jdk/src/share/classes/java/util/concurrent/ThreadLocalRandom.java

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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
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/*
 * This file is available under and governed by the GNU General Public
 * License version 2 only, as published by the Free Software Foundation.
 * However, the following notice accompanied the original version of this
 * file:
 *
 * 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 java.util.concurrent;

import java.io.ObjectStreamField;
import java.util.Random;
import java.util.concurrent.atomic.AtomicInteger;
import java.util.concurrent.atomic.AtomicLong;
import java.util.stream.DoubleStream;
import java.util.stream.IntStream;
import java.util.stream.LongStream;

/**
 * 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 {
    /*
     * This class implements the java.util.Random API (and subclasses
     * Random) using a single static instance that accesses random
     * number state held in class Thread (primarily, field
     * threadLocalRandomSeed). In doing so, it also provides a home
     * for managing package-private utilities that rely on exactly the
     * same state as needed to maintain the ThreadLocalRandom
     * instances. We leverage the need for an initialization flag
     * field to also use it as a "probe" -- a self-adjusting thread
     * hash used for contention avoidance, as well as a secondary
     * simpler (xorShift) random seed that is conservatively used to
     * avoid otherwise surprising users by hijacking the
     * ThreadLocalRandom sequence.  The dual use is a marriage of
     * convenience, but is a simple and efficient way of reducing
     * application-level overhead and footprint of most concurrent
     * programs.
     *
     * Because this class is in a different package than class Thread,
     * field access methods use Unsafe to bypass access control rules.
     * The base functionality of Random methods is conveniently
     * isolated in method next(bits), that just reads and writes the
     * Thread field rather than its own field.  However, to conform to
     * the requirements of the Random superclass constructor, the
     * common static ThreadLocalRandom maintains an "initialized"
     * field for the sake of rejecting user calls to setSeed while
     * still allowing a call from constructor.  Note that
     * serialization is completely unnecessary because there is only a
     * static singleton.  But we generate a serial form containing
     * "rnd" and "initialized" fields to ensure compatibility across
     * versions.
     *
     * Per-thread initialization is similar to that in the no-arg
     * Random constructor, but we avoid correlation among not only
     * initial seeds of those created in different threads, but also
     * those created using class Random itself; while at the same time
     * not changing any statistical properties.  So we use the same
     * underlying multiplicative sequence, but start the sequence far
     * away from the base version, and then merge (xor) current time
     * and per-thread probe bits to generate initial values.
     *
     * The nextLocalGaussian ThreadLocal supports the very rarely used
     * nextGaussian method by providing a holder for the second of a
     * pair of them. As is true for the base class version of this
     * method, this time/space tradeoff is probably never worthwhile,
     * but we provide identical statistical properties.
     */

    // 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;
    private static final int PROBE_INCREMENT = 0x61c88647;

    /** Generates the basis for per-thread initial seed values */
    private static final AtomicLong seedGenerator =
        new AtomicLong(1269533684904616924L);

    /** Generates per-thread initialization/probe field */
    private static final AtomicInteger probeGenerator =
        new AtomicInteger(0xe80f8647);

    /** Rarely-used holder for the second of a pair of Gaussians */
    private static final ThreadLocal<Double> nextLocalGaussian =
        new ThreadLocal<Double>();

    /**
     * Field used only during singleton initialization.
     * True when constructor completes.
     */
    boolean initialized;

    /** Constructor used only for static singleton */
    private ThreadLocalRandom() {
        initialized = true; // false during super() call
    }

    /** The common ThreadLocalRandom */
    static final ThreadLocalRandom instance = new ThreadLocalRandom();

    /**
     * Initialize Thread fields for the current thread.  Called only
     * when Thread.threadLocalRandomProbe is zero, indicating that a
     * thread local seed value needs to be generated. Note that even
     * though the initialization is purely thread-local, we need to
     * rely on (static) atomic generators to initialize the values.
     */
    static final void localInit() {
        int p = probeGenerator.getAndAdd(PROBE_INCREMENT);
        int probe = (p == 0) ? 1 : p; // skip 0
        long current, next;
        do { // same sequence as j.u.Random but different initial value
            current = seedGenerator.get();
            next = current * 181783497276652981L;
        } while (!seedGenerator.compareAndSet(current, next));
        long r = next ^ ((long)probe << 32) ^ System.nanoTime();
        Thread t = Thread.currentThread();
        UNSAFE.putLong(t, SEED, r);
        UNSAFE.putInt(t, PROBE, probe);
    }

    /**
     * Returns the current thread's {@code ThreadLocalRandom}.
     *
     * @return the current thread's {@code ThreadLocalRandom}
     */
    public static ThreadLocalRandom current() {
        if (UNSAFE.getInt(Thread.currentThread(), PROBE) == 0)
            localInit();
        return instance;
    }

    /**
     * Throws {@code UnsupportedOperationException}.  Setting seeds in
     * this generator is not supported.
     *
     * @throws UnsupportedOperationException always
     */
    public void setSeed(long seed) {
        // only allow call from super() constructor
        if (initialized)
            throw new UnsupportedOperationException();
    }

    protected int next(int bits) {
        Thread t; long r; // read and update per-thread seed
        UNSAFE.putLong
            (t = Thread.currentThread(), SEED,
             r = (UNSAFE.getLong(t, SEED) * multiplier + addend) & mask);
        return (int) (r >>> (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);
    }

    @Override
    public IntStream ints() {
        return IntStream.generate(() -> current().nextInt());
    }

    @Override
    public LongStream longs() {
        return LongStream.generate(() -> current().nextLong());
    }

    @Override
    public DoubleStream doubles() {
        return DoubleStream.generate(() -> current().nextDouble());
    }

    @Override
    public DoubleStream gaussians() {
        return DoubleStream.generate(() -> current().nextGaussian());
    }

    /**
     * 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;
    }

    public double nextGaussian() {
        // Use nextLocalGaussian instead of nextGaussian field
        Double d = nextLocalGaussian.get();
        if (d != null) {
            nextLocalGaussian.set(null);
            return d.doubleValue();
        }
        double v1, v2, s;
        do {
            v1 = 2 * nextDouble() - 1; // between -1 and 1
            v2 = 2 * nextDouble() - 1; // between -1 and 1
            s = v1 * v1 + v2 * v2;
        } while (s >= 1 || s == 0);
        double multiplier = StrictMath.sqrt(-2 * StrictMath.log(s)/s);
        nextLocalGaussian.set(new Double(v2 * multiplier));
        return v1 * multiplier;
    }

    // Within-package utilities

    /*
     * Descriptions of the usages of the methods below can be found in
     * the classes that use them. Briefly, a thread's "probe" value is
     * a non-zero hash code that (probably) does not collide with
     * other existing threads with respect to any power of two
     * collision space. When it does collide, it is pseudo-randomly
     * adjusted (using a Marsaglia XorShift). The nextSecondarySeed
     * method is used in the same contexts as ThreadLocalRandom, but
     * only for transient usages such as random adaptive spin/block
     * sequences for which a cheap RNG suffices and for which it could
     * in principle disrupt user-visible statistical properties of the
     * main ThreadLocalRandom if we were to use it.
     *
     * Note: Because of package-protection issues, versions of some
     * these methods also appear in some subpackage classes.
     */

    /**
     * Returns the probe value for the current thread without forcing
     * initialization. Note that invoking ThreadLocalRandom.current()
     * can be used to force initialization on zero return.
     */
    static final int getProbe() {
        return UNSAFE.getInt(Thread.currentThread(), PROBE);
    }

    /**
     * Pseudo-randomly advances and records the given probe value for the
     * given thread.
     */
    static final int advanceProbe(int probe) {
        probe ^= probe << 13;   // xorshift
        probe ^= probe >>> 17;
        probe ^= probe << 5;
        UNSAFE.putInt(Thread.currentThread(), PROBE, probe);
        return probe;
    }

    /**
     * Returns the pseudo-randomly initialized or updated secondary seed.
     */
    static final int nextSecondarySeed() {
        int r;
        Thread t = Thread.currentThread();
        if ((r = UNSAFE.getInt(t, SECONDARY)) != 0) {
            r ^= r << 13;   // xorshift
            r ^= r >>> 17;
            r ^= r << 5;
        }
        else {
            localInit();
            if ((r = (int)UNSAFE.getLong(t, SEED)) == 0)
                r = 1; // avoid zero
        }
        UNSAFE.putInt(t, SECONDARY, r);
        return r;
    }

    // Serialization support

    private static final long serialVersionUID = -5851777807851030925L;

    /**
     * @serialField rnd long
     *              seed for random computations
     * @serialField initialized boolean
     *              always true
     */
    private static final ObjectStreamField[] serialPersistentFields = {
            new ObjectStreamField("rnd", long.class),
            new ObjectStreamField("initialized", boolean.class)
    };

    /**
     * Saves the {@code ThreadLocalRandom} to a stream (that is, serializes it).
     */
    private void writeObject(java.io.ObjectOutputStream out)
        throws java.io.IOException {

        java.io.ObjectOutputStream.PutField fields = out.putFields();
        fields.put("rnd", UNSAFE.getLong(Thread.currentThread(), SEED));
        fields.put("initialized", true);
        out.writeFields();
    }

    /**
     * Returns the {@link #current() current} thread's {@code ThreadLocalRandom}.
     */
    private Object readResolve() {
        return current();
    }

    // Unsafe mechanics
    private static final sun.misc.Unsafe UNSAFE;
    private static final long SEED;
    private static final long PROBE;
    private static final long SECONDARY;
    static {
        try {
            UNSAFE = sun.misc.Unsafe.getUnsafe();
            Class<?> tk = Thread.class;
            SEED = UNSAFE.objectFieldOffset
                (tk.getDeclaredField("threadLocalRandomSeed"));
            PROBE = UNSAFE.objectFieldOffset
                (tk.getDeclaredField("threadLocalRandomProbe"));
            SECONDARY = UNSAFE.objectFieldOffset
                (tk.getDeclaredField("threadLocalRandomSecondarySeed"));
        } catch (Exception e) {
            throw new Error(e);
        }
    }
}