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/licenses/publicdomain
 */

package jsr166y;
import java.util.*;

/**
 * A random number generator with the same properties as class {@link
 * Random} but isolated to the current Thread.  Like the global
 * generator used by the {@link java.lang.Math} class, a
 * ThreadLocalRandom is initialized with an internally generated seed
 * that may not otherwise be modified. When applicable, use of
 * ThreadLocalRandom rather than shared Random objects in concurrent
 * programs will typically encounter much less overhead and
 * contention.  ThreadLocalRandoms are 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 ThreadLocalRandoms 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 ThreadLocalRandom.
     *
     * @return the current Thread's ThreadLocalRandom
     */
    public static ThreadLocalRandom current() {
        return localRandom.get();
    }

    /**
     * Throws UnsupportedOperationException. Setting seeds in this
     * generator is unsupported.
     *
     * @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;
    }

}
Revision:	1.8
Committed:	Thu Jul 23 23:07:57 2009 UTC (14 years, 9 months ago) by jsr166
Branch:	MAIN
Changes since 1.7:	+13 -5 lines
Log Message:	j.u.c. coding standards
#	Content
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 jsr166y;
8	import java.util.*;
9
10	/**
11	* A random number generator with the same properties as class {@link
12	* Random} but isolated to the current Thread. Like the global
13	* generator used by the {@link java.lang.Math} class, a
14	* ThreadLocalRandom is initialized with an internally generated seed
15	* that may not otherwise be modified. When applicable, use of
16	* ThreadLocalRandom rather than shared Random objects in concurrent
17	* programs will typically encounter much less overhead and
18	* contention. ThreadLocalRandoms are particularly appropriate when
19	* multiple tasks (for example, each a {@link ForkJoinTask}), use
20	* random numbers in parallel in thread pools.
21	*
22	* <p>Usages of this class should typically be of the form:
23	* {@code ThreadLocalRandom.current().nextX(...)} (where
24	* {@code X} is {@code Int}, {@code Long}, etc).
25	* When all usages are of this form, it is never possible to
26	* accidently share ThreadLocalRandoms across multiple threads.
27	*
28	* <p>This class also provides additional commonly used bounded random
29	* generation methods.
30	*
31	* @since 1.7
32	* @author Doug Lea
33	*/
34	public class ThreadLocalRandom extends Random {
35	// same constants as Random, but must be redeclared because private
36	private final static long multiplier = 0x5DEECE66DL;
37	private final static long addend = 0xBL;
38	private final static long mask = (1L << 48) - 1;
39
40	/**
41	* The random seed. We can't use super.seed.
42	*/
43	private long rnd;
44
45	/**
46	* Initialization flag to permit the first and only allowed call
47	* to setSeed (inside Random constructor) to succeed. We can't
48	* allow others since it would cause setting seed in one part of a
49	* program to unintentionally impact other usages by the thread.
50	*/
51	boolean initialized;
52
53	// Padding to help avoid memory contention among seed updates in
54	// different TLRs in the common case that they are located near
55	// each other.
56	private long pad0, pad1, pad2, pad3, pad4, pad5, pad6, pad7;
57
58	/**
59	* The actual ThreadLocal
60	*/
61	private static final ThreadLocal<ThreadLocalRandom> localRandom =
62	new ThreadLocal<ThreadLocalRandom>() {
63	protected ThreadLocalRandom initialValue() {
64	return new ThreadLocalRandom();
65	}
66	};
67
68
69	/**
70	* Constructor called only by localRandom.initialValue.
71	* We rely on the fact that the superclass no-arg constructor
72	* invokes setSeed exactly once to initialize.
73	*/
74	ThreadLocalRandom() {
75	super();
76	}
77
78	/**
79	* Returns the current Thread's ThreadLocalRandom.
80	*
81	* @return the current Thread's ThreadLocalRandom
82	*/
83	public static ThreadLocalRandom current() {
84	return localRandom.get();
85	}
86
87	/**
88	* Throws UnsupportedOperationException. Setting seeds in this
89	* generator is unsupported.
90	*
91	* @throws UnsupportedOperationException always
92	*/
93	public void setSeed(long seed) {
94	if (initialized)
95	throw new UnsupportedOperationException();
96	initialized = true;
97	rnd = (seed ^ multiplier) & mask;
98	}
99
100	protected int next(int bits) {
101	rnd = (rnd * multiplier + addend) & mask;
102	return (int) (rnd >>> (48-bits));
103	}
104
105	/**
106	* Returns a pseudorandom, uniformly distributed value between the
107	* given least value (inclusive) and bound (exclusive).
108	*
109	* @param least the least value returned
110	* @param bound the upper bound (exclusive)
111	* @throws IllegalArgumentException if least greater than or equal
112	* to bound
113	* @return the next value
114	*/
115	public int nextInt(int least, int bound) {
116	if (least >= bound)
117	throw new IllegalArgumentException();
118	return nextInt(bound - least) + least;
119	}
120
121	/**
122	* Returns a pseudorandom, uniformly distributed value
123	* between 0 (inclusive) and the specified value (exclusive).
124	*
125	* @param n the bound on the random number to be returned. Must be
126	* positive.
127	* @return the next value
128	* @throws IllegalArgumentException if n is not positive
129	*/
130	public long nextLong(long n) {
131	if (n <= 0)
132	throw new IllegalArgumentException("n must be positive");
133	// Divide n by two until small enough for nextInt. On each
134	// iteration (at most 31 of them but usually much less),
135	// randomly choose both whether to include high bit in result
136	// (offset) and whether to continue with the lower vs upper
137	// half (which makes a difference only if odd).
138	long offset = 0;
139	while (n >= Integer.MAX_VALUE) {
140	int bits = next(2);
141	long half = n >>> 1;
142	long nextn = ((bits & 2) == 0) ? half : n - half;
143	if ((bits & 1) == 0)
144	offset += n - nextn;
145	n = nextn;
146	}
147	return offset + nextInt((int) n);
148	}
149
150	/**
151	* Returns a pseudorandom, uniformly distributed value between the
152	* given least value (inclusive) and bound (exclusive).
153	*
154	* @param least the least value returned
155	* @param bound the upper bound (exclusive)
156	* @return the next value
157	* @throws IllegalArgumentException if least greater than or equal
158	* to bound
159	*/
160	public long nextLong(long least, long bound) {
161	if (least >= bound)
162	throw new IllegalArgumentException();
163	return nextLong(bound - least) + least;
164	}
165
166	/**
167	* Returns a pseudorandom, uniformly distributed {@code double} value
168	* between 0 (inclusive) and the specified value (exclusive).
169	*
170	* @param n the bound on the random number to be returned. Must be
171	* positive.
172	* @return the next value
173	* @throws IllegalArgumentException if n is not positive
174	*/
175	public double nextDouble(double n) {
176	if (n <= 0)
177	throw new IllegalArgumentException("n must be positive");
178	return nextDouble() * n;
179	}
180
181	/**
182	* Returns a pseudorandom, uniformly distributed value between the
183	* given least value (inclusive) and bound (exclusive).
184	*
185	* @param least the least value returned
186	* @param bound the upper bound (exclusive)
187	* @return the next value
188	* @throws IllegalArgumentException if least greater than or equal
189	* to bound
190	*/
191	public double nextDouble(double least, double bound) {
192	if (least >= bound)
193	throw new IllegalArgumentException();
194	return nextDouble() * (bound - least) + least;
195	}
196
197	}