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.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>();

    /*
     * Fields used only during singleton initialization
     */
    private long rnd;    // superclass random value
    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;
    }

    private static final long serialVersionUID = -5851777807851030925L;

    // 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:	1.7
Committed:	Thu Jan 10 15:03:25 2013 UTC (11 years, 4 months ago) by dl
Branch:	MAIN
Changes since 1.6:	+194 -33 lines
Log Message:	Reinvent ThreadLocalRandom and new dependents
#	Content
1	/*
2	* Written by Doug Lea with assistance from members of JCP JSR-166
3	* Expert Group and released to the public domain, as explained at
4	* http://creativecommons.org/publicdomain/zero/1.0/
5	*/
6
7	package java.util.concurrent;
8
9	import java.util.Random;
10	import java.util.concurrent.atomic.AtomicInteger;
11	import java.util.concurrent.atomic.AtomicLong;
12
13	/**
14	* A random number generator isolated to the current thread. Like the
15	* global {@link java.util.Random} generator used by the {@link
16	* java.lang.Math} class, a {@code ThreadLocalRandom} is initialized
17	* with an internally generated seed that may not otherwise be
18	* modified. When applicable, use of {@code ThreadLocalRandom} rather
19	* than shared {@code Random} objects in concurrent programs will
20	* typically encounter much less overhead and contention. Use of
21	* {@code ThreadLocalRandom} is particularly appropriate when multiple
22	* tasks (for example, each a {@link ForkJoinTask}) use random numbers
23	* in parallel in thread pools.
24	*
25	* <p>Usages of this class should typically be of the form:
26	* {@code ThreadLocalRandom.current().nextX(...)} (where
27	* {@code X} is {@code Int}, {@code Long}, etc).
28	* When all usages are of this form, it is never possible to
29	* accidently share a {@code ThreadLocalRandom} across multiple threads.
30	*
31	* <p>This class also provides additional commonly used bounded random
32	* generation methods.
33	*
34	* @since 1.7
35	* @author Doug Lea
36	*/
37	public class ThreadLocalRandom extends Random {
38	/*
39	* This class implements the java.util.Random API (and subclasses
40	* Random) using a single static instance that accesses random
41	* number state held in class Thread (primarily, field
42	* threadLocalRandomSeed). In doing so, it also provides a home
43	* for managing package-private utilities that rely on exactly the
44	* same state as needed to maintain the ThreadLocalRandom
45	* instances. We leverage the need for an initialization flag
46	* field to also use it as a "probe" -- a self-adjusting thread
47	* hash used for contention avoidance, as well as a secondary
48	* simpler (xorShift) random seed that is conservatively used to
49	* avoid otherwise surprising users by hijacking the
50	* ThreadLocalRandom sequence. The dual use is a marriage of
51	* convenience, but is a simple and efficient way of reducing
52	* application-level overhead and footprint of most concurrent
53	* programs.
54	*
55	* Because this class is in a different package than class Thread,
56	* field access methods must use Unsafe to bypass access control
57	* rules. The base functionality of Random methods is
58	* conveniently isolated in method next(bits), that just reads and
59	* writes the Thread field rather than its own field. However, to
60	* conform to the requirements of the Random constructor, during
61	* construction, the common static ThreadLocalRandom must maintain
62	* initialization and value fields, mainly for the sake of
63	* disabling user calls to setSeed while still allowing a call
64	* from constructor. For serialization compatibility, these
65	* fields are left with the same declarations as used in the
66	* previous ThreadLocal-based version of this class, that used
67	* them differently. Note that serialization is completely
68	* unnecessary because there is only a static singleton. But these
69	* mechanics still ensure compatibility across versions.
70	*
71	* Per-instance initialization is similar to that in the no-arg
72	* Random constructor, but we avoid correlation among not only
73	* initial seeds of those created in different threads, but also
74	* those created using class Random itself; while at the same time
75	* not changing any statistical properties. So we use the same
76	* underlying multiplicative sequence, but start the sequence far
77	* away from the base version, and then merge (xor) current time
78	* and per-thread probe bits to generate initial values.
79	*
80	* The nextLocalGaussian ThreadLocal supports the very rarely used
81	* nextGaussian method by providing a holder for the second of a
82	* pair of them. As is true for the base class version of this
83	* method, this time/space tradeoff is probably never worthwhile,
84	* but we provide identical statistical properties.
85	*/
86
87	// same constants as Random, but must be redeclared because private
88	private static final long multiplier = 0x5DEECE66DL;
89	private static final long addend = 0xBL;
90	private static final long mask = (1L << 48) - 1;
91	private static final int PROBE_INCREMENT = 0x61c88647;
92
93	/** Generates the basis for per-thread initial seed values */
94	private static final AtomicLong seedGenerator =
95	new AtomicLong(1269533684904616924L);
96
97	/** Generates per-thread initialization/probe field */
98	private static final AtomicInteger probeGenerator =
99	new AtomicInteger(0xe80f8647);
100
101	/** Rarely-used holder for the second of a pair of Gaussians */
102	private static final ThreadLocal<Double> nextLocalGaussian =
103	new ThreadLocal<Double>();
104
105	/*
106	* Fields used only during singleton initialization
107	*/
108	private long rnd; // superclass random value
109	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
116	/** The common ThreadLocalRandom */
117	static final ThreadLocalRandom instance = new ThreadLocalRandom();
118
119	/**
120	* 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	}
139
140	/**
141	* Returns the current thread's {@code ThreadLocalRandom}.
142	*
143	* @return the current thread's {@code ThreadLocalRandom}
144	*/
145	public static ThreadLocalRandom current() {
146	if (UNSAFE.getInt(Thread.currentThread(), PROBE) == 0)
147	localInit();
148	return instance;
149	}
150
151	/**
152	* Throws {@code UnsupportedOperationException}. Setting seeds in
153	* this generator is not supported.
154	*
155	* @throws UnsupportedOperationException always
156	*/
157	public void setSeed(long seed) {
158	if (initialized) // allow call from super() constructor
159	throw new UnsupportedOperationException();
160	}
161
162	protected int next(int bits) {
163	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	}
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	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	* these methods also appear in some subpackage classes.
297	*/
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	private static final long serialVersionUID = -5851777807851030925L;
338
339	// Unsafe mechanics
340	private static final sun.misc.Unsafe UNSAFE;
341	private static final long SEED;
342	private static final long PROBE;
343	private static final long SECONDARY;
344	static {
345	try {
346	UNSAFE = sun.misc.Unsafe.getUnsafe();
347	Class<?> tk = Thread.class;
348	SEED = UNSAFE.objectFieldOffset
349	(tk.getDeclaredField("threadLocalRandomSeed"));
350	PROBE = UNSAFE.objectFieldOffset
351	(tk.getDeclaredField("threadLocalRandomProbe"));
352	SECONDARY = UNSAFE.objectFieldOffset
353	(tk.getDeclaredField("threadLocalRandomSecondarySeed"));
354	} catch (Exception e) {
355	throw new Error(e);
356	}
357	}
358	}