1 /*
2 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
3 *
4 * This code is free software; you can redistribute it and/or modify it
5 * under the terms of the GNU General Public License version 2 only, as
6 * published by the Free Software Foundation. Oracle designates this
7 * particular file as subject to the "Classpath" exception as provided
8 * by Oracle in the LICENSE file that accompanied this code.
9 *
10 * This code is distributed in the hope that it will be useful, but WITHOUT
11 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
12 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
13 * version 2 for more details (a copy is included in the LICENSE file that
14 * accompanied this code).
15 *
16 * You should have received a copy of the GNU General Public License version
17 * 2 along with this work; if not, write to the Free Software Foundation,
18 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
19 *
20 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
21 * or visit www.oracle.com if you need additional information or have any
22 * questions.
23 */
24
25 /*
26 * This file is available under and governed by the GNU General Public
27 * License version 2 only, as published by the Free Software Foundation.
28 * However, the following notice accompanied the original version of this
29 * file:
30 *
31 * Written by Doug Lea with assistance from members of JCP JSR-166
32 * Expert Group and released to the public domain, as explained at
33 * http://creativecommons.org/publicdomain/zero/1.0/
34 */
35
36 package java.util.concurrent.locks;
37
38 import jdk.internal.misc.Unsafe;
39
40 /**
41 * Basic thread blocking primitives for creating locks and other
42 * synchronization classes.
43 *
44 * <p>This class associates, with each thread that uses it, a permit
45 * (in the sense of the {@link java.util.concurrent.Semaphore
46 * Semaphore} class). A call to {@code park} will return immediately
47 * if the permit is available, consuming it in the process; otherwise
48 * it <em>may</em> block. A call to {@code unpark} makes the permit
49 * available, if it was not already available. (Unlike with Semaphores
50 * though, permits do not accumulate. There is at most one.)
51 * Reliable usage requires the use of volatile (or atomic) variables
52 * to control when to park or unpark. Orderings of calls to these
53 * methods are maintained with respect to volatile variable accesses,
54 * but not necessarily non-volatile variable accesses.
55 *
56 * <p>Methods {@code park} and {@code unpark} provide efficient
57 * means of blocking and unblocking threads that do not encounter the
58 * problems that cause the deprecated methods {@code Thread.suspend}
59 * and {@code Thread.resume} to be unusable for such purposes: Races
60 * between one thread invoking {@code park} and another thread trying
61 * to {@code unpark} it will preserve liveness, due to the
62 * permit. Additionally, {@code park} will return if the caller's
63 * thread was interrupted, and timeout versions are supported. The
64 * {@code park} method may also return at any other time, for "no
65 * reason", so in general must be invoked within a loop that rechecks
66 * conditions upon return. In this sense {@code park} serves as an
67 * optimization of a "busy wait" that does not waste as much time
68 * spinning, but must be paired with an {@code unpark} to be
69 * effective.
70 *
71 * <p>The three forms of {@code park} each also support a
72 * {@code blocker} object parameter. This object is recorded while
73 * the thread is blocked to permit monitoring and diagnostic tools to
74 * identify the reasons that threads are blocked. (Such tools may
75 * access blockers using method {@link #getBlocker(Thread)}.)
76 * The use of these forms rather than the original forms without this
77 * parameter is strongly encouraged. The normal argument to supply as
78 * a {@code blocker} within a lock implementation is {@code this}.
79 *
80 * <p>These methods are designed to be used as tools for creating
81 * higher-level synchronization utilities, and are not in themselves
82 * useful for most concurrency control applications. The {@code park}
83 * method is designed for use only in constructions of the form:
84 *
85 * <pre> {@code
86 * while (!canProceed()) {
87 * // ensure request to unpark is visible to other threads
88 * ...
89 * LockSupport.park(this);
90 * }}</pre>
91 *
92 * where no actions by the thread publishing a request to unpark,
93 * prior to the call to {@code park}, entail locking or blocking.
94 * Because only one permit is associated with each thread, any
95 * intermediary uses of {@code park}, including implicitly via class
96 * loading, could lead to an unresponsive thread (a "lost unpark").
97 *
98 * <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out
99 * non-reentrant lock class:
100 * <pre> {@code
101 * class FIFOMutex {
102 * private final AtomicBoolean locked = new AtomicBoolean(false);
103 * private final Queue<Thread> waiters
104 * = new ConcurrentLinkedQueue<>();
105 *
106 * public void lock() {
107 * boolean wasInterrupted = false;
108 * // publish current thread for unparkers
109 * waiters.add(Thread.currentThread());
110 *
111 * // Block while not first in queue or cannot acquire lock
112 * while (waiters.peek() != Thread.currentThread() ||
113 * !locked.compareAndSet(false, true)) {
114 * LockSupport.park(this);
115 * // ignore interrupts while waiting
116 * if (Thread.interrupted())
117 * wasInterrupted = true;
118 * }
119 *
120 * waiters.remove();
121 * // ensure correct interrupt status on return
122 * if (wasInterrupted)
123 * Thread.currentThread().interrupt();
124 * }
125 *
126 * public void unlock() {
127 * locked.set(false);
128 * LockSupport.unpark(waiters.peek());
129 * }
130 *
131 * static {
132 * // Reduce the risk of "lost unpark" due to classloading
133 * Class<?> ensureLoaded = LockSupport.class;
134 * }
135 * }}</pre>
136 *
137 * @since 1.5
138 */
139 public class LockSupport {
140 private LockSupport() {} // Cannot be instantiated.
141
142 private static void setBlocker(Thread t, Object arg) {
143 U.putReferenceOpaque(t, PARKBLOCKER, arg);
144 }
145
146 /**
147 * Sets the object to be returned by invocations of {@link
148 * #getBlocker getBlocker} for the current thread. This method may
149 * be used before invoking the no-argument version of {@link
150 * LockSupport#park() park()} from non-public objects, allowing
151 * more helpful diagnostics, or retaining compatibility with
152 * previous implementations of blocking methods. Previous values
153 * of the blocker are not automatically restored after blocking.
154 * To obtain the effects of {@code park(b}}, use {@code
155 * setCurrentBlocker(b); park(); setCurrentBlocker(null);}
156 *
157 * @param blocker the blocker object
158 * @since 14
159 */
160 public static void setCurrentBlocker(Object blocker) {
161 U.putReferenceOpaque(Thread.currentThread(), PARKBLOCKER, blocker);
162 }
163
164 /**
165 * Makes available the permit for the given thread, if it
166 * was not already available. If the thread was blocked on
167 * {@code park} then it will unblock. Otherwise, its next call
168 * to {@code park} is guaranteed not to block. This operation
169 * is not guaranteed to have any effect at all if the given
170 * thread has not been started.
171 *
172 * @param thread the thread to unpark, or {@code null}, in which case
173 * this operation has no effect
174 */
175 public static void unpark(Thread thread) {
176 if (thread != null)
177 U.unpark(thread);
178 }
179
180 /**
181 * Disables the current thread for thread scheduling purposes unless the
182 * permit is available.
183 *
184 * <p>If the permit is available then it is consumed and the call returns
185 * immediately; otherwise
186 * the current thread becomes disabled for thread scheduling
187 * purposes and lies dormant until one of three things happens:
188 *
189 * <ul>
190 * <li>Some other thread invokes {@link #unpark unpark} with the
191 * current thread as the target; or
192 *
193 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
194 * the current thread; or
195 *
196 * <li>The call spuriously (that is, for no reason) returns.
197 * </ul>
198 *
199 * <p>This method does <em>not</em> report which of these caused the
200 * method to return. Callers should re-check the conditions which caused
201 * the thread to park in the first place. Callers may also determine,
202 * for example, the interrupt status of the thread upon return.
203 *
204 * @param blocker the synchronization object responsible for this
205 * thread parking
206 * @since 1.6
207 */
208 public static void park(Object blocker) {
209 Thread t = Thread.currentThread();
210 setBlocker(t, blocker);
211 U.park(false, 0L);
212 setBlocker(t, null);
213 }
214
215 /**
216 * Disables the current thread for thread scheduling purposes, for up to
217 * the specified waiting time, unless the permit is available.
218 *
219 * <p>If the specified waiting time is zero or negative, the
220 * method does nothing. Otherwise, if the permit is available then
221 * it is consumed and the call returns immediately; otherwise the
222 * current thread becomes disabled for thread scheduling purposes
223 * and lies dormant until one of four things happens:
224 *
225 * <ul>
226 * <li>Some other thread invokes {@link #unpark unpark} with the
227 * current thread as the target; or
228 *
229 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
230 * the current thread; or
231 *
232 * <li>The specified waiting time elapses; or
233 *
234 * <li>The call spuriously (that is, for no reason) returns.
235 * </ul>
236 *
237 * <p>This method does <em>not</em> report which of these caused the
238 * method to return. Callers should re-check the conditions which caused
239 * the thread to park in the first place. Callers may also determine,
240 * for example, the interrupt status of the thread, or the elapsed time
241 * upon return.
242 *
243 * @param blocker the synchronization object responsible for this
244 * thread parking
245 * @param nanos the maximum number of nanoseconds to wait
246 * @since 1.6
247 */
248 public static void parkNanos(Object blocker, long nanos) {
249 if (nanos > 0) {
250 Thread t = Thread.currentThread();
251 setBlocker(t, blocker);
252 U.park(false, nanos);
253 setBlocker(t, null);
254 }
255 }
256
257 /**
258 * Disables the current thread for thread scheduling purposes, until
259 * the specified deadline, unless the permit is available.
260 *
261 * <p>If the permit is available then it is consumed and the call
262 * returns immediately; otherwise the current thread becomes disabled
263 * for thread scheduling purposes and lies dormant until one of four
264 * things happens:
265 *
266 * <ul>
267 * <li>Some other thread invokes {@link #unpark unpark} with the
268 * current thread as the target; or
269 *
270 * <li>Some other thread {@linkplain Thread#interrupt interrupts} the
271 * current thread; or
272 *
273 * <li>The specified deadline passes; or
274 *
275 * <li>The call spuriously (that is, for no reason) returns.
276 * </ul>
277 *
278 * <p>This method does <em>not</em> report which of these caused the
279 * method to return. Callers should re-check the conditions which caused
280 * the thread to park in the first place. Callers may also determine,
281 * for example, the interrupt status of the thread, or the current time
282 * upon return.
283 *
284 * @param blocker the synchronization object responsible for this
285 * thread parking
286 * @param deadline the absolute time, in milliseconds from the Epoch,
287 * to wait until
288 * @since 1.6
289 */
290 public static void parkUntil(Object blocker, long deadline) {
291 Thread t = Thread.currentThread();
292 setBlocker(t, blocker);
293 U.park(true, deadline);
294 setBlocker(t, null);
295 }
296
297 /**
298 * Returns the blocker object supplied to the most recent
299 * invocation of a park method that has not yet unblocked, or null
300 * if not blocked. The value returned is just a momentary
301 * snapshot -- the thread may have since unblocked or blocked on a
302 * different blocker object.
303 *
304 * @param t the thread
305 * @return the blocker
306 * @throws NullPointerException if argument is null
307 * @since 1.6
308 */
309 public static Object getBlocker(Thread t) {
310 if (t == null)
311 throw new NullPointerException();
312 return U.getReferenceOpaque(t, PARKBLOCKER);
313 }
314
315 /**
316 * Disables the current thread for thread scheduling purposes unless the
317 * permit is available.
318 *
319 * <p>If the permit is available then it is consumed and the call
320 * returns immediately; otherwise the current thread becomes disabled
321 * for thread scheduling purposes and lies dormant until one of three
322 * things happens:
323 *
324 * <ul>
325 *
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 call spuriously (that is, for no reason) returns.
333 * </ul>
334 *
335 * <p>This method does <em>not</em> report which of these caused the
336 * method to return. Callers should re-check the conditions which caused
337 * the thread to park in the first place. Callers may also determine,
338 * for example, the interrupt status of the thread upon return.
339 */
340 public static void park() {
341 U.park(false, 0L);
342 }
343
344 /**
345 * Disables the current thread for thread scheduling purposes, for up to
346 * the specified waiting time, unless the permit is available.
347 *
348 * <p>If the specified waiting time is zero or negative, the
349 * method does nothing. Otherwise, if the permit is available then
350 * it is consumed and the call returns immediately; otherwise the
351 * current thread becomes disabled for thread scheduling purposes
352 * and lies dormant until one of four things happens:
353 *
354 * <ul>
355 * <li>Some other thread invokes {@link #unpark unpark} with the
356 * current thread as the target; or
357 *
358 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
359 * the current thread; or
360 *
361 * <li>The specified waiting time elapses; or
362 *
363 * <li>The call spuriously (that is, for no reason) returns.
364 * </ul>
365 *
366 * <p>This method does <em>not</em> report which of these caused the
367 * method to return. Callers should re-check the conditions which caused
368 * the thread to park in the first place. Callers may also determine,
369 * for example, the interrupt status of the thread, or the elapsed time
370 * upon return.
371 *
372 * @param nanos the maximum number of nanoseconds to wait
373 */
374 public static void parkNanos(long nanos) {
375 if (nanos > 0)
376 U.park(false, nanos);
377 }
378
379 /**
380 * Disables the current thread for thread scheduling purposes, until
381 * the specified deadline, unless the permit is available.
382 *
383 * <p>If the permit is available then it is consumed and the call
384 * returns immediately; otherwise the current thread becomes disabled
385 * for thread scheduling purposes and lies dormant until one of four
386 * things happens:
387 *
388 * <ul>
389 * <li>Some other thread invokes {@link #unpark unpark} with the
390 * current thread as the target; or
391 *
392 * <li>Some other thread {@linkplain Thread#interrupt interrupts}
393 * the current thread; or
394 *
395 * <li>The specified deadline passes; or
396 *
397 * <li>The call spuriously (that is, for no reason) returns.
398 * </ul>
399 *
400 * <p>This method does <em>not</em> report which of these caused the
401 * method to return. Callers should re-check the conditions which caused
402 * the thread to park in the first place. Callers may also determine,
403 * for example, the interrupt status of the thread, or the current time
404 * upon return.
405 *
406 * @param deadline the absolute time, in milliseconds from the Epoch,
407 * to wait until
408 */
409 public static void parkUntil(long deadline) {
410 U.park(true, deadline);
411 }
412
413 /**
414 * Returns the thread id for the given thread. We must access
415 * this directly rather than via method Thread.getId() because
416 * getId() has been known to be overridden in ways that do not
417 * preserve unique mappings.
418 */
419 static final long getThreadId(Thread thread) {
420 return U.getLong(thread, TID);
421 }
422
423 // Hotspot implementation via intrinsics API
424 private static final Unsafe U = Unsafe.getUnsafe();
425 private static final long PARKBLOCKER
426 = U.objectFieldOffset(Thread.class, "parkBlocker");
427 private static final long TID
428 = U.objectFieldOffset(Thread.class, "tid");
429
430 }