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;

import jdk.internal.misc.Unsafe;

/**
 * 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) {
        U.putReference(t, PARKBLOCKER, arg);
    }

    /**
     * Sets the object to be returned by invocations of {@link
     * #getBlocker getBlocker} for the current thread. This method may
     * be used before invoking the no-argument version of {@link
     * LockSupport#park() park()} from non-public objects, allowing
     * more helpful diagnostics, or retaining compatibility with
     * previous implementations of blocking methods.  Previous values
     * of the blocker are not automatically restored after blocking.
     * To obtain the effects of {@code park(b}}, use {@code
     * setCurrentBlocker(b); park(); setCurrentBlocker(null);}
     *
     * @param blocker the blocker object
     * @since 14
     */
    public static void setCurrentBlocker(Object blocker) {
        U.putReference(Thread.currentThread(), PARKBLOCKER, blocker);
    }

    /**
     * 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 specified waiting time is zero or negative, the
     * method does nothing. Otherwise, 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.getReference(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 specified waiting time is zero or negative, the
     * method does nothing. Otherwise, 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 thread id for the given thread.  We must access
     * this directly rather than via method Thread.getId() because
     * getId() has been known to be overridden in ways that do not
     * preserve unique mappings.
     */
    static final long getThreadId(Thread thread) {
        return U.getLong(thread, TID);
    }

    // Hotspot implementation via intrinsics API
    private static final Unsafe U = Unsafe.getUnsafe();
    private static final long PARKBLOCKER
        = U.objectFieldOffset(Thread.class, "parkBlocker");
    private static final long TID
        = U.objectFieldOffset(Thread.class, "tid");

}
Revision:	1.66
Committed:	Fri Mar 18 16:00:16 2022 UTC (2 years, 2 months ago) by dl
Branch:	MAIN
CVS Tags:	HEAD
Changes since 1.65:	+3 -3 lines
Log Message:	Faster queue check
#	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	import jdk.internal.misc.Unsafe;
10
11	/**
12	* Basic thread blocking primitives for creating locks and other
13	* synchronization classes.
14	*
15	* <p>This class associates, with each thread that uses it, a permit
16	* (in the sense of the {@link java.util.concurrent.Semaphore
17	* Semaphore} class). A call to {@code park} will return immediately
18	* if the permit is available, consuming it in the process; otherwise
19	* it <em>may</em> block. A call to {@code unpark} makes the permit
20	* available, if it was not already available. (Unlike with Semaphores
21	* though, permits do not accumulate. There is at most one.)
22	* Reliable usage requires the use of volatile (or atomic) variables
23	* to control when to park or unpark. Orderings of calls to these
24	* methods are maintained with respect to volatile variable accesses,
25	* but not necessarily non-volatile variable accesses.
26	*
27	* <p>Methods {@code park} and {@code unpark} provide efficient
28	* means of blocking and unblocking threads that do not encounter the
29	* problems that cause the deprecated methods {@code Thread.suspend}
30	* and {@code Thread.resume} to be unusable for such purposes: Races
31	* between one thread invoking {@code park} and another thread trying
32	* to {@code unpark} it will preserve liveness, due to the
33	* permit. Additionally, {@code park} will return if the caller's
34	* thread was interrupted, and timeout versions are supported. The
35	* {@code park} method may also return at any other time, for "no
36	* reason", so in general must be invoked within a loop that rechecks
37	* conditions upon return. In this sense {@code park} serves as an
38	* optimization of a "busy wait" that does not waste as much time
39	* spinning, but must be paired with an {@code unpark} to be
40	* effective.
41	*
42	* <p>The three forms of {@code park} each also support a
43	* {@code blocker} object parameter. This object is recorded while
44	* the thread is blocked to permit monitoring and diagnostic tools to
45	* identify the reasons that threads are blocked. (Such tools may
46	* access blockers using method {@link #getBlocker(Thread)}.)
47	* The use of these forms rather than the original forms without this
48	* parameter is strongly encouraged. The normal argument to supply as
49	* a {@code blocker} within a lock implementation is {@code this}.
50	*
51	* <p>These methods are designed to be used as tools for creating
52	* higher-level synchronization utilities, and are not in themselves
53	* useful for most concurrency control applications. The {@code park}
54	* method is designed for use only in constructions of the form:
55	*
56	* <pre> {@code
57	* while (!canProceed()) {
58	* // ensure request to unpark is visible to other threads
59	* ...
60	* LockSupport.park(this);
61	* }}</pre>
62	*
63	* where no actions by the thread publishing a request to unpark,
64	* prior to the call to {@code park}, entail locking or blocking.
65	* Because only one permit is associated with each thread, any
66	* intermediary uses of {@code park}, including implicitly via class
67	* loading, could lead to an unresponsive thread (a "lost unpark").
68	*
69	* <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out
70	* non-reentrant lock class:
71	* <pre> {@code
72	* class FIFOMutex {
73	* private final AtomicBoolean locked = new AtomicBoolean(false);
74	* private final Queue<Thread> waiters
75	* = new ConcurrentLinkedQueue<>();
76	*
77	* public void lock() {
78	* boolean wasInterrupted = false;
79	* // publish current thread for unparkers
80	* waiters.add(Thread.currentThread());
81	*
82	* // Block while not first in queue or cannot acquire lock
83	* while (waiters.peek() != Thread.currentThread() \|\|
84	* !locked.compareAndSet(false, true)) {
85	* LockSupport.park(this);
86	* // ignore interrupts while waiting
87	* if (Thread.interrupted())
88	* wasInterrupted = true;
89	* }
90	*
91	* waiters.remove();
92	* // ensure correct interrupt status on return
93	* if (wasInterrupted)
94	* Thread.currentThread().interrupt();
95	* }
96	*
97	* public void unlock() {
98	* locked.set(false);
99	* LockSupport.unpark(waiters.peek());
100	* }
101	*
102	* static {
103	* // Reduce the risk of "lost unpark" due to classloading
104	* Class<?> ensureLoaded = LockSupport.class;
105	* }
106	* }}</pre>
107	*
108	* @since 1.5
109	*/
110	public class LockSupport {
111	private LockSupport() {} // Cannot be instantiated.
112
113	private static void setBlocker(Thread t, Object arg) {
114	U.putReference(t, PARKBLOCKER, arg);
115	}
116
117	/**
118	* Sets the object to be returned by invocations of {@link
119	* #getBlocker getBlocker} for the current thread. This method may
120	* be used before invoking the no-argument version of {@link
121	* LockSupport#park() park()} from non-public objects, allowing
122	* more helpful diagnostics, or retaining compatibility with
123	* previous implementations of blocking methods. Previous values
124	* of the blocker are not automatically restored after blocking.
125	* To obtain the effects of {@code park(b}}, use {@code
126	* setCurrentBlocker(b); park(); setCurrentBlocker(null);}
127	*
128	* @param blocker the blocker object
129	* @since 14
130	*/
131	public static void setCurrentBlocker(Object blocker) {
132	U.putReference(Thread.currentThread(), PARKBLOCKER, blocker);
133	}
134
135	/**
136	* Makes available the permit for the given thread, if it
137	* was not already available. If the thread was blocked on
138	* {@code park} then it will unblock. Otherwise, its next call
139	* to {@code park} is guaranteed not to block. This operation
140	* is not guaranteed to have any effect at all if the given
141	* thread has not been started.
142	*
143	* @param thread the thread to unpark, or {@code null}, in which case
144	* this operation has no effect
145	*/
146	public static void unpark(Thread thread) {
147	if (thread != null)
148	U.unpark(thread);
149	}
150
151	/**
152	* Disables the current thread for thread scheduling purposes unless the
153	* permit is available.
154	*
155	* <p>If the permit is available then it is consumed and the call returns
156	* immediately; otherwise
157	* the current thread becomes disabled for thread scheduling
158	* purposes and lies dormant until one of three things happens:
159	*
160	* <ul>
161	* <li>Some other thread invokes {@link #unpark unpark} with the
162	* current thread as the target; or
163	*
164	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
165	* the current thread; or
166	*
167	* <li>The call spuriously (that is, for no reason) returns.
168	* </ul>
169	*
170	* <p>This method does <em>not</em> report which of these caused the
171	* method to return. Callers should re-check the conditions which caused
172	* the thread to park in the first place. Callers may also determine,
173	* for example, the interrupt status of the thread upon return.
174	*
175	* @param blocker the synchronization object responsible for this
176	* thread parking
177	* @since 1.6
178	*/
179	public static void park(Object blocker) {
180	Thread t = Thread.currentThread();
181	setBlocker(t, blocker);
182	U.park(false, 0L);
183	setBlocker(t, null);
184	}
185
186	/**
187	* Disables the current thread for thread scheduling purposes, for up to
188	* the specified waiting time, unless the permit is available.
189	*
190	* <p>If the specified waiting time is zero or negative, the
191	* method does nothing. Otherwise, if the permit is available then
192	* it is consumed and the call returns immediately; otherwise the
193	* current thread becomes disabled for thread scheduling purposes
194	* and lies dormant until one of four things happens:
195	*
196	* <ul>
197	* <li>Some other thread invokes {@link #unpark unpark} with the
198	* current thread as the target; or
199	*
200	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
201	* the current thread; or
202	*
203	* <li>The specified waiting time elapses; or
204	*
205	* <li>The call spuriously (that is, for no reason) returns.
206	* </ul>
207	*
208	* <p>This method does <em>not</em> report which of these caused the
209	* method to return. Callers should re-check the conditions which caused
210	* the thread to park in the first place. Callers may also determine,
211	* for example, the interrupt status of the thread, or the elapsed time
212	* upon return.
213	*
214	* @param blocker the synchronization object responsible for this
215	* thread parking
216	* @param nanos the maximum number of nanoseconds to wait
217	* @since 1.6
218	*/
219	public static void parkNanos(Object blocker, long nanos) {
220	if (nanos > 0) {
221	Thread t = Thread.currentThread();
222	setBlocker(t, blocker);
223	U.park(false, nanos);
224	setBlocker(t, null);
225	}
226	}
227
228	/**
229	* Disables the current thread for thread scheduling purposes, until
230	* the specified deadline, unless the permit is available.
231	*
232	* <p>If the permit is available then it is consumed and the call
233	* returns immediately; otherwise the current thread becomes disabled
234	* for thread scheduling purposes and lies dormant until one of four
235	* things happens:
236	*
237	* <ul>
238	* <li>Some other thread invokes {@link #unpark unpark} with the
239	* current thread as the target; or
240	*
241	* <li>Some other thread {@linkplain Thread#interrupt interrupts} the
242	* current thread; or
243	*
244	* <li>The specified deadline passes; or
245	*
246	* <li>The call spuriously (that is, for no reason) returns.
247	* </ul>
248	*
249	* <p>This method does <em>not</em> report which of these caused the
250	* method to return. Callers should re-check the conditions which caused
251	* the thread to park in the first place. Callers may also determine,
252	* for example, the interrupt status of the thread, or the current time
253	* upon return.
254	*
255	* @param blocker the synchronization object responsible for this
256	* thread parking
257	* @param deadline the absolute time, in milliseconds from the Epoch,
258	* to wait until
259	* @since 1.6
260	*/
261	public static void parkUntil(Object blocker, long deadline) {
262	Thread t = Thread.currentThread();
263	setBlocker(t, blocker);
264	U.park(true, deadline);
265	setBlocker(t, null);
266	}
267
268	/**
269	* Returns the blocker object supplied to the most recent
270	* invocation of a park method that has not yet unblocked, or null
271	* if not blocked. The value returned is just a momentary
272	* snapshot -- the thread may have since unblocked or blocked on a
273	* different blocker object.
274	*
275	* @param t the thread
276	* @return the blocker
277	* @throws NullPointerException if argument is null
278	* @since 1.6
279	*/
280	public static Object getBlocker(Thread t) {
281	if (t == null)
282	throw new NullPointerException();
283	return U.getReference(t, PARKBLOCKER);
284	}
285
286	/**
287	* Disables the current thread for thread scheduling purposes unless the
288	* permit is available.
289	*
290	* <p>If the permit is available then it is consumed and the call
291	* returns immediately; otherwise the current thread becomes disabled
292	* for thread scheduling purposes and lies dormant until one of three
293	* things happens:
294	*
295	* <ul>
296	*
297	* <li>Some other thread invokes {@link #unpark unpark} with the
298	* current thread as the target; or
299	*
300	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
301	* the current thread; or
302	*
303	* <li>The call spuriously (that is, for no reason) returns.
304	* </ul>
305	*
306	* <p>This method does <em>not</em> report which of these caused the
307	* method to return. Callers should re-check the conditions which caused
308	* the thread to park in the first place. Callers may also determine,
309	* for example, the interrupt status of the thread upon return.
310	*/
311	public static void park() {
312	U.park(false, 0L);
313	}
314
315	/**
316	* Disables the current thread for thread scheduling purposes, for up to
317	* the specified waiting time, unless the permit is available.
318	*
319	* <p>If the specified waiting time is zero or negative, the
320	* method does nothing. Otherwise, if the permit is available then
321	* it is consumed and the call returns immediately; otherwise the
322	* current thread becomes disabled for thread scheduling purposes
323	* and lies dormant until one of four things happens:
324	*
325	* <ul>
326	* <li>Some other thread invokes {@link #unpark unpark} with the
327	* current thread as the target; or
328	*
329	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
330	* the current thread; or
331	*
332	* <li>The specified waiting time elapses; or
333	*
334	* <li>The call spuriously (that is, for no reason) returns.
335	* </ul>
336	*
337	* <p>This method does <em>not</em> report which of these caused the
338	* method to return. Callers should re-check the conditions which caused
339	* the thread to park in the first place. Callers may also determine,
340	* for example, the interrupt status of the thread, or the elapsed time
341	* upon return.
342	*
343	* @param nanos the maximum number of nanoseconds to wait
344	*/
345	public static void parkNanos(long nanos) {
346	if (nanos > 0)
347	U.park(false, nanos);
348	}
349
350	/**
351	* Disables the current thread for thread scheduling purposes, until
352	* the specified deadline, unless the permit is available.
353	*
354	* <p>If the permit is available then it is consumed and the call
355	* returns immediately; otherwise the current thread becomes disabled
356	* for thread scheduling purposes and lies dormant until one of four
357	* things happens:
358	*
359	* <ul>
360	* <li>Some other thread invokes {@link #unpark unpark} with the
361	* current thread as the target; or
362	*
363	* <li>Some other thread {@linkplain Thread#interrupt interrupts}
364	* the current thread; or
365	*
366	* <li>The specified deadline passes; or
367	*
368	* <li>The call spuriously (that is, for no reason) returns.
369	* </ul>
370	*
371	* <p>This method does <em>not</em> report which of these caused the
372	* method to return. Callers should re-check the conditions which caused
373	* the thread to park in the first place. Callers may also determine,
374	* for example, the interrupt status of the thread, or the current time
375	* upon return.
376	*
377	* @param deadline the absolute time, in milliseconds from the Epoch,
378	* to wait until
379	*/
380	public static void parkUntil(long deadline) {
381	U.park(true, deadline);
382	}
383
384	/**
385	* Returns the thread id for the given thread. We must access
386	* this directly rather than via method Thread.getId() because
387	* getId() has been known to be overridden in ways that do not
388	* preserve unique mappings.
389	*/
390	static final long getThreadId(Thread thread) {
391	return U.getLong(thread, TID);
392	}
393
394	// Hotspot implementation via intrinsics API
395	private static final Unsafe U = Unsafe.getUnsafe();
396	private static final long PARKBLOCKER
397	= U.objectFieldOffset(Thread.class, "parkBlocker");
398	private static final long TID
399	= U.objectFieldOffset(Thread.class, "tid");
400
401	}