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.
     * 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) {
        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 {
            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);
        }
    }
}
Revision:	1.14
Committed:	Mon Feb 25 22:26:49 2013 UTC (11 years, 3 months ago) by jsr166
Branch:	MAIN
Changes since 1.13:	+23 -0 lines
Log Message:	reinstate @serialFields and writeObject, but un-padded
#	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	jsr166	1.11	/**
107			* Field used only during singleton initialization.
108			* True when constructor completes.
109	jsr166	1.1	*/
110	jsr166	1.11	boolean initialized;
111	dl	1.7
112			/** Constructor used only for static singleton */
113			private ThreadLocalRandom() {
114			initialized = true; // false during super() call
115			}
116	jsr166	1.1
117	dl	1.7	/** The common ThreadLocalRandom */
118			static final ThreadLocalRandom instance = new ThreadLocalRandom();
119	jsr166	1.1
120			/**
121	dl	1.7	* Initialize Thread fields for the current thread. Called only
122			* when Thread.threadLocalRandomProbe is zero, indicating that a
123			* thread local seed value needs to be generated. Note that even
124			* though the initialization is purely thread-local, we need to
125			* rely on (static) atomic generators to initialize the values.
126			*/
127			static final void localInit() {
128			int p = probeGenerator.getAndAdd(PROBE_INCREMENT);
129			int probe = (p == 0) ? 1 : p; // skip 0
130			long current, next;
131			do { // same sequence as j.u.Random but different initial value
132			current = seedGenerator.get();
133			next = current * 181783497276652981L;
134			} while (!seedGenerator.compareAndSet(current, next));
135			long r = next ^ ((long)probe << 32) ^ System.nanoTime();
136			Thread t = Thread.currentThread();
137			UNSAFE.putLong(t, SEED, r);
138			UNSAFE.putInt(t, PROBE, probe);
139	jsr166	1.1	}
140
141			/**
142	jsr166	1.3	* Returns the current thread's {@code ThreadLocalRandom}.
143	jsr166	1.1	*
144	jsr166	1.3	* @return the current thread's {@code ThreadLocalRandom}
145	jsr166	1.1	*/
146			public static ThreadLocalRandom current() {
147	dl	1.7	if (UNSAFE.getInt(Thread.currentThread(), PROBE) == 0)
148			localInit();
149			return instance;
150	jsr166	1.1	}
151
152			/**
153	jsr166	1.3	* Throws {@code UnsupportedOperationException}. Setting seeds in
154			* this generator is not supported.
155	jsr166	1.1	*
156			* @throws UnsupportedOperationException always
157			*/
158			public void setSeed(long seed) {
159	dl	1.7	if (initialized) // allow call from super() constructor
160	jsr166	1.1	throw new UnsupportedOperationException();
161			}
162
163			protected int next(int bits) {
164	dl	1.7	Thread t; long r; // read and update per-thread seed
165			UNSAFE.putLong
166			(t = Thread.currentThread(), SEED,
167			r = (UNSAFE.getLong(t, SEED) * multiplier + addend) & mask);
168			return (int) (r >>> (48-bits));
169	jsr166	1.1	}
170
171			/**
172			* Returns a pseudorandom, uniformly distributed value between the
173			* given least value (inclusive) and bound (exclusive).
174			*
175			* @param least the least value returned
176			* @param bound the upper bound (exclusive)
177			* @throws IllegalArgumentException if least greater than or equal
178			* to bound
179			* @return the next value
180			*/
181			public int nextInt(int least, int bound) {
182			if (least >= bound)
183			throw new IllegalArgumentException();
184			return nextInt(bound - least) + least;
185			}
186
187			/**
188			* Returns a pseudorandom, uniformly distributed value
189			* between 0 (inclusive) and the specified value (exclusive).
190			*
191			* @param n the bound on the random number to be returned. Must be
192			* positive.
193			* @return the next value
194			* @throws IllegalArgumentException if n is not positive
195			*/
196			public long nextLong(long n) {
197			if (n <= 0)
198			throw new IllegalArgumentException("n must be positive");
199			// Divide n by two until small enough for nextInt. On each
200			// iteration (at most 31 of them but usually much less),
201			// randomly choose both whether to include high bit in result
202			// (offset) and whether to continue with the lower vs upper
203			// half (which makes a difference only if odd).
204			long offset = 0;
205			while (n >= Integer.MAX_VALUE) {
206			int bits = next(2);
207			long half = n >>> 1;
208			long nextn = ((bits & 2) == 0) ? half : n - half;
209			if ((bits & 1) == 0)
210			offset += n - nextn;
211			n = nextn;
212			}
213			return offset + nextInt((int) n);
214			}
215
216			/**
217			* Returns a pseudorandom, uniformly distributed value between the
218			* given least value (inclusive) and bound (exclusive).
219			*
220			* @param least the least value returned
221			* @param bound the upper bound (exclusive)
222			* @return the next value
223			* @throws IllegalArgumentException if least greater than or equal
224			* to bound
225			*/
226			public long nextLong(long least, long bound) {
227			if (least >= bound)
228			throw new IllegalArgumentException();
229			return nextLong(bound - least) + least;
230			}
231
232			/**
233			* Returns a pseudorandom, uniformly distributed {@code double} value
234			* between 0 (inclusive) and the specified value (exclusive).
235			*
236			* @param n the bound on the random number to be returned. Must be
237			* positive.
238			* @return the next value
239			* @throws IllegalArgumentException if n is not positive
240			*/
241			public double nextDouble(double n) {
242			if (n <= 0)
243			throw new IllegalArgumentException("n must be positive");
244			return nextDouble() * n;
245			}
246
247			/**
248			* Returns a pseudorandom, uniformly distributed value between the
249			* given least value (inclusive) and bound (exclusive).
250			*
251			* @param least the least value returned
252			* @param bound the upper bound (exclusive)
253			* @return the next value
254			* @throws IllegalArgumentException if least greater than or equal
255			* to bound
256			*/
257			public double nextDouble(double least, double bound) {
258			if (least >= bound)
259			throw new IllegalArgumentException();
260			return nextDouble() * (bound - least) + least;
261			}
262
263	dl	1.7	public double nextGaussian() {
264			// Use nextLocalGaussian instead of nextGaussian field
265			Double d = nextLocalGaussian.get();
266			if (d != null) {
267			nextLocalGaussian.set(null);
268			return d.doubleValue();
269			}
270			double v1, v2, s;
271			do {
272			v1 = 2 * nextDouble() - 1; // between -1 and 1
273			v2 = 2 * nextDouble() - 1; // between -1 and 1
274			s = v1 * v1 + v2 * v2;
275			} while (s >= 1 \|\| s == 0);
276			double multiplier = StrictMath.sqrt(-2 * StrictMath.log(s)/s);
277			nextLocalGaussian.set(new Double(v2 * multiplier));
278			return v1 * multiplier;
279			}
280
281			// Within-package utilities
282
283			/*
284			* Descriptions of the usages of the methods below can be found in
285			* the classes that use them. Briefly, a thread's "probe" value is
286			* a non-zero hash code that (probably) does not collide with
287			* other existing threads with respect to any power of two
288			* collision space. When it does collide, it is pseudo-randomly
289			* adjusted (using a Marsaglia XorShift). The nextSecondarySeed
290			* method is used in the same contexts as ThreadLocalRandom, but
291			* only for transient usages such as random adaptive spin/block
292			* sequences for which a cheap RNG suffices and for which it could
293			* in principle disrupt user-visible statistical properties of the
294			* main ThreadLocalRandom if we were to use it.
295			*
296			* Note: Because of package-protection issues, versions of some
297	dl	1.9	* these methods also appear in some subpackage classes.
298	dl	1.7	*/
299
300			/**
301			* Returns the probe value for the current thread without forcing
302			* initialization. Note that invoking ThreadLocalRandom.current()
303			* can be used to force initialization on zero return.
304			*/
305			static final int getProbe() {
306			return UNSAFE.getInt(Thread.currentThread(), PROBE);
307			}
308
309			/**
310			* Pseudo-randomly advances and records the given probe value for the
311			* given thread.
312			*/
313			static final int advanceProbe(int probe) {
314			probe ^= probe << 13; // xorshift
315			probe ^= probe >>> 17;
316			probe ^= probe << 5;
317			UNSAFE.putInt(Thread.currentThread(), PROBE, probe);
318			return probe;
319			}
320
321			/**
322			* Returns the pseudo-randomly initialized or updated secondary seed.
323			*/
324			static final int nextSecondarySeed() {
325			int r;
326			Thread t = Thread.currentThread();
327			if ((r = UNSAFE.getInt(t, SECONDARY)) != 0) {
328			r ^= r << 13; // xorshift
329			r ^= r >>> 17;
330			r ^= r << 5;
331			}
332	dl	1.10	else {
333			localInit();
334			if ((r = (int)UNSAFE.getLong(t, SEED)) == 0)
335			r = 1; // avoid zero
336			}
337	dl	1.7	UNSAFE.putInt(t, SECONDARY, r);
338			return r;
339			}
340
341	jsr166	1.13	// Serialization support
342	dl	1.9
343	jsr166	1.1	private static final long serialVersionUID = -5851777807851030925L;
344	dl	1.7
345	dl	1.9	/**
346	jsr166	1.14	* @serialField rnd long
347			* seed for random computations
348			* @serialField initialized boolean
349			* always true
350			*/
351			private static final ObjectStreamField[] serialPersistentFields = {
352			new ObjectStreamField("rnd", long.class),
353			new ObjectStreamField("initialized", boolean.class),
354			};
355
356			/**
357			* Saves the {@code ThreadLocalRandom} to a stream (that is, serializes it).
358			*/
359			private void writeObject(java.io.ObjectOutputStream out)
360			throws java.io.IOException {
361
362			java.io.ObjectOutputStream.PutField fields = out.putFields();
363			fields.put("rnd", UNSAFE.getLong(Thread.currentThread(), SEED));
364			fields.put("initialized", true);
365			out.writeFields();
366			}
367
368			/**
369	dl	1.9	* Returns the {@link #current() current} thread's {@code ThreadLocalRandom}.
370			*/
371			private Object readResolve() {
372			return current();
373			}
374
375	dl	1.7	// Unsafe mechanics
376			private static final sun.misc.Unsafe UNSAFE;
377			private static final long SEED;
378			private static final long PROBE;
379			private static final long SECONDARY;
380			static {
381			try {
382			UNSAFE = sun.misc.Unsafe.getUnsafe();
383			Class<?> tk = Thread.class;
384			SEED = UNSAFE.objectFieldOffset
385			(tk.getDeclaredField("threadLocalRandomSeed"));
386			PROBE = UNSAFE.objectFieldOffset
387			(tk.getDeclaredField("threadLocalRandomProbe"));
388			SECONDARY = UNSAFE.objectFieldOffset
389			(tk.getDeclaredField("threadLocalRandomSecondarySeed"));
390			} catch (Exception e) {
391			throw new Error(e);
392			}
393			}
394	jsr166	1.1	}