code Random} objects in concurrent programs will
 * typically encounter much less overhead and contention.  Use of
 * {@code ThreadLocalRandom} is particularly appropriate when multiple
 * tasks (for example, each a {@link ForkJoinTask}) use random numbers
 * in parallel in thread pools.
 *
 * <p>Usages of this class should typically be of the form:
 * {@code ThreadLocalRandom.current().nextX(...)} (where
 * {@code X} is {@code Int}, {@code Long}, etc).
 * When all usages are of this form, it is never possible to
 * accidently share a {@code ThreadLocalRandom} across multiple threads.
 *
 * <p>This class also provides additional commonly used bounded random
 * generation methods.
 *
 * @since 1.7
 * @author Doug Lea
 */
public class ThreadLocalRandom extends Random {
    // same constants as Random, but must be redeclared because private
    private final static long multiplier = 0x5DEECE66DL;
    private final static long addend = 0xBL;
    private final static long mask = (1L << 48) - 1;

    /**
     * The random seed. We can't use super.seed.
     */
    private long rnd;

    /**
     * Initialization flag to permit the first and only allowed call
     * to setSeed (inside Random constructor) to succeed.  We can't
     * allow others since it would cause setting seed in one part of a
     * program to unintentionally impact other usages by the thread.
     */
    boolean initialized;

    // Padding to help avoid memory contention among seed updates in
    // different TLRs in the common case that they are located near
    // each other.
    private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;

    /**
     * The actual ThreadLocal
     */
    private static final ThreadLocal<ThreadLocalRandom> localRandom =
        new ThreadLocal<ThreadLocalRandom>() {
            protected ThreadLocalRandom initialValue() {
                return new ThreadLocalRandom();
            }
    };


    /**
     * Constructor called only by localRandom.initialValue.
     * We rely on the fact that the superclass no-arg constructor
     * invokes setSeed exactly once to initialize.
     */
    ThreadLocalRandom() {
        super();
    }

    /**
     * Returns the current thread's {@code ThreadLocalRandom}.
     *
     * @return the current thread's {@code ThreadLocalRandom}
     */
    public static ThreadLocalRandom current() {
        return localRandom.get();
    }

    /**
     * Throws {@code UnsupportedOperationException}.  Setting seeds in
     * this generator is not supported.
     *
     * @throws UnsupportedOperationException always
     */
    public void setSeed(long seed) {
        if (initialized)
            throw new UnsupportedOperationException();
        initialized = true;
        rnd = (seed ^ multiplier) & mask;
    }

    protected int next(int bits) {
        rnd = (rnd * multiplier + addend) & mask;
        return (int) (rnd >>> (48-bits));
    }

    /**
     * Returns a pseudorandom, uniformly distributed value between the
     * given least value (inclusive) and bound (exclusive).
     *
     * @param least the least value returned
     * @param bound the upper bound (exclusive)
     * @throws IllegalArgumentException if least greater than or equal
     * to bound
     * @return the next value
     */
    public int nextInt(int least, int bound) {
        if (least >= bound)
            throw new IllegalArgumentException();
        return nextInt(bound - least) + least;
    }

    /**
     * Returns a pseudorandom, uniformly distributed value
     * between 0 (inclusive) and the specified value (exclusive).
     *
     * @param n the bound on the random number to be returned.  Must be
     *        positive.
     * @return the next value
     * @throws IllegalArgumentException if n is not positive
     */
    public long nextLong(long n) {
        if (n <= 0)
            throw new IllegalArgumentException("n must be positive");
        // Divide n by two until small enough for nextInt. On each
        // iteration (at most 31 of them but usually much less),
        // randomly choose both whether to include high bit in result
        // (offset) and whether to continue with the lower vs upper
        // half (which makes a difference only if odd).
        long offset = 0;
        while (n >= Integer.MAX_VALUE) {
            int bits = next(2);
            long half = n >>> 1;
            long nextn = ((bits & 2) == 0) ? half : n - half;
            if ((bits & 1) == 0)
                offset += n - nextn;
            n = nextn;
        }
        return offset + nextInt((int) n);
    }

    /**
     * Returns a pseudorandom, uniformly distributed value between the
     * given least value (inclusive) and bound (exclusive).
     *
     * @param least the least value returned
     * @param bound the upper bound (exclusive)
     * @return the next value
     * @throws IllegalArgumentException if least greater than or equal
     * to bound
     */
    public long nextLong(long least, long bound) {
        if (least >= bound)
            throw new IllegalArgumentException();
        return nextLong(bound - least) + least;
    }

    /**
     * Returns a pseudorandom, uniformly distributed {@code double} value
     * between 0 (inclusive) and the specified value (exclusive).
     *
     * @param n the bound on the random number to be returned.  Must be
     *        positive.
     * @return the next value
     * @throws IllegalArgumentException if n is not positive
     */
    public double nextDouble(double n) {
        if (n <= 0)
            throw new IllegalArgumentException("n must be positive");
        return nextDouble() * n;
    }

    /**
     * Returns a pseudorandom, uniformly distributed value between the
     * given least value (inclusive) and bound (exclusive).
     *
     * @param least the least value returned
     * @param bound the upper bound (exclusive)
     * @return the next value
     * @throws IllegalArgumentException if least greater than or equal
     * to bound
     */
    public double nextDouble(double least, double bound) {
        if (least >= bound)
            throw new IllegalArgumentException();
        return nextDouble() * (bound - least) + least;
    }

    private static final long serialVersionUID = -5851777807851030925L;
}
Revision:	1.3
Committed:	Wed Aug 19 18:07:36 2009 UTC (14 years, 9 months ago) by jsr166
Branch:	MAIN
Changes since 1.2:	+14 -13 lines
Log Message:	sync with jsr166 package
#	User	Rev	Content
1	jsr166	1.1	/*
2			* Written by Doug Lea with assistance from members of JCP JSR-166
3			* Expert Group and released to the public domain, as explained at
4			* http://creativecommons.org/licenses/publicdomain
5			*/
6
7			package java.util.concurrent;
8
9			import java.util.Random;
10
11			/**
12	jsr166	1.3	* A random number generator isolated to the current thread. Like the
13	jsr166	1.2	* global {@link java.util.Random} generator used by the {@link
14	jsr166	1.3	* java.lang.Math} class, a {@code ThreadLocalRandom} is initialized
15			* with an internally generated seed that may not otherwise be
16			* modified. When applicable, use of {@code ThreadLocalRandom} rather
17			* than shared {@code Random} objects in concurrent programs will
18			* typically encounter much less overhead and contention. Use of
19			* {@code ThreadLocalRandom} is particularly appropriate when multiple
20			* tasks (for example, each a {@link ForkJoinTask}) use random numbers
21			* in parallel in thread pools.
22	jsr166	1.1	*
23			* <p>Usages of this class should typically be of the form:
24			* {@code ThreadLocalRandom.current().nextX(...)} (where
25			* {@code X} is {@code Int}, {@code Long}, etc).
26			* When all usages are of this form, it is never possible to
27	jsr166	1.3	* accidently share a {@code ThreadLocalRandom} across multiple threads.
28	jsr166	1.1	*
29			* <p>This class also provides additional commonly used bounded random
30			* generation methods.
31			*
32			* @since 1.7
33			* @author Doug Lea
34			*/
35			public class ThreadLocalRandom extends Random {
36			// same constants as Random, but must be redeclared because private
37			private final static long multiplier = 0x5DEECE66DL;
38			private final static long addend = 0xBL;
39			private final static long mask = (1L << 48) - 1;
40
41			/**
42			* The random seed. We can't use super.seed.
43			*/
44			private long rnd;
45
46			/**
47			* Initialization flag to permit the first and only allowed call
48			* to setSeed (inside Random constructor) to succeed. We can't
49			* allow others since it would cause setting seed in one part of a
50			* program to unintentionally impact other usages by the thread.
51			*/
52			boolean initialized;
53
54			// Padding to help avoid memory contention among seed updates in
55			// different TLRs in the common case that they are located near
56			// each other.
57			private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;
58
59			/**
60			* The actual ThreadLocal
61			*/
62			private static final ThreadLocal<ThreadLocalRandom> localRandom =
63			new ThreadLocal<ThreadLocalRandom>() {
64			protected ThreadLocalRandom initialValue() {
65			return new ThreadLocalRandom();
66			}
67			};
68
69
70			/**
71			* Constructor called only by localRandom.initialValue.
72			* We rely on the fact that the superclass no-arg constructor
73			* invokes setSeed exactly once to initialize.
74			*/
75			ThreadLocalRandom() {
76			super();
77			}
78
79			/**
80	jsr166	1.3	* Returns the current thread's {@code ThreadLocalRandom}.
81	jsr166	1.1	*
82	jsr166	1.3	* @return the current thread's {@code ThreadLocalRandom}
83	jsr166	1.1	*/
84			public static ThreadLocalRandom current() {
85			return localRandom.get();
86			}
87
88			/**
89	jsr166	1.3	* Throws {@code UnsupportedOperationException}. Setting seeds in
90			* this generator is not supported.
91	jsr166	1.1	*
92			* @throws UnsupportedOperationException always
93			*/
94			public void setSeed(long seed) {
95			if (initialized)
96			throw new UnsupportedOperationException();
97			initialized = true;
98			rnd = (seed ^ multiplier) & mask;
99			}
100
101			protected int next(int bits) {
102			rnd = (rnd * multiplier + addend) & mask;
103			return (int) (rnd >>> (48-bits));
104			}
105
106			/**
107			* Returns a pseudorandom, uniformly distributed value between the
108			* given least value (inclusive) and bound (exclusive).
109			*
110			* @param least the least value returned
111			* @param bound the upper bound (exclusive)
112			* @throws IllegalArgumentException if least greater than or equal
113			* to bound
114			* @return the next value
115			*/
116			public int nextInt(int least, int bound) {
117			if (least >= bound)
118			throw new IllegalArgumentException();
119			return nextInt(bound - least) + least;
120			}
121
122			/**
123			* Returns a pseudorandom, uniformly distributed value
124			* between 0 (inclusive) and the specified value (exclusive).
125			*
126			* @param n the bound on the random number to be returned. Must be
127			* positive.
128			* @return the next value
129			* @throws IllegalArgumentException if n is not positive
130			*/
131			public long nextLong(long n) {
132			if (n <= 0)
133			throw new IllegalArgumentException("n must be positive");
134			// Divide n by two until small enough for nextInt. On each
135			// iteration (at most 31 of them but usually much less),
136			// randomly choose both whether to include high bit in result
137			// (offset) and whether to continue with the lower vs upper
138			// half (which makes a difference only if odd).
139			long offset = 0;
140			while (n >= Integer.MAX_VALUE) {
141			int bits = next(2);
142			long half = n >>> 1;
143			long nextn = ((bits & 2) == 0) ? half : n - half;
144			if ((bits & 1) == 0)
145			offset += n - nextn;
146			n = nextn;
147			}
148			return offset + nextInt((int) n);
149			}
150
151			/**
152			* Returns a pseudorandom, uniformly distributed value between the
153			* given least value (inclusive) and bound (exclusive).
154			*
155			* @param least the least value returned
156			* @param bound the upper bound (exclusive)
157			* @return the next value
158			* @throws IllegalArgumentException if least greater than or equal
159			* to bound
160			*/
161			public long nextLong(long least, long bound) {
162			if (least >= bound)
163			throw new IllegalArgumentException();
164			return nextLong(bound - least) + least;
165			}
166
167			/**
168			* Returns a pseudorandom, uniformly distributed {@code double} value
169			* between 0 (inclusive) and the specified value (exclusive).
170			*
171			* @param n the bound on the random number to be returned. Must be
172			* positive.
173			* @return the next value
174			* @throws IllegalArgumentException if n is not positive
175			*/
176			public double nextDouble(double n) {
177			if (n <= 0)
178			throw new IllegalArgumentException("n must be positive");
179			return nextDouble() * n;
180			}
181
182			/**
183			* Returns a pseudorandom, uniformly distributed value between the
184			* given least value (inclusive) and bound (exclusive).
185			*
186			* @param least the least value returned
187			* @param bound the upper bound (exclusive)
188			* @return the next value
189			* @throws IllegalArgumentException if least greater than or equal
190			* to bound
191			*/
192			public double nextDouble(double least, double bound) {
193			if (least >= bound)
194			throw new IllegalArgumentException();
195			return nextDouble() * (bound - least) + least;
196			}
197
198			private static final long serialVersionUID = -5851777807851030925L;
199			}