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;

    /**
     * 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:	1.13
Committed:	Fri Feb 22 13:47:07 2013 UTC (11 years, 3 months ago) by jsr166
Branch:	MAIN
Changes since 1.12:	+1 -1 lines
Log Message:	remove stale comment
#	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.io.ObjectStreamField;
10	import java.util.Random;
11	import java.util.concurrent.atomic.AtomicInteger;
12	import java.util.concurrent.atomic.AtomicLong;
13
14	/**
15	* A random number generator isolated to the current thread. Like the
16	* global {@link java.util.Random} generator used by the {@link
17	* 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	*
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	* accidently share a {@code ThreadLocalRandom} across multiple threads.
31	*
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	/*
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	// same constants as Random, but must be redeclared because private
89	private static final long multiplier = 0x5DEECE66DL;
90	private static final long addend = 0xBL;
91	private static final long mask = (1L << 48) - 1;
92	private static final int PROBE_INCREMENT = 0x61c88647;
93
94	/** Generates the basis for per-thread initial seed values */
95	private static final AtomicLong seedGenerator =
96	new AtomicLong(1269533684904616924L);
97
98	/** Generates per-thread initialization/probe field */
99	private static final AtomicInteger probeGenerator =
100	new AtomicInteger(0xe80f8647);
101
102	/** Rarely-used holder for the second of a pair of Gaussians */
103	private static final ThreadLocal<Double> nextLocalGaussian =
104	new ThreadLocal<Double>();
105
106	/**
107	* Field used only during singleton initialization.
108	* True when constructor completes.
109	*/
110	boolean initialized;
111
112	/** Constructor used only for static singleton */
113	private ThreadLocalRandom() {
114	initialized = true; // false during super() call
115	}
116
117	/** The common ThreadLocalRandom */
118	static final ThreadLocalRandom instance = new ThreadLocalRandom();
119
120	/**
121	* 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	}
140
141	/**
142	* Returns the current thread's {@code ThreadLocalRandom}.
143	*
144	* @return the current thread's {@code ThreadLocalRandom}
145	*/
146	public static ThreadLocalRandom current() {
147	if (UNSAFE.getInt(Thread.currentThread(), PROBE) == 0)
148	localInit();
149	return instance;
150	}
151
152	/**
153	* Throws {@code UnsupportedOperationException}. Setting seeds in
154	* this generator is not supported.
155	*
156	* @throws UnsupportedOperationException always
157	*/
158	public void setSeed(long seed) {
159	if (initialized) // allow call from super() constructor
160	throw new UnsupportedOperationException();
161	}
162
163	protected int next(int bits) {
164	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	}
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	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	* these methods also appear in some subpackage classes.
298	*/
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	else {
333	localInit();
334	if ((r = (int)UNSAFE.getLong(t, SEED)) == 0)
335	r = 1; // avoid zero
336	}
337	UNSAFE.putInt(t, SECONDARY, r);
338	return r;
339	}
340
341	// Serialization support
342
343	private static final long serialVersionUID = -5851777807851030925L;
344
345	/**
346	* Returns the {@link #current() current} thread's {@code ThreadLocalRandom}.
347	*/
348	private Object readResolve() {
349	return current();
350	}
351
352	// Unsafe mechanics
353	private static final sun.misc.Unsafe UNSAFE;
354	private static final long SEED;
355	private static final long PROBE;
356	private static final long SECONDARY;
357	static {
358	try {
359	UNSAFE = sun.misc.Unsafe.getUnsafe();
360	Class<?> tk = Thread.class;
361	SEED = UNSAFE.objectFieldOffset
362	(tk.getDeclaredField("threadLocalRandomSeed"));
363	PROBE = UNSAFE.objectFieldOffset
364	(tk.getDeclaredField("threadLocalRandomProbe"));
365	SECONDARY = UNSAFE.objectFieldOffset
366	(tk.getDeclaredField("threadLocalRandomSecondarySeed"));
367	} catch (Exception e) {
368	throw new Error(e);
369	}
370	}
371	}