concurrent/locks/LockSupport.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.locks;

/**
 * Basic thread blocking primitives for creating locks and other
 * synchronization classes.
 *
 * <p>This class associates, with each thread that uses it, a permit
 * (in the sense of the {@link java.util.concurrent.Semaphore
 * Semaphore} class). A call to {@code park} will return immediately
 * if the permit is available, consuming it in the process; otherwise
 * it <em>may</em> block.  A call to {@code unpark} makes the permit
 * available, if it was not already available. (Unlike with Semaphores
 * though, permits do not accumulate. There is at most one.)
 * Reliable usage requires the use of volatile (or atomic) variables
 * to control when to park or unpark.  Orderings of calls to these
 * methods are maintained with respect to volatile variable accesses,
 * but not necessarily non-volatile variable accesses.
 *
 * <p>Methods {@code park} and {@code unpark} provide efficient
 * means of blocking and unblocking threads that do not encounter the
 * problems that cause the deprecated methods {@code Thread.suspend}
 * and {@code Thread.resume} to be unusable for such purposes: Races
 * between one thread invoking {@code park} and another thread trying
 * to {@code unpark} it will preserve liveness, due to the
 * permit. Additionally, {@code park} will return if the caller's
 * thread was interrupted, and timeout versions are supported. The
 * {@code park} method may also return at any other time, for "no
 * reason", so in general must be invoked within a loop that rechecks
 * conditions upon return. In this sense {@code park} serves as an
 * optimization of a "busy wait" that does not waste as much time
 * spinning, but must be paired with an {@code unpark} to be
 * effective.
 *
 * <p>The three forms of {@code park} each also support a
 * {@code blocker} object parameter. This object is recorded while
 * the thread is blocked to permit monitoring and diagnostic tools to
 * identify the reasons that threads are blocked. (Such tools may
 * access blockers using method {@link #getBlocker(Thread)}.)
 * The use of these forms rather than the original forms without this
 * parameter is strongly encouraged. The normal argument to supply as
 * a {@code blocker} within a lock implementation is {@code this}.
 *
 * <p>These methods are designed to be used as tools for creating
 * higher-level synchronization utilities, and are not in themselves
 * useful for most concurrency control applications.  The {@code park}
 * method is designed for use only in constructions of the form:
 *
 * <pre> {@code
 * while (!canProceed()) {
 *   // ensure request to unpark is visible to other threads
 *   ...
 *   LockSupport.park(this);
 * }}</pre>
 *
 * where no actions by the thread publishing a request to unpark,
 * prior to the call to {@code park}, entail locking or blocking.
 * Because only one permit is associated with each thread, any
 * intermediary uses of {@code park}, including implicitly via class
 * loading, could lead to an unresponsive thread (a "lost unpark").
 *
 * <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out
 * non-reentrant lock class:
 * <pre> {@code
 * class FIFOMutex {
 *   private final AtomicBoolean locked = new AtomicBoolean(false);
 *   private final Queue<Thread> waiters
 *     = new ConcurrentLinkedQueue<>();
 *
 *   public void lock() {
 *     boolean wasInterrupted = false;
 *     // publish current thread for unparkers
 *     waiters.add(Thread.currentThread());
 *
 *     // Block while not first in queue or cannot acquire lock
 *     while (waiters.peek() != Thread.currentThread() ||
 *            !locked.compareAndSet(false, true)) {
 *       LockSupport.park(this);
 *       // ignore interrupts while waiting
 *       if (Thread.interrupted())
 *         wasInterrupted = true;
 *     }
 *
 *     waiters.remove();
 *     // ensure correct interrupt status on return
 *     if (wasInterrupted)
 *       Thread.currentThread().interrupt();
 *   }
 *
 *   public void unlock() {
 *     locked.set(false);
 *     LockSupport.unpark(waiters.peek());
 *   }
 *
 *   static {
 *     // Reduce the risk of "lost unpark" due to classloading
 *     Class<?> ensureLoaded = LockSupport.class;
 *   }
 * }}</pre>
 *
 * @since 1.5
 */
public class LockSupport {
    private LockSupport() {} // Cannot be instantiated.

    private static void setBlocker(Thread t, Object arg) {
        // Even though volatile, hotspot doesn't need a write barrier here.
        U.putObject(t, PARKBLOCKER, arg);
    }

    /**
     * Makes available the permit for the given thread, if it
     * was not already available.  If the thread was blocked on
     * {@code park} then it will unblock.  Otherwise, its next call
     * to {@code park} is guaranteed not to block. This operation
     * is not guaranteed to have any effect at all if the given
     * thread has not been started.
     *
     * @param thread the thread to unpark, or {@code null}, in which case
     *        this operation has no effect
     */
    public static void unpark(Thread thread) {
        if (thread != null)
            U.unpark(thread);
    }

    /**
     * Disables the current thread for thread scheduling purposes unless the
     * permit is available.
     *
     * <p>If the permit is available then it is consumed and the call returns
     * immediately; otherwise
     * the current thread becomes disabled for thread scheduling
     * purposes and lies dormant until one of three things happens:
     *
     * <ul>
     * <li>Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     *
     * <li>The call spuriously (that is, for no reason) returns.
     * </ul>
     *
     * <p>This method does <em>not</em> report which of these caused the
     * method to return. Callers should re-check the conditions which caused
     * the thread to park in the first place. Callers may also determine,
     * for example, the interrupt status of the thread upon return.
     *
     * @param blocker the synchronization object responsible for this
     *        thread parking
     * @since 1.6
     */
    public static void park(Object blocker) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        U.park(false, 0L);
        setBlocker(t, null);
    }

    /**
     * Disables the current thread for thread scheduling purposes, for up to
     * the specified waiting time, unless the permit is available.
     *
     * <p>If the permit is available then it is consumed and the call
     * returns immediately; otherwise the current thread becomes disabled
     * for thread scheduling purposes and lies dormant until one of four
     * things happens:
     *
     * <ul>
     * <li>Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     *
     * <li>The specified waiting time elapses; or
     *
     * <li>The call spuriously (that is, for no reason) returns.
     * </ul>
     *
     * <p>This method does <em>not</em> report which of these caused the
     * method to return. Callers should re-check the conditions which caused
     * the thread to park in the first place. Callers may also determine,
     * for example, the interrupt status of the thread, or the elapsed time
     * upon return.
     *
     * @param blocker the synchronization object responsible for this
     *        thread parking
     * @param nanos the maximum number of nanoseconds to wait
     * @since 1.6
     */
    public static void parkNanos(Object blocker, long nanos) {
        if (nanos > 0) {
            Thread t = Thread.currentThread();
            setBlocker(t, blocker);
            U.park(false, nanos);
            setBlocker(t, null);
        }
    }

    /**
     * Disables the current thread for thread scheduling purposes, until
     * the specified deadline, unless the permit is available.
     *
     * <p>If the permit is available then it is consumed and the call
     * returns immediately; otherwise the current thread becomes disabled
     * for thread scheduling purposes and lies dormant until one of four
     * things happens:
     *
     * <ul>
     * <li>Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
     * current thread; or
     *
     * <li>The specified deadline passes; or
     *
     * <li>The call spuriously (that is, for no reason) returns.
     * </ul>
     *
     * <p>This method does <em>not</em> report which of these caused the
     * method to return. Callers should re-check the conditions which caused
     * the thread to park in the first place. Callers may also determine,
     * for example, the interrupt status of the thread, or the current time
     * upon return.
     *
     * @param blocker the synchronization object responsible for this
     *        thread parking
     * @param deadline the absolute time, in milliseconds from the Epoch,
     *        to wait until
     * @since 1.6
     */
    public static void parkUntil(Object blocker, long deadline) {
        Thread t = Thread.currentThread();
        setBlocker(t, blocker);
        U.park(true, deadline);
        setBlocker(t, null);
    }

    /**
     * Returns the blocker object supplied to the most recent
     * invocation of a park method that has not yet unblocked, or null
     * if not blocked.  The value returned is just a momentary
     * snapshot -- the thread may have since unblocked or blocked on a
     * different blocker object.
     *
     * @param t the thread
     * @return the blocker
     * @throws NullPointerException if argument is null
     * @since 1.6
     */
    public static Object getBlocker(Thread t) {
        if (t == null)
            throw new NullPointerException();
        return U.getObjectVolatile(t, PARKBLOCKER);
    }

    /**
     * Disables the current thread for thread scheduling purposes unless the
     * permit is available.
     *
     * <p>If the permit is available then it is consumed and the call
     * returns immediately; otherwise the current thread becomes disabled
     * for thread scheduling purposes and lies dormant until one of three
     * things happens:
     *
     * <ul>
     *
     * <li>Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     *
     * <li>The call spuriously (that is, for no reason) returns.
     * </ul>
     *
     * <p>This method does <em>not</em> report which of these caused the
     * method to return. Callers should re-check the conditions which caused
     * the thread to park in the first place. Callers may also determine,
     * for example, the interrupt status of the thread upon return.
     */
    public static void park() {
        U.park(false, 0L);
    }

    /**
     * Disables the current thread for thread scheduling purposes, for up to
     * the specified waiting time, unless the permit is available.
     *
     * <p>If the permit is available then it is consumed and the call
     * returns immediately; otherwise the current thread becomes disabled
     * for thread scheduling purposes and lies dormant until one of four
     * things happens:
     *
     * <ul>
     * <li>Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     *
     * <li>The specified waiting time elapses; or
     *
     * <li>The call spuriously (that is, for no reason) returns.
     * </ul>
     *
     * <p>This method does <em>not</em> report which of these caused the
     * method to return. Callers should re-check the conditions which caused
     * the thread to park in the first place. Callers may also determine,
     * for example, the interrupt status of the thread, or the elapsed time
     * upon return.
     *
     * @param nanos the maximum number of nanoseconds to wait
     */
    public static void parkNanos(long nanos) {
        if (nanos > 0)
            U.park(false, nanos);
    }

    /**
     * Disables the current thread for thread scheduling purposes, until
     * the specified deadline, unless the permit is available.
     *
     * <p>If the permit is available then it is consumed and the call
     * returns immediately; otherwise the current thread becomes disabled
     * for thread scheduling purposes and lies dormant until one of four
     * things happens:
     *
     * <ul>
     * <li>Some other thread invokes {@link #unpark unpark} with the
     * current thread as the target; or
     *
     * <li>Some other thread {@linkplain Thread#interrupt interrupts}
     * the current thread; or
     *
     * <li>The specified deadline passes; or
     *
     * <li>The call spuriously (that is, for no reason) returns.
     * </ul>
     *
     * <p>This method does <em>not</em> report which of these caused the
     * method to return. Callers should re-check the conditions which caused
     * the thread to park in the first place. Callers may also determine,
     * for example, the interrupt status of the thread, or the current time
     * upon return.
     *
     * @param deadline the absolute time, in milliseconds from the Epoch,
     *        to wait until
     */
    public static void parkUntil(long deadline) {
        U.park(true, deadline);
    }

    /**
     * Returns the pseudo-randomly initialized or updated secondary seed.
     * Copied from ThreadLocalRandom due to package access restrictions.
     */
    static final int nextSecondarySeed() {
        int r;
        Thread t = Thread.currentThread();
        if ((r = U.getInt(t, SECONDARY)) != 0) {
            r ^= r << 13;   // xorshift
            r ^= r >>> 17;
            r ^= r << 5;
        }
        else if ((r = java.util.concurrent.ThreadLocalRandom.current().nextInt()) == 0)
            r = 1; // avoid zero
        U.putInt(t, SECONDARY, r);
        return r;
    }

    // Hotspot implementation via intrinsics API
    private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
    private static final long PARKBLOCKER;
    private static final long SECONDARY;
    static {
        try {
            PARKBLOCKER = U.objectFieldOffset
                (Thread.class.getDeclaredField("parkBlocker"));
            SECONDARY = U.objectFieldOffset
                (Thread.class.getDeclaredField("threadLocalRandomSecondarySeed"));
        } catch (ReflectiveOperationException e) {
            throw new Error(e);
        }
    }

}
Revision:	1.2
Committed:	Thu Jun 15 22:48:08 2017 UTC (6 years, 11 months ago) by jsr166
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.1:	+2 -0 lines
Log Message:	8181082: class-level since tag issues in java.base & java.datatransfer module
#	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.locks;
8
9	/**
10	* Basic thread blocking primitives for creating locks and other
11	* synchronization classes.
12	*
13	* <p>This class associates, with each thread that uses it, a permit
14	* (in the sense of the {@link java.util.concurrent.Semaphore
15	* Semaphore} class). A call to {@code park} will return immediately
16	* if the permit is available, consuming it in the process; otherwise
17	* it <em>may</em> block. A call to {@code unpark} makes the permit
18	* available, if it was not already available. (Unlike with Semaphores
19	* though, permits do not accumulate. There is at most one.)
20	* Reliable usage requires the use of volatile (or atomic) variables
21	* to control when to park or unpark. Orderings of calls to these
22	* methods are maintained with respect to volatile variable accesses,
23	* but not necessarily non-volatile variable accesses.
24	*
25	* <p>Methods {@code park} and {@code unpark} provide efficient
26	* means of blocking and unblocking threads that do not encounter the
27	* problems that cause the deprecated methods {@code Thread.suspend}
28	* and {@code Thread.resume} to be unusable for such purposes: Races
29	* between one thread invoking {@code park} and another thread trying
30	* to {@code unpark} it will preserve liveness, due to the
31	* permit. Additionally, {@code park} will return if the caller's
32	* thread was interrupted, and timeout versions are supported. The
33	* {@code park} method may also return at any other time, for "no
34	* reason", so in general must be invoked within a loop that rechecks
35	* conditions upon return. In this sense {@code park} serves as an
36	* optimization of a "busy wait" that does not waste as much time
37	* spinning, but must be paired with an {@code unpark} to be
38	* effective.
39	*
40	* <p>The three forms of {@code park} each also support a
41	* {@code blocker} object parameter. This object is recorded while
42	* the thread is blocked to permit monitoring and diagnostic tools to
43	* identify the reasons that threads are blocked. (Such tools may
44	* access blockers using method {@link #getBlocker(Thread)}.)
45	* The use of these forms rather than the original forms without this
46	* parameter is strongly encouraged. The normal argument to supply as
47	* a {@code blocker} within a lock implementation is {@code this}.
48	*
49	* <p>These methods are designed to be used as tools for creating
50	* higher-level synchronization utilities, and are not in themselves
51	* useful for most concurrency control applications. The {@code park}
52	* method is designed for use only in constructions of the form:
53	*
54	* <pre> {@code
55	* while (!canProceed()) {
56	* // ensure request to unpark is visible to other threads
57	* ...
58	* LockSupport.park(this);
59	* }}</pre>
60	*
61	* where no actions by the thread publishing a request to unpark,
62	* prior to the call to {@code park}, entail locking or blocking.
63	* Because only one permit is associated with each thread, any
64	* intermediary uses of {@code park}, including implicitly via class
65	* loading, could lead to an unresponsive thread (a "lost unpark").
66	*
67	* <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out
68	* non-reentrant lock class:
69	* <pre> {@code
70	* class FIFOMutex {
71	* private final AtomicBoolean locked = new AtomicBoolean(false);
72	* private final Queue<Thread> waiters
73	* = new ConcurrentLinkedQueue<>();
74	*
75	* public void lock() {
76	* boolean wasInterrupted = false;
77	* // publish current thread for unparkers
78	* waiters.add(Thread.currentThread());
79	*
80	* // Block while not first in queue or cannot acquire lock
81	* while (waiters.peek() != Thread.currentThread() \|\|
82	* !locked.compareAndSet(false, true)) {
83	* LockSupport.park(this);
84	* // ignore interrupts while waiting
85	* if (Thread.interrupted())
86	* wasInterrupted = true;
87	* }
88	*
89	* waiters.remove();
90	* // ensure correct interrupt status on return
91	* if (wasInterrupted)
92	* Thread.currentThread().interrupt();
93	* }
94	*
95	* public void unlock() {
96	* locked.set(false);
97	* LockSupport.unpark(waiters.peek());
98	* }
99	*
100	* static {
101	* // Reduce the risk of "lost unpark" due to classloading
102	* Class<?> ensureLoaded = LockSupport.class;
103	* }
104	* }}</pre>
105	*
106	* @since 1.5
107	*/
108	public class LockSupport {
109	private LockSupport() {} // Cannot be instantiated.
110
111	private static void setBlocker(Thread t, Object arg) {
112	// Even though volatile, hotspot doesn't need a write barrier here.
113	U.putObject(t, PARKBLOCKER, arg);
114	}
115
116	/**
117	* Makes available the permit for the given thread, if it
118	* was not already available. If the thread was blocked on
119	* {@code park} then it will unblock. Otherwise, its next call
120	* to {@code park} is guaranteed not to block. This operation
121	* is not guaranteed to have any effect at all if the given
122	* thread has not been started.
123	*
124	* @param thread the thread to unpark, or {@code null}, in which case
125	* this operation has no effect
126	*/
127	public static void unpark(Thread thread) {
128	if (thread != null)
129	U.unpark(thread);
130	}
131
132	/**
133	* Disables the current thread for thread scheduling purposes unless the
134	* permit is available.
135	*
136	* <p>If the permit is available then it is consumed and the call returns
137	* immediately; otherwise
138	* the current thread becomes disabled for thread scheduling
139	* purposes and lies dormant until one of three things happens:
140	*
141	* <ul>
142	* <li>Some other thread invokes {@link #unpark unpark} with the
143	* current thread as the target; or
144	*
145	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
146	* the current thread; or
147	*
148	* <li>The call spuriously (that is, for no reason) returns.
149	* </ul>
150	*
151	* <p>This method does <em>not</em> report which of these caused the
152	* method to return. Callers should re-check the conditions which caused
153	* the thread to park in the first place. Callers may also determine,
154	* for example, the interrupt status of the thread upon return.
155	*
156	* @param blocker the synchronization object responsible for this
157	* thread parking
158	* @since 1.6
159	*/
160	public static void park(Object blocker) {
161	Thread t = Thread.currentThread();
162	setBlocker(t, blocker);
163	U.park(false, 0L);
164	setBlocker(t, null);
165	}
166
167	/**
168	* Disables the current thread for thread scheduling purposes, for up to
169	* the specified waiting time, unless the permit is available.
170	*
171	* <p>If the permit is available then it is consumed and the call
172	* returns immediately; otherwise the current thread becomes disabled
173	* for thread scheduling purposes and lies dormant until one of four
174	* things happens:
175	*
176	* <ul>
177	* <li>Some other thread invokes {@link #unpark unpark} with the
178	* current thread as the target; or
179	*
180	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
181	* the current thread; or
182	*
183	* <li>The specified waiting time elapses; or
184	*
185	* <li>The call spuriously (that is, for no reason) returns.
186	* </ul>
187	*
188	* <p>This method does <em>not</em> report which of these caused the
189	* method to return. Callers should re-check the conditions which caused
190	* the thread to park in the first place. Callers may also determine,
191	* for example, the interrupt status of the thread, or the elapsed time
192	* upon return.
193	*
194	* @param blocker the synchronization object responsible for this
195	* thread parking
196	* @param nanos the maximum number of nanoseconds to wait
197	* @since 1.6
198	*/
199	public static void parkNanos(Object blocker, long nanos) {
200	if (nanos > 0) {
201	Thread t = Thread.currentThread();
202	setBlocker(t, blocker);
203	U.park(false, nanos);
204	setBlocker(t, null);
205	}
206	}
207
208	/**
209	* Disables the current thread for thread scheduling purposes, until
210	* the specified deadline, unless the permit is available.
211	*
212	* <p>If the permit is available then it is consumed and the call
213	* returns immediately; otherwise the current thread becomes disabled
214	* for thread scheduling purposes and lies dormant until one of four
215	* things happens:
216	*
217	* <ul>
218	* <li>Some other thread invokes {@link #unpark unpark} with the
219	* current thread as the target; or
220	*
221	* <li>Some other thread {@linkplain Thread#interrupt interrupts} the
222	* current thread; or
223	*
224	* <li>The specified deadline passes; or
225	*
226	* <li>The call spuriously (that is, for no reason) returns.
227	* </ul>
228	*
229	* <p>This method does <em>not</em> report which of these caused the
230	* method to return. Callers should re-check the conditions which caused
231	* the thread to park in the first place. Callers may also determine,
232	* for example, the interrupt status of the thread, or the current time
233	* upon return.
234	*
235	* @param blocker the synchronization object responsible for this
236	* thread parking
237	* @param deadline the absolute time, in milliseconds from the Epoch,
238	* to wait until
239	* @since 1.6
240	*/
241	public static void parkUntil(Object blocker, long deadline) {
242	Thread t = Thread.currentThread();
243	setBlocker(t, blocker);
244	U.park(true, deadline);
245	setBlocker(t, null);
246	}
247
248	/**
249	* Returns the blocker object supplied to the most recent
250	* invocation of a park method that has not yet unblocked, or null
251	* if not blocked. The value returned is just a momentary
252	* snapshot -- the thread may have since unblocked or blocked on a
253	* different blocker object.
254	*
255	* @param t the thread
256	* @return the blocker
257	* @throws NullPointerException if argument is null
258	* @since 1.6
259	*/
260	public static Object getBlocker(Thread t) {
261	if (t == null)
262	throw new NullPointerException();
263	return U.getObjectVolatile(t, PARKBLOCKER);
264	}
265
266	/**
267	* Disables the current thread for thread scheduling purposes unless the
268	* permit is available.
269	*
270	* <p>If the permit is available then it is consumed and the call
271	* returns immediately; otherwise the current thread becomes disabled
272	* for thread scheduling purposes and lies dormant until one of three
273	* things happens:
274	*
275	* <ul>
276	*
277	* <li>Some other thread invokes {@link #unpark unpark} with the
278	* current thread as the target; or
279	*
280	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
281	* the current thread; or
282	*
283	* <li>The call spuriously (that is, for no reason) returns.
284	* </ul>
285	*
286	* <p>This method does <em>not</em> report which of these caused the
287	* method to return. Callers should re-check the conditions which caused
288	* the thread to park in the first place. Callers may also determine,
289	* for example, the interrupt status of the thread upon return.
290	*/
291	public static void park() {
292	U.park(false, 0L);
293	}
294
295	/**
296	* Disables the current thread for thread scheduling purposes, for up to
297	* the specified waiting time, unless the permit is available.
298	*
299	* <p>If the permit is available then it is consumed and the call
300	* returns immediately; otherwise the current thread becomes disabled
301	* for thread scheduling purposes and lies dormant until one of four
302	* things happens:
303	*
304	* <ul>
305	* <li>Some other thread invokes {@link #unpark unpark} with the
306	* current thread as the target; or
307	*
308	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
309	* the current thread; or
310	*
311	* <li>The specified waiting time elapses; or
312	*
313	* <li>The call spuriously (that is, for no reason) returns.
314	* </ul>
315	*
316	* <p>This method does <em>not</em> report which of these caused the
317	* method to return. Callers should re-check the conditions which caused
318	* the thread to park in the first place. Callers may also determine,
319	* for example, the interrupt status of the thread, or the elapsed time
320	* upon return.
321	*
322	* @param nanos the maximum number of nanoseconds to wait
323	*/
324	public static void parkNanos(long nanos) {
325	if (nanos > 0)
326	U.park(false, nanos);
327	}
328
329	/**
330	* Disables the current thread for thread scheduling purposes, until
331	* the specified deadline, unless the permit is available.
332	*
333	* <p>If the permit is available then it is consumed and the call
334	* returns immediately; otherwise the current thread becomes disabled
335	* for thread scheduling purposes and lies dormant until one of four
336	* things happens:
337	*
338	* <ul>
339	* <li>Some other thread invokes {@link #unpark unpark} with the
340	* current thread as the target; or
341	*
342	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
343	* the current thread; or
344	*
345	* <li>The specified deadline passes; or
346	*
347	* <li>The call spuriously (that is, for no reason) returns.
348	* </ul>
349	*
350	* <p>This method does <em>not</em> report which of these caused the
351	* method to return. Callers should re-check the conditions which caused
352	* the thread to park in the first place. Callers may also determine,
353	* for example, the interrupt status of the thread, or the current time
354	* upon return.
355	*
356	* @param deadline the absolute time, in milliseconds from the Epoch,
357	* to wait until
358	*/
359	public static void parkUntil(long deadline) {
360	U.park(true, deadline);
361	}
362
363	/**
364	* Returns the pseudo-randomly initialized or updated secondary seed.
365	* Copied from ThreadLocalRandom due to package access restrictions.
366	*/
367	static final int nextSecondarySeed() {
368	int r;
369	Thread t = Thread.currentThread();
370	if ((r = U.getInt(t, SECONDARY)) != 0) {
371	r ^= r << 13; // xorshift
372	r ^= r >>> 17;
373	r ^= r << 5;
374	}
375	else if ((r = java.util.concurrent.ThreadLocalRandom.current().nextInt()) == 0)
376	r = 1; // avoid zero
377	U.putInt(t, SECONDARY, r);
378	return r;
379	}
380
381	// Hotspot implementation via intrinsics API
382	private static final sun.misc.Unsafe U = sun.misc.Unsafe.getUnsafe();
383	private static final long PARKBLOCKER;
384	private static final long SECONDARY;
385	static {
386	try {
387	PARKBLOCKER = U.objectFieldOffset
388	(Thread.class.getDeclaredField("parkBlocker"));
389	SECONDARY = U.objectFieldOffset
390	(Thread.class.getDeclaredField("threadLocalRandomSecondarySeed"));
391	} catch (ReflectiveOperationException e) {
392	throw new Error(e);
393	}
394	}
395
396	}