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/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;

/**
 * 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 must 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 constructor, during
     * construction, the common static ThreadLocalRandom must maintain
     * initialization and value fields, mainly for the sake of
     * disabling user calls to setSeed while still allowing a call
     * from constructor.  For serialization compatibility, these
     * fields are left with the same declarations as used in the
     * previous ThreadLocal-based version of this class, that used
     * them differently. Note that serialization is completely
     * unnecessary because there is only a static singleton. But these
     * mechanics still ensure compatibility across versions.
     *
     * Per-instance 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
     */
    boolean initialized; // true when constructor completes

    /** 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) {
        if (initialized) // allow call from super() constructor
            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);
    }

    /**
     * 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 if ((r = (int)UNSAFE.getLong(t, SEED)) == 0)
            r = 1; // avoid zero
        UNSAFE.putInt(t, SECONDARY, r);
        return r;
    }

    // Serialization support, maintains original persistent form.

    private static final long serialVersionUID = -5851777807851030925L;

    /**
     * @serialField rnd long
     * @serialField initialized boolean
     * @serialField pad0 long
     * @serialField pad1 long
     * @serialField pad2 long
     * @serialField pad3 long
     * @serialField pad4 long
     * @serialField pad5 long
     * @serialField pad6 long
     * @serialField pad7 long
     */
    private static final ObjectStreamField[] serialPersistentFields = {
            new ObjectStreamField("rnd", long.class),
            new ObjectStreamField("initialized", boolean.class),
            new ObjectStreamField("pad0", long.class),
            new ObjectStreamField("pad1", long.class),
            new ObjectStreamField("pad2", long.class),
            new ObjectStreamField("pad3", long.class),
            new ObjectStreamField("pad4", long.class),
            new ObjectStreamField("pad5", long.class),
            new ObjectStreamField("pad6", long.class),
            new ObjectStreamField("pad7", long.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", 0L);
        fields.put("initialized", true);
        fields.put("pad0", 0L);
        fields.put("pad1", 0L);
        fields.put("pad2", 0L);
        fields.put("pad3", 0L);
        fields.put("pad4", 0L);
        fields.put("pad5", 0L);
        fields.put("pad6", 0L);
        fields.put("pad7", 0L);
        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);
        }
    }
}
Revision:	1.9
Committed:	Wed Jan 16 15:04:04 2013 UTC (11 years, 4 months ago) by dl
Branch:	MAIN
Changes since 1.8:	+56 -3 lines
Log Message:	lambda-lib support
#	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	jsr166	1.6	* http://creativecommons.org/publicdomain/zero/1.0/
5	jsr166	1.1	*/
6
7			package java.util.concurrent;
8
9	dl	1.9	import java.io.ObjectStreamField;
10	jsr166	1.1	import java.util.Random;
11	dl	1.7	import java.util.concurrent.atomic.AtomicInteger;
12			import java.util.concurrent.atomic.AtomicLong;
13	jsr166	1.1
14			/**
15	jsr166	1.3	* A random number generator isolated to the current thread. Like the
16	jsr166	1.2	* global {@link java.util.Random} generator used by the {@link
17	jsr166	1.3	* java.lang.Math} class, a {@code ThreadLocalRandom} is initialized
18			* with an internally generated seed that may not otherwise be
19			* modified. When applicable, use of {@code ThreadLocalRandom} rather
20			* than shared {@code Random} objects in concurrent programs will
21			* typically encounter much less overhead and contention. Use of
22			* {@code ThreadLocalRandom} is particularly appropriate when multiple
23			* tasks (for example, each a {@link ForkJoinTask}) use random numbers
24			* in parallel in thread pools.
25	jsr166	1.1	*
26			* <p>Usages of this class should typically be of the form:
27			* {@code ThreadLocalRandom.current().nextX(...)} (where
28			* {@code X} is {@code Int}, {@code Long}, etc).
29			* When all usages are of this form, it is never possible to
30	jsr166	1.3	* accidently share a {@code ThreadLocalRandom} across multiple threads.
31	jsr166	1.1	*
32			* <p>This class also provides additional commonly used bounded random
33			* generation methods.
34			*
35			* @since 1.7
36			* @author Doug Lea
37			*/
38			public class ThreadLocalRandom extends Random {
39	dl	1.7	/*
40			* This class implements the java.util.Random API (and subclasses
41			* Random) using a single static instance that accesses random
42			* number state held in class Thread (primarily, field
43			* threadLocalRandomSeed). In doing so, it also provides a home
44			* for managing package-private utilities that rely on exactly the
45			* same state as needed to maintain the ThreadLocalRandom
46			* instances. We leverage the need for an initialization flag
47			* field to also use it as a "probe" -- a self-adjusting thread
48			* hash used for contention avoidance, as well as a secondary
49			* simpler (xorShift) random seed that is conservatively used to
50			* avoid otherwise surprising users by hijacking the
51			* ThreadLocalRandom sequence. The dual use is a marriage of
52			* convenience, but is a simple and efficient way of reducing
53			* application-level overhead and footprint of most concurrent
54			* programs.
55			*
56			* Because this class is in a different package than class Thread,
57			* field access methods must use Unsafe to bypass access control
58			* rules. The base functionality of Random methods is
59			* conveniently isolated in method next(bits), that just reads and
60			* writes the Thread field rather than its own field. However, to
61			* conform to the requirements of the Random constructor, during
62			* construction, the common static ThreadLocalRandom must maintain
63			* initialization and value fields, mainly for the sake of
64			* disabling user calls to setSeed while still allowing a call
65			* from constructor. For serialization compatibility, these
66			* fields are left with the same declarations as used in the
67			* previous ThreadLocal-based version of this class, that used
68			* them differently. Note that serialization is completely
69			* unnecessary because there is only a static singleton. But these
70			* mechanics still ensure compatibility across versions.
71			*
72			* Per-instance initialization is similar to that in the no-arg
73			* Random constructor, but we avoid correlation among not only
74			* initial seeds of those created in different threads, but also
75			* those created using class Random itself; while at the same time
76			* not changing any statistical properties. So we use the same
77			* underlying multiplicative sequence, but start the sequence far
78			* away from the base version, and then merge (xor) current time
79			* and per-thread probe bits to generate initial values.
80			*
81			* The nextLocalGaussian ThreadLocal supports the very rarely used
82			* nextGaussian method by providing a holder for the second of a
83			* pair of them. As is true for the base class version of this
84			* method, this time/space tradeoff is probably never worthwhile,
85			* but we provide identical statistical properties.
86			*/
87
88	jsr166	1.1	// same constants as Random, but must be redeclared because private
89	jsr166	1.5	private static final long multiplier = 0x5DEECE66DL;
90			private static final long addend = 0xBL;
91			private static final long mask = (1L << 48) - 1;
92	dl	1.7	private static final int PROBE_INCREMENT = 0x61c88647;
93	jsr166	1.1
94	dl	1.7	/** Generates the basis for per-thread initial seed values */
95			private static final AtomicLong seedGenerator =
96			new AtomicLong(1269533684904616924L);
97	jsr166	1.1
98	dl	1.7	/** Generates per-thread initialization/probe field */
99			private static final AtomicInteger probeGenerator =
100			new AtomicInteger(0xe80f8647);
101	jsr166	1.1
102	dl	1.7	/** Rarely-used holder for the second of a pair of Gaussians */
103			private static final ThreadLocal<Double> nextLocalGaussian =
104			new ThreadLocal<Double>();
105	jsr166	1.1
106	dl	1.7	/*
107	dl	1.9	* Field used only during singleton initialization
108	jsr166	1.1	*/
109	dl	1.7	boolean initialized; // true when constructor completes
110
111			/** Constructor used only for static singleton */
112			private ThreadLocalRandom() {
113			initialized = true; // false during super() call
114			}
115	jsr166	1.1
116	dl	1.7	/** The common ThreadLocalRandom */
117			static final ThreadLocalRandom instance = new ThreadLocalRandom();
118	jsr166	1.1
119			/**
120	dl	1.7	* Initialize Thread fields for the current thread. Called only
121			* when Thread.threadLocalRandomProbe is zero, indicating that a
122			* thread local seed value needs to be generated. Note that even
123			* though the initialization is purely thread-local, we need to
124			* rely on (static) atomic generators to initialize the values.
125			*/
126			static final void localInit() {
127			int p = probeGenerator.getAndAdd(PROBE_INCREMENT);
128			int probe = (p == 0) ? 1 : p; // skip 0
129			long current, next;
130			do { // same sequence as j.u.Random but different initial value
131			current = seedGenerator.get();
132			next = current * 181783497276652981L;
133			} while (!seedGenerator.compareAndSet(current, next));
134			long r = next ^ ((long)probe << 32) ^ System.nanoTime();
135			Thread t = Thread.currentThread();
136			UNSAFE.putLong(t, SEED, r);
137			UNSAFE.putInt(t, PROBE, probe);
138	jsr166	1.1	}
139
140			/**
141	jsr166	1.3	* Returns the current thread's {@code ThreadLocalRandom}.
142	jsr166	1.1	*
143	jsr166	1.3	* @return the current thread's {@code ThreadLocalRandom}
144	jsr166	1.1	*/
145			public static ThreadLocalRandom current() {
146	dl	1.7	if (UNSAFE.getInt(Thread.currentThread(), PROBE) == 0)
147			localInit();
148			return instance;
149	jsr166	1.1	}
150
151			/**
152	jsr166	1.3	* Throws {@code UnsupportedOperationException}. Setting seeds in
153			* this generator is not supported.
154	jsr166	1.1	*
155			* @throws UnsupportedOperationException always
156			*/
157			public void setSeed(long seed) {
158	dl	1.7	if (initialized) // allow call from super() constructor
159	jsr166	1.1	throw new UnsupportedOperationException();
160			}
161
162			protected int next(int bits) {
163	dl	1.7	Thread t; long r; // read and update per-thread seed
164			UNSAFE.putLong
165			(t = Thread.currentThread(), SEED,
166			r = (UNSAFE.getLong(t, SEED) * multiplier + addend) & mask);
167			return (int) (r >>> (48-bits));
168	jsr166	1.1	}
169
170			/**
171			* Returns a pseudorandom, uniformly distributed value between the
172			* given least value (inclusive) and bound (exclusive).
173			*
174			* @param least the least value returned
175			* @param bound the upper bound (exclusive)
176			* @throws IllegalArgumentException if least greater than or equal
177			* to bound
178			* @return the next value
179			*/
180			public int nextInt(int least, int bound) {
181			if (least >= bound)
182			throw new IllegalArgumentException();
183			return nextInt(bound - least) + least;
184			}
185
186			/**
187			* Returns a pseudorandom, uniformly distributed value
188			* between 0 (inclusive) and the specified value (exclusive).
189			*
190			* @param n the bound on the random number to be returned. Must be
191			* positive.
192			* @return the next value
193			* @throws IllegalArgumentException if n is not positive
194			*/
195			public long nextLong(long n) {
196			if (n <= 0)
197			throw new IllegalArgumentException("n must be positive");
198			// Divide n by two until small enough for nextInt. On each
199			// iteration (at most 31 of them but usually much less),
200			// randomly choose both whether to include high bit in result
201			// (offset) and whether to continue with the lower vs upper
202			// half (which makes a difference only if odd).
203			long offset = 0;
204			while (n >= Integer.MAX_VALUE) {
205			int bits = next(2);
206			long half = n >>> 1;
207			long nextn = ((bits & 2) == 0) ? half : n - half;
208			if ((bits & 1) == 0)
209			offset += n - nextn;
210			n = nextn;
211			}
212			return offset + nextInt((int) n);
213			}
214
215			/**
216			* Returns a pseudorandom, uniformly distributed value between the
217			* given least value (inclusive) and bound (exclusive).
218			*
219			* @param least the least value returned
220			* @param bound the upper bound (exclusive)
221			* @return the next value
222			* @throws IllegalArgumentException if least greater than or equal
223			* to bound
224			*/
225			public long nextLong(long least, long bound) {
226			if (least >= bound)
227			throw new IllegalArgumentException();
228			return nextLong(bound - least) + least;
229			}
230
231			/**
232			* Returns a pseudorandom, uniformly distributed {@code double} value
233			* between 0 (inclusive) and the specified value (exclusive).
234			*
235			* @param n the bound on the random number to be returned. Must be
236			* positive.
237			* @return the next value
238			* @throws IllegalArgumentException if n is not positive
239			*/
240			public double nextDouble(double n) {
241			if (n <= 0)
242			throw new IllegalArgumentException("n must be positive");
243			return nextDouble() * n;
244			}
245
246			/**
247			* Returns a pseudorandom, uniformly distributed value between the
248			* given least value (inclusive) and bound (exclusive).
249			*
250			* @param least the least value returned
251			* @param bound the upper bound (exclusive)
252			* @return the next value
253			* @throws IllegalArgumentException if least greater than or equal
254			* to bound
255			*/
256			public double nextDouble(double least, double bound) {
257			if (least >= bound)
258			throw new IllegalArgumentException();
259			return nextDouble() * (bound - least) + least;
260			}
261
262	dl	1.7	public double nextGaussian() {
263			// Use nextLocalGaussian instead of nextGaussian field
264			Double d = nextLocalGaussian.get();
265			if (d != null) {
266			nextLocalGaussian.set(null);
267			return d.doubleValue();
268			}
269			double v1, v2, s;
270			do {
271			v1 = 2 * nextDouble() - 1; // between -1 and 1
272			v2 = 2 * nextDouble() - 1; // between -1 and 1
273			s = v1 * v1 + v2 * v2;
274			} while (s >= 1 \|\| s == 0);
275			double multiplier = StrictMath.sqrt(-2 * StrictMath.log(s)/s);
276			nextLocalGaussian.set(new Double(v2 * multiplier));
277			return v1 * multiplier;
278			}
279
280			// Within-package utilities
281
282			/*
283			* Descriptions of the usages of the methods below can be found in
284			* the classes that use them. Briefly, a thread's "probe" value is
285			* a non-zero hash code that (probably) does not collide with
286			* other existing threads with respect to any power of two
287			* collision space. When it does collide, it is pseudo-randomly
288			* adjusted (using a Marsaglia XorShift). The nextSecondarySeed
289			* method is used in the same contexts as ThreadLocalRandom, but
290			* only for transient usages such as random adaptive spin/block
291			* sequences for which a cheap RNG suffices and for which it could
292			* in principle disrupt user-visible statistical properties of the
293			* main ThreadLocalRandom if we were to use it.
294			*
295			* Note: Because of package-protection issues, versions of some
296	dl	1.9	* these methods also appear in some subpackage classes.
297	dl	1.7	*/
298
299			/**
300			* Returns the probe value for the current thread without forcing
301			* initialization. Note that invoking ThreadLocalRandom.current()
302			* can be used to force initialization on zero return.
303			*/
304			static final int getProbe() {
305			return UNSAFE.getInt(Thread.currentThread(), PROBE);
306			}
307
308			/**
309			* Pseudo-randomly advances and records the given probe value for the
310			* given thread.
311			*/
312			static final int advanceProbe(int probe) {
313			probe ^= probe << 13; // xorshift
314			probe ^= probe >>> 17;
315			probe ^= probe << 5;
316			UNSAFE.putInt(Thread.currentThread(), PROBE, probe);
317			return probe;
318			}
319
320			/**
321			* Returns the pseudo-randomly initialized or updated secondary seed.
322			*/
323			static final int nextSecondarySeed() {
324			int r;
325			Thread t = Thread.currentThread();
326			if ((r = UNSAFE.getInt(t, SECONDARY)) != 0) {
327			r ^= r << 13; // xorshift
328			r ^= r >>> 17;
329			r ^= r << 5;
330			}
331			else if ((r = (int)UNSAFE.getLong(t, SEED)) == 0)
332			r = 1; // avoid zero
333			UNSAFE.putInt(t, SECONDARY, r);
334			return r;
335			}
336
337	dl	1.9	// Serialization support, maintains original persistent form.
338
339	jsr166	1.1	private static final long serialVersionUID = -5851777807851030925L;
340	dl	1.7
341	dl	1.9	/**
342			* @serialField rnd long
343			* @serialField initialized boolean
344			* @serialField pad0 long
345			* @serialField pad1 long
346			* @serialField pad2 long
347			* @serialField pad3 long
348			* @serialField pad4 long
349			* @serialField pad5 long
350			* @serialField pad6 long
351			* @serialField pad7 long
352			*/
353			private static final ObjectStreamField[] serialPersistentFields = {
354			new ObjectStreamField("rnd", long.class),
355			new ObjectStreamField("initialized", boolean.class),
356			new ObjectStreamField("pad0", long.class),
357			new ObjectStreamField("pad1", long.class),
358			new ObjectStreamField("pad2", long.class),
359			new ObjectStreamField("pad3", long.class),
360			new ObjectStreamField("pad4", long.class),
361			new ObjectStreamField("pad5", long.class),
362			new ObjectStreamField("pad6", long.class),
363			new ObjectStreamField("pad7", long.class) };
364
365			/**
366			* Saves the {@code ThreadLocalRandom} to a stream (that is, serializes it).
367			*/
368			private void writeObject(java.io.ObjectOutputStream out)
369			throws java.io.IOException {
370
371			java.io.ObjectOutputStream.PutField fields = out.putFields();
372			fields.put("rnd", 0L);
373			fields.put("initialized", true);
374			fields.put("pad0", 0L);
375			fields.put("pad1", 0L);
376			fields.put("pad2", 0L);
377			fields.put("pad3", 0L);
378			fields.put("pad4", 0L);
379			fields.put("pad5", 0L);
380			fields.put("pad6", 0L);
381			fields.put("pad7", 0L);
382			out.writeFields();
383			}
384
385			/**
386			* Returns the {@link #current() current} thread's {@code ThreadLocalRandom}.
387			*/
388			private Object readResolve() {
389			return current();
390			}
391
392	dl	1.7	// Unsafe mechanics
393			private static final sun.misc.Unsafe UNSAFE;
394			private static final long SEED;
395			private static final long PROBE;
396			private static final long SECONDARY;
397			static {
398			try {
399			UNSAFE = sun.misc.Unsafe.getUnsafe();
400			Class<?> tk = Thread.class;
401			SEED = UNSAFE.objectFieldOffset
402			(tk.getDeclaredField("threadLocalRandomSeed"));
403			PROBE = UNSAFE.objectFieldOffset
404			(tk.getDeclaredField("threadLocalRandomProbe"));
405			SECONDARY = UNSAFE.objectFieldOffset
406			(tk.getDeclaredField("threadLocalRandomSecondarySeed"));
407			} catch (Exception e) {
408			throw new Error(e);
409			}
410			}
411	jsr166	1.1	}