src/jsr166y/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/publicdomain/zero/1.0/
 */

package jsr166y;

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 static final long multiplier = 0x5DEECE66DL;
    private static final long addend = 0xBL;
    private static final long mask = (1L << 48) - 1;

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

    /**
     * Initialization flag to permit calls to setSeed to succeed only
     * while executing the Random constructor.  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.
     */
    ThreadLocalRandom() {
        super();
        initialized = true;
    }

    /**
     * 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();
        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)
     * @return the next value
     * @throws IllegalArgumentException if least greater than or equal
     * to bound
     */
    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:	1.17
Committed:	Fri Jul 19 19:34:43 2013 UTC (10 years, 10 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.16:	+1 -1 lines
Log Message:	enforce standard javadoc tag order
#	User	Rev	Content
1	dl	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	jsr166	1.15	* http://creativecommons.org/publicdomain/zero/1.0/
5	dl	1.1	*/
6
7			package jsr166y;
8	jsr166	1.10
9			import java.util.Random;
10	dl	1.1
11			/**
12	jsr166	1.13	* A random number generator isolated to the current thread. Like the
13	dl	1.11	* global {@link java.util.Random} generator used by the {@link
14	jsr166	1.13	* 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	1.3	*
23	dl	1.2	* <p>Usages of this class should typically be of the form:
24	jsr166	1.4	* {@code ThreadLocalRandom.current().nextX(...)} (where
25			* {@code X} is {@code Int}, {@code Long}, etc).
26	dl	1.2	* When all usages are of this form, it is never possible to
27	jsr166	1.13	* accidently share a {@code ThreadLocalRandom} across multiple threads.
28	dl	1.1	*
29			* <p>This class also provides additional commonly used bounded random
30			* generation methods.
31	jsr166	1.6	*
32			* @since 1.7
33			* @author Doug Lea
34	dl	1.1	*/
35			public class ThreadLocalRandom extends Random {
36			// same constants as Random, but must be redeclared because private
37	jsr166	1.16	private static final long multiplier = 0x5DEECE66DL;
38			private static final long addend = 0xBL;
39			private static final long mask = (1L << 48) - 1;
40	dl	1.1
41			/**
42	jsr166	1.8	* The random seed. We can't use super.seed.
43	dl	1.1	*/
44	jsr166	1.3	private long rnd;
45	dl	1.1
46			/**
47	jsr166	1.14	* Initialization flag to permit calls to setSeed to succeed only
48			* while executing the Random constructor. We can't allow others
49			* since it would cause setting seed in one part of a program to
50			* unintentionally impact other usages by the thread.
51	dl	1.1	*/
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	dl	1.2	/**
60			* The actual ThreadLocal
61			*/
62	dl	1.1	private static final ThreadLocal<ThreadLocalRandom> localRandom =
63			new ThreadLocal<ThreadLocalRandom>() {
64			protected ThreadLocalRandom initialValue() {
65			return new ThreadLocalRandom();
66			}
67			};
68
69	dl	1.2
70			/**
71			* Constructor called only by localRandom.initialValue.
72			*/
73			ThreadLocalRandom() {
74			super();
75	jsr166	1.14	initialized = true;
76	dl	1.1	}
77
78			/**
79	jsr166	1.13	* Returns the current thread's {@code ThreadLocalRandom}.
80	jsr166	1.8	*
81	jsr166	1.13	* @return the current thread's {@code ThreadLocalRandom}
82	dl	1.1	*/
83			public static ThreadLocalRandom current() {
84			return localRandom.get();
85			}
86
87			/**
88	jsr166	1.12	* Throws {@code UnsupportedOperationException}. Setting seeds in
89			* this generator is not supported.
90	jsr166	1.8	*
91	jsr166	1.3	* @throws UnsupportedOperationException always
92	dl	1.1	*/
93	jsr166	1.3	public void setSeed(long seed) {
94	dl	1.1	if (initialized)
95			throw new UnsupportedOperationException();
96			rnd = (seed ^ multiplier) & mask;
97			}
98
99			protected int next(int bits) {
100	jsr166	1.8	rnd = (rnd * multiplier + addend) & mask;
101			return (int) (rnd >>> (48-bits));
102	dl	1.1	}
103
104			/**
105			* Returns a pseudorandom, uniformly distributed value between the
106			* given least value (inclusive) and bound (exclusive).
107	jsr166	1.8	*
108	dl	1.1	* @param least the least value returned
109			* @param bound the upper bound (exclusive)
110	jsr166	1.17	* @return the next value
111	dl	1.1	* @throws IllegalArgumentException if least greater than or equal
112			* to bound
113			*/
114			public int nextInt(int least, int bound) {
115			if (least >= bound)
116			throw new IllegalArgumentException();
117			return nextInt(bound - least) + least;
118			}
119
120			/**
121			* Returns a pseudorandom, uniformly distributed value
122	jsr166	1.8	* between 0 (inclusive) and the specified value (exclusive).
123			*
124	dl	1.1	* @param n the bound on the random number to be returned. Must be
125			* positive.
126			* @return the next value
127			* @throws IllegalArgumentException if n is not positive
128			*/
129			public long nextLong(long n) {
130			if (n <= 0)
131			throw new IllegalArgumentException("n must be positive");
132	dl	1.2	// Divide n by two until small enough for nextInt. On each
133			// iteration (at most 31 of them but usually much less),
134			// randomly choose both whether to include high bit in result
135			// (offset) and whether to continue with the lower vs upper
136	jsr166	1.3	// half (which makes a difference only if odd).
137	dl	1.1	long offset = 0;
138	dl	1.2	while (n >= Integer.MAX_VALUE) {
139	jsr166	1.3	int bits = next(2);
140	dl	1.1	long half = n >>> 1;
141	jsr166	1.8	long nextn = ((bits & 2) == 0) ? half : n - half;
142	dl	1.1	if ((bits & 1) == 0)
143			offset += n - nextn;
144			n = nextn;
145			}
146	jsr166	1.7	return offset + nextInt((int) n);
147	dl	1.1	}
148
149			/**
150			* Returns a pseudorandom, uniformly distributed value between the
151			* given least value (inclusive) and bound (exclusive).
152	jsr166	1.8	*
153	dl	1.1	* @param least the least value returned
154			* @param bound the upper bound (exclusive)
155			* @return the next value
156			* @throws IllegalArgumentException if least greater than or equal
157			* to bound
158			*/
159			public long nextLong(long least, long bound) {
160			if (least >= bound)
161			throw new IllegalArgumentException();
162			return nextLong(bound - least) + least;
163			}
164
165			/**
166			* Returns a pseudorandom, uniformly distributed {@code double} value
167	jsr166	1.8	* between 0 (inclusive) and the specified value (exclusive).
168			*
169	dl	1.1	* @param n the bound on the random number to be returned. Must be
170			* positive.
171			* @return the next value
172			* @throws IllegalArgumentException if n is not positive
173			*/
174			public double nextDouble(double n) {
175			if (n <= 0)
176			throw new IllegalArgumentException("n must be positive");
177			return nextDouble() * n;
178			}
179
180			/**
181			* Returns a pseudorandom, uniformly distributed value between the
182			* given least value (inclusive) and bound (exclusive).
183	jsr166	1.8	*
184	dl	1.1	* @param least the least value returned
185			* @param bound the upper bound (exclusive)
186			* @return the next value
187			* @throws IllegalArgumentException if least greater than or equal
188			* to bound
189			*/
190			public double nextDouble(double least, double bound) {
191			if (least >= bound)
192			throw new IllegalArgumentException();
193			return nextDouble() * (bound - least) + least;
194			}
195
196	jsr166	1.9	private static final long serialVersionUID = -5851777807851030925L;
197	jsr166	1.3	}