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 use Unsafe to bypass access control rules.
     * The base functionality of Random methods is conveniently
     * isolated in method next(bits), that just reads and writes the
     * Thread field rather than its own field.  However, to conform to
     * the requirements of the Random superclass constructor, the
     * common static ThreadLocalRandom maintains an "initialized"
     * field for the sake of rejecting user calls to setSeed while
     * still allowing a call from constructor.  Note that
     * serialization is completely unnecessary because there is only a
     * static singleton.  But we generate a serial form containing
     * "rnd" and "initialized" fields to ensure compatibility across
     * versions.
     *
     * Per-thread initialization is similar to that in the no-arg
     * Random constructor, but we avoid correlation among not only
     * initial seeds of those created in different threads, but also
     * those created using class Random itself; while at the same time
     * not changing any statistical properties.  So we use the same
     * underlying multiplicative sequence, but start the sequence far
     * away from the base version, and then merge (xor) current time
     * and per-thread probe bits to generate initial values.
     *
     * The nextLocalGaussian ThreadLocal supports the very rarely used
     * nextGaussian method by providing a holder for the second of a
     * pair of them. As is true for the base class version of this
     * method, this time/space tradeoff is probably never worthwhile,
     * but we provide identical statistical properties.
     */

    // same constants as Random, but must be redeclared because private
    private static final long multiplier = 0x5DEECE66DL;
    private static final long addend = 0xBL;
    private static final long mask = (1L << 48) - 1;
    private static final int PROBE_INCREMENT = 0x61c88647;

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

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

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

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

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

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

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

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

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

    protected int next(int bits) {
        Thread t; long r; // read and update per-thread seed
        UNSAFE.putLong
            (t = Thread.currentThread(), SEED,
             r = (UNSAFE.getLong(t, SEED) * multiplier + addend) & mask);
        return (int) (r >>> (48-bits));
    }

    /**
     * Returns a pseudorandom, uniformly distributed value between the
     * given least value (inclusive) and bound (exclusive).
     *
     * @param least the least value returned
     * @param bound the upper bound (exclusive)
     * @return the next value
     * @throws IllegalArgumentException if least greater than or equal
     * to bound
     */
    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).
     * @param s the stream
     * @throws java.io.IOException if an I/O error occurs
     */
    private void writeObject(java.io.ObjectOutputStream s)
        throws java.io.IOException {

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

    /**
     * Returns the {@link #current() current} thread's {@code ThreadLocalRandom}.
     * @return 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:	1.18
Committed:	Fri Jul 19 19:34:43 2013 UTC (10 years, 10 months ago) by jsr166
Branch:	MAIN
Changes since 1.17:	+1 -1 lines
Log Message:	enforce standard javadoc tag order
#	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 use Unsafe to bypass access control rules.
58	* The base functionality of Random methods is conveniently
59	* isolated in method next(bits), that just reads and writes the
60	* Thread field rather than its own field. However, to conform to
61	* the requirements of the Random superclass constructor, the
62	* common static ThreadLocalRandom maintains an "initialized"
63	* field for the sake of rejecting user calls to setSeed while
64	* still allowing a call from constructor. Note that
65	* serialization is completely unnecessary because there is only a
66	* static singleton. But we generate a serial form containing
67	* "rnd" and "initialized" fields to ensure compatibility across
68	* versions.
69	*
70	* Per-thread initialization is similar to that in the no-arg
71	* Random constructor, but we avoid correlation among not only
72	* initial seeds of those created in different threads, but also
73	* those created using class Random itself; while at the same time
74	* not changing any statistical properties. So we use the same
75	* underlying multiplicative sequence, but start the sequence far
76	* away from the base version, and then merge (xor) current time
77	* and per-thread probe bits to generate initial values.
78	*
79	* The nextLocalGaussian ThreadLocal supports the very rarely used
80	* nextGaussian method by providing a holder for the second of a
81	* pair of them. As is true for the base class version of this
82	* method, this time/space tradeoff is probably never worthwhile,
83	* but we provide identical statistical properties.
84	*/
85
86	// same constants as Random, but must be redeclared because private
87	private static final long multiplier = 0x5DEECE66DL;
88	private static final long addend = 0xBL;
89	private static final long mask = (1L << 48) - 1;
90	private static final int PROBE_INCREMENT = 0x61c88647;
91
92	/** Generates the basis for per-thread initial seed values */
93	private static final AtomicLong seedGenerator =
94	new AtomicLong(1269533684904616924L);
95
96	/** Generates per-thread initialization/probe field */
97	private static final AtomicInteger probeGenerator =
98	new AtomicInteger(0xe80f8647);
99
100	/** Rarely-used holder for the second of a pair of Gaussians */
101	private static final ThreadLocal<Double> nextLocalGaussian =
102	new ThreadLocal<Double>();
103
104	/**
105	* Field used only during singleton initialization.
106	* True when constructor completes.
107	*/
108	boolean initialized;
109
110	/** Constructor used only for static singleton */
111	private ThreadLocalRandom() {
112	initialized = true; // false during super() call
113	}
114
115	/** The common ThreadLocalRandom */
116	static final ThreadLocalRandom instance = new ThreadLocalRandom();
117
118	/**
119	* Initialize Thread fields for the current thread. Called only
120	* when Thread.threadLocalRandomProbe is zero, indicating that a
121	* thread local seed value needs to be generated. Note that even
122	* though the initialization is purely thread-local, we need to
123	* rely on (static) atomic generators to initialize the values.
124	*/
125	static final void localInit() {
126	int p = probeGenerator.getAndAdd(PROBE_INCREMENT);
127	int probe = (p == 0) ? 1 : p; // skip 0
128	long current, next;
129	do { // same sequence as j.u.Random but different initial value
130	current = seedGenerator.get();
131	next = current * 181783497276652981L;
132	} while (!seedGenerator.compareAndSet(current, next));
133	long r = next ^ ((long)probe << 32) ^ System.nanoTime();
134	Thread t = Thread.currentThread();
135	UNSAFE.putLong(t, SEED, r);
136	UNSAFE.putInt(t, PROBE, probe);
137	}
138
139	/**
140	* Returns the current thread's {@code ThreadLocalRandom}.
141	*
142	* @return the current thread's {@code ThreadLocalRandom}
143	*/
144	public static ThreadLocalRandom current() {
145	if (UNSAFE.getInt(Thread.currentThread(), PROBE) == 0)
146	localInit();
147	return instance;
148	}
149
150	/**
151	* Throws {@code UnsupportedOperationException}. Setting seeds in
152	* this generator is not supported.
153	*
154	* @throws UnsupportedOperationException always
155	*/
156	public void setSeed(long seed) {
157	// only allow call from super() constructor
158	if (initialized)
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	* @return the next value
177	* @throws IllegalArgumentException if least greater than or equal
178	* to bound
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 {
332	localInit();
333	if ((r = (int)UNSAFE.getLong(t, SEED)) == 0)
334	r = 1; // avoid zero
335	}
336	UNSAFE.putInt(t, SECONDARY, r);
337	return r;
338	}
339
340	// Serialization support
341
342	private static final long serialVersionUID = -5851777807851030925L;
343
344	/**
345	* @serialField rnd long
346	* seed for random computations
347	* @serialField initialized boolean
348	* always true
349	*/
350	private static final ObjectStreamField[] serialPersistentFields = {
351	new ObjectStreamField("rnd", long.class),
352	new ObjectStreamField("initialized", boolean.class),
353	};
354
355	/**
356	* Saves the {@code ThreadLocalRandom} to a stream (that is, serializes it).
357	* @param s the stream
358	* @throws java.io.IOException if an I/O error occurs
359	*/
360	private void writeObject(java.io.ObjectOutputStream s)
361	throws java.io.IOException {
362
363	java.io.ObjectOutputStream.PutField fields = s.putFields();
364	fields.put("rnd", UNSAFE.getLong(Thread.currentThread(), SEED));
365	fields.put("initialized", true);
366	s.writeFields();
367	}
368
369	/**
370	* Returns the {@link #current() current} thread's {@code ThreadLocalRandom}.
371	* @return the {@link #current() current} thread's {@code ThreadLocalRandom}
372	*/
373	private Object readResolve() {
374	return current();
375	}
376
377	// Unsafe mechanics
378	private static final sun.misc.Unsafe UNSAFE;
379	private static final long SEED;
380	private static final long PROBE;
381	private static final long SECONDARY;
382	static {
383	try {
384	UNSAFE = sun.misc.Unsafe.getUnsafe();
385	Class<?> tk = Thread.class;
386	SEED = UNSAFE.objectFieldOffset
387	(tk.getDeclaredField("threadLocalRandomSeed"));
388	PROBE = UNSAFE.objectFieldOffset
389	(tk.getDeclaredField("threadLocalRandomProbe"));
390	SECONDARY = UNSAFE.objectFieldOffset
391	(tk.getDeclaredField("threadLocalRandomSecondarySeed"));
392	} catch (Exception e) {
393	throw new Error(e);
394	}
395	}
396	}