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) {
        return (int)((rnd = (rnd * multiplier + addend) & mask) >>> (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:	1.7
Committed:	Thu Jul 23 19:25:45 2009 UTC (14 years, 9 months ago) by jsr166
Branch:	MAIN
Changes since 1.6:	+1 -1 lines
Log Message:	j.u.c. coding standards
#	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			* http://creativecommons.org/licenses/publicdomain
5			*/
6
7			package jsr166y;
8			import java.util.*;
9
10			/**
11	dl	1.2	* A random number generator with the same properties as class {@link
12	dl	1.1	* Random} but isolated to the current Thread. Like the global
13	dl	1.2	* 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	jsr166	1.3	*
22	dl	1.2	* <p>Usages of this class should typically be of the form:
23	jsr166	1.4	* {@code ThreadLocalRandom.current().nextX(...)} (where
24			* {@code X} is {@code Int}, {@code Long}, etc).
25	dl	1.2	* When all usages are of this form, it is never possible to
26			* accidently share ThreadLocalRandoms across multiple threads.
27	dl	1.1	*
28			* <p>This class also provides additional commonly used bounded random
29			* generation methods.
30	jsr166	1.6	*
31			* @since 1.7
32			* @author Doug Lea
33	dl	1.1	*/
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	jsr166	1.3	private long rnd;
44	dl	1.1
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	dl	1.2	* program to unintentionally impact other usages by the thread.
50	dl	1.1	*/
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	dl	1.2	/**
59			* The actual ThreadLocal
60			*/
61	dl	1.1	private static final ThreadLocal<ThreadLocalRandom> localRandom =
62			new ThreadLocal<ThreadLocalRandom>() {
63			protected ThreadLocalRandom initialValue() {
64			return new ThreadLocalRandom();
65			}
66			};
67
68	dl	1.2
69			/**
70			* Constructor called only by localRandom.initialValue.
71	jsr166	1.3	* We rely on the fact that the superclass no-arg constructor
72	dl	1.2	* invokes setSeed exactly once to initialize.
73			*/
74			ThreadLocalRandom() {
75			super();
76	dl	1.1	}
77
78			/**
79	jsr166	1.5	* Returns the current Thread's ThreadLocalRandom.
80	dl	1.1	* @return the current Thread's ThreadLocalRandom
81			*/
82			public static ThreadLocalRandom current() {
83			return localRandom.get();
84			}
85
86			/**
87			* Throws UnsupportedOperationException. Setting seeds in this
88			* generator is unsupported.
89	jsr166	1.3	* @throws UnsupportedOperationException always
90	dl	1.1	*/
91	jsr166	1.3	public void setSeed(long seed) {
92	dl	1.1	if (initialized)
93			throw new UnsupportedOperationException();
94			initialized = true;
95			rnd = (seed ^ multiplier) & mask;
96			}
97
98			protected int next(int bits) {
99			return (int)((rnd = (rnd * multiplier + addend) & mask) >>> (48-bits));
100			}
101
102			/**
103			* Returns a pseudorandom, uniformly distributed value between the
104			* given least value (inclusive) and bound (exclusive).
105			* @param least the least value returned
106			* @param bound the upper bound (exclusive)
107			* @throws IllegalArgumentException if least greater than or equal
108			* to bound
109			* @return the next value
110			*/
111			public int nextInt(int least, int bound) {
112			if (least >= bound)
113			throw new IllegalArgumentException();
114			return nextInt(bound - least) + least;
115			}
116
117			/**
118			* Returns a pseudorandom, uniformly distributed value
119			* between 0 (inclusive) and the specified value (exclusive)
120			* @param n the bound on the random number to be returned. Must be
121			* positive.
122			* @return the next value
123			* @throws IllegalArgumentException if n is not positive
124			*/
125			public long nextLong(long n) {
126			if (n <= 0)
127			throw new IllegalArgumentException("n must be positive");
128	dl	1.2	// Divide n by two until small enough for nextInt. On each
129			// iteration (at most 31 of them but usually much less),
130			// randomly choose both whether to include high bit in result
131			// (offset) and whether to continue with the lower vs upper
132	jsr166	1.3	// half (which makes a difference only if odd).
133	dl	1.1	long offset = 0;
134	dl	1.2	while (n >= Integer.MAX_VALUE) {
135	jsr166	1.3	int bits = next(2);
136	dl	1.1	long half = n >>> 1;
137			long nextn = ((bits & 2) == 0)? half : n - half;
138			if ((bits & 1) == 0)
139			offset += n - nextn;
140			n = nextn;
141			}
142	jsr166	1.7	return offset + nextInt((int) n);
143	dl	1.1	}
144
145			/**
146			* Returns a pseudorandom, uniformly distributed value between the
147			* given least value (inclusive) and bound (exclusive).
148			* @param least the least value returned
149			* @param bound the upper bound (exclusive)
150			* @return the next value
151			* @throws IllegalArgumentException if least greater than or equal
152			* to bound
153			*/
154			public long nextLong(long least, long bound) {
155			if (least >= bound)
156			throw new IllegalArgumentException();
157			return nextLong(bound - least) + least;
158			}
159
160			/**
161			* Returns a pseudorandom, uniformly distributed {@code double} value
162			* between 0 (inclusive) and the specified value (exclusive)
163			* @param n the bound on the random number to be returned. Must be
164			* positive.
165			* @return the next value
166			* @throws IllegalArgumentException if n is not positive
167			*/
168			public double nextDouble(double n) {
169			if (n <= 0)
170			throw new IllegalArgumentException("n must be positive");
171			return nextDouble() * n;
172			}
173
174			/**
175			* Returns a pseudorandom, uniformly distributed value between the
176			* given least value (inclusive) and bound (exclusive).
177			* @param least the least value returned
178			* @param bound the upper bound (exclusive)
179			* @return the next value
180			* @throws IllegalArgumentException if least greater than or equal
181			* to bound
182			*/
183			public double nextDouble(double least, double bound) {
184			if (least >= bound)
185			throw new IllegalArgumentException();
186			return nextDouble() * (bound - least) + least;
187			}
188
189	jsr166	1.3	}