J. Duke | 319a3b9 | 2007-12-01 00:00:00 +0000 | [diff] [blame^] | 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. Sun designates this |
| 7 | * particular file as subject to the "Classpath" exception as provided |
| 8 | * by Sun 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 Sun Microsystems, Inc., 4150 Network Circle, Santa Clara, |
| 21 | * CA 95054 USA or visit www.sun.com if you need additional information or |
| 22 | * have any 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/licenses/publicdomain |
| 34 | */ |
| 35 | |
| 36 | package java.util.concurrent.locks; |
| 37 | import java.util.concurrent.*; |
| 38 | import sun.misc.Unsafe; |
| 39 | |
| 40 | |
| 41 | /** |
| 42 | * Basic thread blocking primitives for creating locks and other |
| 43 | * synchronization classes. |
| 44 | * |
| 45 | * <p>This class associates, with each thread that uses it, a permit |
| 46 | * (in the sense of the {@link java.util.concurrent.Semaphore |
| 47 | * Semaphore} class). A call to {@code park} will return immediately |
| 48 | * if the permit is available, consuming it in the process; otherwise |
| 49 | * it <em>may</em> block. A call to {@code unpark} makes the permit |
| 50 | * available, if it was not already available. (Unlike with Semaphores |
| 51 | * though, permits do not accumulate. There is at most one.) |
| 52 | * |
| 53 | * <p>Methods {@code park} and {@code unpark} provide efficient |
| 54 | * means of blocking and unblocking threads that do not encounter the |
| 55 | * problems that cause the deprecated methods {@code Thread.suspend} |
| 56 | * and {@code Thread.resume} to be unusable for such purposes: Races |
| 57 | * between one thread invoking {@code park} and another thread trying |
| 58 | * to {@code unpark} it will preserve liveness, due to the |
| 59 | * permit. Additionally, {@code park} will return if the caller's |
| 60 | * thread was interrupted, and timeout versions are supported. The |
| 61 | * {@code park} method may also return at any other time, for "no |
| 62 | * reason", so in general must be invoked within a loop that rechecks |
| 63 | * conditions upon return. In this sense {@code park} serves as an |
| 64 | * optimization of a "busy wait" that does not waste as much time |
| 65 | * spinning, but must be paired with an {@code unpark} to be |
| 66 | * effective. |
| 67 | * |
| 68 | * <p>The three forms of {@code park} each also support a |
| 69 | * {@code blocker} object parameter. This object is recorded while |
| 70 | * the thread is blocked to permit monitoring and diagnostic tools to |
| 71 | * identify the reasons that threads are blocked. (Such tools may |
| 72 | * access blockers using method {@link #getBlocker}.) The use of these |
| 73 | * forms rather than the original forms without this parameter is |
| 74 | * strongly encouraged. The normal argument to supply as a |
| 75 | * {@code blocker} within a lock implementation is {@code this}. |
| 76 | * |
| 77 | * <p>These methods are designed to be used as tools for creating |
| 78 | * higher-level synchronization utilities, and are not in themselves |
| 79 | * useful for most concurrency control applications. The {@code park} |
| 80 | * method is designed for use only in constructions of the form: |
| 81 | * <pre>while (!canProceed()) { ... LockSupport.park(this); }</pre> |
| 82 | * where neither {@code canProceed} nor any other actions prior to the |
| 83 | * call to {@code park} entail locking or blocking. Because only one |
| 84 | * permit is associated with each thread, any intermediary uses of |
| 85 | * {@code park} could interfere with its intended effects. |
| 86 | * |
| 87 | * <p><b>Sample Usage.</b> Here is a sketch of a first-in-first-out |
| 88 | * non-reentrant lock class: |
| 89 | * <pre>{@code |
| 90 | * class FIFOMutex { |
| 91 | * private final AtomicBoolean locked = new AtomicBoolean(false); |
| 92 | * private final Queue<Thread> waiters |
| 93 | * = new ConcurrentLinkedQueue<Thread>(); |
| 94 | * |
| 95 | * public void lock() { |
| 96 | * boolean wasInterrupted = false; |
| 97 | * Thread current = Thread.currentThread(); |
| 98 | * waiters.add(current); |
| 99 | * |
| 100 | * // Block while not first in queue or cannot acquire lock |
| 101 | * while (waiters.peek() != current || |
| 102 | * !locked.compareAndSet(false, true)) { |
| 103 | * LockSupport.park(this); |
| 104 | * if (Thread.interrupted()) // ignore interrupts while waiting |
| 105 | * wasInterrupted = true; |
| 106 | * } |
| 107 | * |
| 108 | * waiters.remove(); |
| 109 | * if (wasInterrupted) // reassert interrupt status on exit |
| 110 | * current.interrupt(); |
| 111 | * } |
| 112 | * |
| 113 | * public void unlock() { |
| 114 | * locked.set(false); |
| 115 | * LockSupport.unpark(waiters.peek()); |
| 116 | * } |
| 117 | * }}</pre> |
| 118 | */ |
| 119 | |
| 120 | public class LockSupport { |
| 121 | private LockSupport() {} // Cannot be instantiated. |
| 122 | |
| 123 | // Hotspot implementation via intrinsics API |
| 124 | private static final Unsafe unsafe = Unsafe.getUnsafe(); |
| 125 | private static final long parkBlockerOffset; |
| 126 | |
| 127 | static { |
| 128 | try { |
| 129 | parkBlockerOffset = unsafe.objectFieldOffset |
| 130 | (java.lang.Thread.class.getDeclaredField("parkBlocker")); |
| 131 | } catch (Exception ex) { throw new Error(ex); } |
| 132 | } |
| 133 | |
| 134 | private static void setBlocker(Thread t, Object arg) { |
| 135 | // Even though volatile, hotspot doesn't need a write barrier here. |
| 136 | unsafe.putObject(t, parkBlockerOffset, arg); |
| 137 | } |
| 138 | |
| 139 | /** |
| 140 | * Makes available the permit for the given thread, if it |
| 141 | * was not already available. If the thread was blocked on |
| 142 | * {@code park} then it will unblock. Otherwise, its next call |
| 143 | * to {@code park} is guaranteed not to block. This operation |
| 144 | * is not guaranteed to have any effect at all if the given |
| 145 | * thread has not been started. |
| 146 | * |
| 147 | * @param thread the thread to unpark, or {@code null}, in which case |
| 148 | * this operation has no effect |
| 149 | */ |
| 150 | public static void unpark(Thread thread) { |
| 151 | if (thread != null) |
| 152 | unsafe.unpark(thread); |
| 153 | } |
| 154 | |
| 155 | /** |
| 156 | * Disables the current thread for thread scheduling purposes unless the |
| 157 | * permit is available. |
| 158 | * |
| 159 | * <p>If the permit is available then it is consumed and the call returns |
| 160 | * immediately; otherwise |
| 161 | * the current thread becomes disabled for thread scheduling |
| 162 | * purposes and lies dormant until one of three things happens: |
| 163 | * |
| 164 | * <ul> |
| 165 | * <li>Some other thread invokes {@link #unpark unpark} with the |
| 166 | * current thread as the target; or |
| 167 | * |
| 168 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| 169 | * the current thread; or |
| 170 | * |
| 171 | * <li>The call spuriously (that is, for no reason) returns. |
| 172 | * </ul> |
| 173 | * |
| 174 | * <p>This method does <em>not</em> report which of these caused the |
| 175 | * method to return. Callers should re-check the conditions which caused |
| 176 | * the thread to park in the first place. Callers may also determine, |
| 177 | * for example, the interrupt status of the thread upon return. |
| 178 | * |
| 179 | * @param blocker the synchronization object responsible for this |
| 180 | * thread parking |
| 181 | * @since 1.6 |
| 182 | */ |
| 183 | public static void park(Object blocker) { |
| 184 | Thread t = Thread.currentThread(); |
| 185 | setBlocker(t, blocker); |
| 186 | unsafe.park(false, 0L); |
| 187 | setBlocker(t, null); |
| 188 | } |
| 189 | |
| 190 | /** |
| 191 | * Disables the current thread for thread scheduling purposes, for up to |
| 192 | * the specified waiting time, unless the permit is available. |
| 193 | * |
| 194 | * <p>If the permit is available then it is consumed and the call |
| 195 | * returns immediately; otherwise the current thread becomes disabled |
| 196 | * for thread scheduling purposes and lies dormant until one of four |
| 197 | * things happens: |
| 198 | * |
| 199 | * <ul> |
| 200 | * <li>Some other thread invokes {@link #unpark unpark} with the |
| 201 | * current thread as the target; or |
| 202 | * |
| 203 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} the current |
| 204 | * thread; or |
| 205 | * |
| 206 | * <li>The specified waiting time elapses; or |
| 207 | * |
| 208 | * <li>The call spuriously (that is, for no reason) returns. |
| 209 | * </ul> |
| 210 | * |
| 211 | * <p>This method does <em>not</em> report which of these caused the |
| 212 | * method to return. Callers should re-check the conditions which caused |
| 213 | * the thread to park in the first place. Callers may also determine, |
| 214 | * for example, the interrupt status of the thread, or the elapsed time |
| 215 | * upon return. |
| 216 | * |
| 217 | * @param blocker the synchronization object responsible for this |
| 218 | * thread parking |
| 219 | * @param nanos the maximum number of nanoseconds to wait |
| 220 | * @since 1.6 |
| 221 | */ |
| 222 | public static void parkNanos(Object blocker, long nanos) { |
| 223 | if (nanos > 0) { |
| 224 | Thread t = Thread.currentThread(); |
| 225 | setBlocker(t, blocker); |
| 226 | unsafe.park(false, nanos); |
| 227 | setBlocker(t, null); |
| 228 | } |
| 229 | } |
| 230 | |
| 231 | /** |
| 232 | * Disables the current thread for thread scheduling purposes, until |
| 233 | * the specified deadline, unless the permit is available. |
| 234 | * |
| 235 | * <p>If the permit is available then it is consumed and the call |
| 236 | * returns immediately; otherwise the current thread becomes disabled |
| 237 | * for thread scheduling purposes and lies dormant until one of four |
| 238 | * things happens: |
| 239 | * |
| 240 | * <ul> |
| 241 | * <li>Some other thread invokes {@link #unpark unpark} with the |
| 242 | * current thread as the target; or |
| 243 | * |
| 244 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} the |
| 245 | * current thread; or |
| 246 | * |
| 247 | * <li>The specified deadline passes; or |
| 248 | * |
| 249 | * <li>The call spuriously (that is, for no reason) returns. |
| 250 | * </ul> |
| 251 | * |
| 252 | * <p>This method does <em>not</em> report which of these caused the |
| 253 | * method to return. Callers should re-check the conditions which caused |
| 254 | * the thread to park in the first place. Callers may also determine, |
| 255 | * for example, the interrupt status of the thread, or the current time |
| 256 | * upon return. |
| 257 | * |
| 258 | * @param blocker the synchronization object responsible for this |
| 259 | * thread parking |
| 260 | * @param deadline the absolute time, in milliseconds from the Epoch, |
| 261 | * to wait until |
| 262 | * @since 1.6 |
| 263 | */ |
| 264 | public static void parkUntil(Object blocker, long deadline) { |
| 265 | Thread t = Thread.currentThread(); |
| 266 | setBlocker(t, blocker); |
| 267 | unsafe.park(true, deadline); |
| 268 | setBlocker(t, null); |
| 269 | } |
| 270 | |
| 271 | /** |
| 272 | * Returns the blocker object supplied to the most recent |
| 273 | * invocation of a park method that has not yet unblocked, or null |
| 274 | * if not blocked. The value returned is just a momentary |
| 275 | * snapshot -- the thread may have since unblocked or blocked on a |
| 276 | * different blocker object. |
| 277 | * |
| 278 | * @return the blocker |
| 279 | * @since 1.6 |
| 280 | */ |
| 281 | public static Object getBlocker(Thread t) { |
| 282 | return unsafe.getObjectVolatile(t, parkBlockerOffset); |
| 283 | } |
| 284 | |
| 285 | /** |
| 286 | * Disables the current thread for thread scheduling purposes unless the |
| 287 | * permit is available. |
| 288 | * |
| 289 | * <p>If the permit is available then it is consumed and the call |
| 290 | * returns immediately; otherwise the current thread becomes disabled |
| 291 | * for thread scheduling purposes and lies dormant until one of three |
| 292 | * things happens: |
| 293 | * |
| 294 | * <ul> |
| 295 | * |
| 296 | * <li>Some other thread invokes {@link #unpark unpark} with the |
| 297 | * current thread as the target; or |
| 298 | * |
| 299 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| 300 | * the current thread; or |
| 301 | * |
| 302 | * <li>The call spuriously (that is, for no reason) returns. |
| 303 | * </ul> |
| 304 | * |
| 305 | * <p>This method does <em>not</em> report which of these caused the |
| 306 | * method to return. Callers should re-check the conditions which caused |
| 307 | * the thread to park in the first place. Callers may also determine, |
| 308 | * for example, the interrupt status of the thread upon return. |
| 309 | */ |
| 310 | public static void park() { |
| 311 | unsafe.park(false, 0L); |
| 312 | } |
| 313 | |
| 314 | /** |
| 315 | * Disables the current thread for thread scheduling purposes, for up to |
| 316 | * the specified waiting time, unless the permit is available. |
| 317 | * |
| 318 | * <p>If the permit is available then it is consumed and the call |
| 319 | * returns immediately; otherwise the current thread becomes disabled |
| 320 | * for thread scheduling purposes and lies dormant until one of four |
| 321 | * things happens: |
| 322 | * |
| 323 | * <ul> |
| 324 | * <li>Some other thread invokes {@link #unpark unpark} with the |
| 325 | * current thread as the target; or |
| 326 | * |
| 327 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| 328 | * the current thread; or |
| 329 | * |
| 330 | * <li>The specified waiting time elapses; 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, or the elapsed time |
| 339 | * upon return. |
| 340 | * |
| 341 | * @param nanos the maximum number of nanoseconds to wait |
| 342 | */ |
| 343 | public static void parkNanos(long nanos) { |
| 344 | if (nanos > 0) |
| 345 | unsafe.park(false, nanos); |
| 346 | } |
| 347 | |
| 348 | /** |
| 349 | * Disables the current thread for thread scheduling purposes, until |
| 350 | * the specified deadline, unless the permit is available. |
| 351 | * |
| 352 | * <p>If the permit is available then it is consumed and the call |
| 353 | * returns immediately; otherwise the current thread becomes disabled |
| 354 | * for thread scheduling purposes and lies dormant until one of four |
| 355 | * things happens: |
| 356 | * |
| 357 | * <ul> |
| 358 | * <li>Some other thread invokes {@link #unpark unpark} with the |
| 359 | * current thread as the target; or |
| 360 | * |
| 361 | * <li>Some other thread {@linkplain Thread#interrupt interrupts} |
| 362 | * the current thread; or |
| 363 | * |
| 364 | * <li>The specified deadline passes; or |
| 365 | * |
| 366 | * <li>The call spuriously (that is, for no reason) returns. |
| 367 | * </ul> |
| 368 | * |
| 369 | * <p>This method does <em>not</em> report which of these caused the |
| 370 | * method to return. Callers should re-check the conditions which caused |
| 371 | * the thread to park in the first place. Callers may also determine, |
| 372 | * for example, the interrupt status of the thread, or the current time |
| 373 | * upon return. |
| 374 | * |
| 375 | * @param deadline the absolute time, in milliseconds from the Epoch, |
| 376 | * to wait until |
| 377 | */ |
| 378 | public static void parkUntil(long deadline) { |
| 379 | unsafe.park(true, deadline); |
| 380 | } |
| 381 | } |