1 /*
2 * Copyright (c) 2016, Oracle and/or its affiliates. All rights reserved.
3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
4 *
5 * This code is free software; you can redistribute it and/or modify it
6 * under the terms of the GNU General Public License version 2 only, as
7 * published by the Free Software Foundation. Oracle designates this
8 * particular file as subject to the "Classpath" exception as provided
9 * by Oracle in the LICENSE file that accompanied this code.
10 *
11 * This code is distributed in the hope that it will be useful, but WITHOUT
12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
14 * version 2 for more details (a copy is included in the LICENSE file that
15 * accompanied this code).
16 *
17 * You should have received a copy of the GNU General Public License version
18 * 2 along with this work; if not, write to the Free Software Foundation,
19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
20 *
21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
22 * or visit www.oracle.com if you need additional information or have any
23 * questions.
24 */
25
26 package java.lang;
27
28 import jdk.internal.misc.Unsafe;
29 import jdk.internal.vm.annotation.ForceInline;
30 import jdk.internal.vm.annotation.Stable;
31
32 import java.lang.ref.PhantomReference;
33 import java.lang.ref.ReferenceQueue;
34 import java.lang.ref.WeakReference;
35 import java.util.concurrent.atomic.AtomicBoolean;
36 import java.util.concurrent.atomic.AtomicInteger;
37 import java.util.concurrent.atomic.AtomicReference;
38
39 /**
40 * Lazily associate a computed value with (potentially) every type.
41 * For example, if a dynamic language needs to construct a message dispatch
42 * table for each class encountered at a message send call site,
43 * it can use a {@code ClassValue} to cache information needed to
44 * perform the message send quickly, for each class encountered.
45 * @author John Rose, JSR 292 EG
46 * @since 1.7
47 */
48 public abstract class ClassValue<T> {
49 /**
50 * Sole constructor. (For invocation by subclass constructors, typically
51 * implicit.)
52 */
53 protected ClassValue() {
54 }
55
56 /**
57 * Computes the given class's derived value for this {@code ClassValue}.
58 * <p>
59 * This method will be invoked within the first thread that accesses
60 * the value with the {@link #get get} method.
61 * <p>
62 * Normally, this method is invoked at most once per class,
63 * but it may be invoked again if there has been a call to
64 * {@link #remove remove}.
65 * <p>
66 * If this method throws an exception, the corresponding call to {@code get}
67 * will terminate abnormally with that exception, and no class value will be recorded.
68 *
69 * @param type the type whose class value must be computed
70 * @return the newly computed value associated with this {@code ClassValue}, for the given class or interface
71 * @see #get
72 * @see #remove
73 */
74 protected abstract T computeValue(Class<?> type);
75
76 /**
77 * Returns the value for the given class.
78 * If no value has yet been computed, it is obtained by
79 * an invocation of the {@link #computeValue computeValue} method.
80 * <p>
81 * The actual installation of the value on the class
82 * is performed atomically.
83 * At that point, if several racing threads have
84 * computed values, one is chosen, and returned to
85 * all the racing threads.
86 * <p>
87 * The {@code type} parameter is typically a class, but it may be any type,
88 * such as an interface, a primitive type (like {@code int.class}), or {@code void.class}.
89 * <p>
90 * In the absence of {@code remove} calls, a class value has a simple
91 * state diagram: uninitialized and initialized.
92 * When {@code remove} calls are made,
93 * the rules for value observation are more complex.
94 * See the documentation for {@link #remove remove} for more information.
95 *
96 * @param type the type whose class value must be computed or retrieved
97 * @return the current value associated with this {@code ClassValue}, for the given class or interface
98 * @throws NullPointerException if the argument is null
99 * @see #remove
100 * @see #computeValue
101 */
102 @SuppressWarnings("unchecked")
103 @ForceInline
104 public T get(Class<?> type) {
105 ClassValueMap map = type.classValueMap;
106 Object value;
107 if (map != null) {
108 value = map.get(key);
109 if (value != null && !(value instanceof Removed)) {
110 return (value == NULL_MASK) ? null : (T) value;
111 }
112 } else {
113 value = null;
114 }
115 return getSlow(type, value);
116 }
117 // ...the rest in separate method
118 @SuppressWarnings("unchecked")
119 private T getSlow(Class<?> type, Object oldValue) {
120 ClassValueMap.expungeStaleEntries();
121 ClassValueMap map = getMap(type);
122 while (true) {
123 // assert oldValue == null || oldValue instanceof Removed;
124 T newValue = computeValue(type);
125 if (oldValue == null) {
126 oldValue = map.putIfAbsent(key, (newValue == null) ? NULL_MASK : newValue);
127 if (oldValue == null) {
128 return newValue;
129 } else {
130 // another thread has beaten us
131 if (!(oldValue instanceof Removed)) {
132 return (oldValue == NULL_MASK) ? null : (T) oldValue;
133 }
134 }
135 } else {
136 // assert oldValue instanceof Removed;
137 if (map.replace(key, oldValue, (newValue == null) ? NULL_MASK : newValue)) {
138 // this only succeeds if oldValue is still the same instance of
139 // Removed as observed before computeValue invocation
140 return newValue;
141 } else {
142 // another thread has beaten us
143 oldValue = map.get(key);
144 if (oldValue != null && !(oldValue instanceof Removed)) {
145 return (oldValue == NULL_MASK) ? null : (T) oldValue;
146 }
147 }
148 }
149 // retry compute since we observed another version of Removed
150 // as was current before compute which means that there was
151 // at least one value present in between time which means
152 // that the result of our compute is stale.
153 }
154 }
155
156 /**
157 * Removes the associated value for the given class.
158 * If this value is subsequently {@linkplain #get read} for the same class,
159 * its value will be reinitialized by invoking its {@link #computeValue computeValue} method.
160 * This may result in an additional invocation of the
161 * {@code computeValue} method for the given class.
162 * <p>
163 * In order to explain the interaction between {@code get} and {@code remove} calls,
164 * we must model the state transitions of a class value to take into account
165 * the alternation between uninitialized and initialized states.
166 * To do this, number these states sequentially from zero, and note that
167 * uninitialized (or removed) states are numbered with even numbers,
168 * while initialized (or re-initialized) states have odd numbers.
169 * <p>
170 * When a thread {@code T} removes a class value in state {@code 2N},
171 * nothing happens, since the class value is already uninitialized.
172 * Otherwise, the state is advanced atomically to {@code 2N+1}.
173 * <p>
174 * When a thread {@code T} queries a class value in state {@code 2N},
175 * the thread first attempts to initialize the class value to state {@code 2N+1}
176 * by invoking {@code computeValue} and installing the resulting value.
177 * <p>
178 * When {@code T} attempts to install the newly computed value,
179 * if the state is still at {@code 2N}, the class value will be initialized
180 * with the computed value, advancing it to state {@code 2N+1}.
181 * <p>
182 * Otherwise, whether the new state is even or odd,
183 * {@code T} will discard the newly computed value
184 * and retry the {@code get} operation.
185 * <p>
186 * Discarding and retrying is an important proviso,
187 * since otherwise {@code T} could potentially install
188 * a disastrously stale value. For example:
189 * <ul>
190 * <li>{@code T} calls {@code CV.get(C)} and sees state {@code 2N}
191 * <li>{@code T} quickly computes a time-dependent value {@code V0} and gets ready to install it
192 * <li>{@code T} is hit by an unlucky paging or scheduling event, and goes to sleep for a long time
193 * <li>...meanwhile, {@code T2} also calls {@code CV.get(C)} and sees state {@code 2N}
194 * <li>{@code T2} quickly computes a similar time-dependent value {@code V1} and installs it on {@code CV.get(C)}
195 * <li>{@code T2} (or a third thread) then calls {@code CV.remove(C)}, undoing {@code T2}'s work
196 * <li> the previous actions of {@code T2} are repeated several times
197 * <li> also, the relevant computed values change over time: {@code V1}, {@code V2}, ...
198 * <li>...meanwhile, {@code T} wakes up and attempts to install {@code V0}; <em>this must fail</em>
199 * </ul>
200 * We can assume in the above scenario that {@code CV.computeValue} uses locks to properly
201 * observe the time-dependent states as it computes {@code V1}, etc.
202 * This does not remove the threat of a stale value, since there is a window of time
203 * between the return of {@code computeValue} in {@code T} and the installation
204 * of the new value. No user synchronization is possible during this time.
205 *
206 * @param type the type whose class value must be removed
207 * @throws NullPointerException if the argument is null
208 */
209 public void remove(Class<?> type) {
210 ClassValueMap.expungeStaleEntries();
211 ClassValueMap map = type.classValueMap;
212 if (map != null) {
213 Object value, removed = null;
214 do {
215 value = map.get(key);
216 if (value == null || value instanceof Removed) {
217 // non-existence is treated as Removed(V0)
218 // invoking remove() when state is Removed(VN) does not
219 // change it to Removed(VN+1)
220 break;
221 }
222 if (removed == null) {
223 removed = new Removed();
224 }
225 } while (!map.replace(key, value, removed));
226 }
227 }
228
229 // Possible functionality for JSR 292 MR 1
230 /*public*/ void put(Class<?> type, T value) {
231 ClassValueMap.expungeStaleEntries();
232 ClassValueMap map = getMap(type);
233 map.put(key, (value == null) ? NULL_MASK : value);
234 }
235
236 /// --------
237 /// Implementation...
238 /// --------
239
240 /**
241 * The key used in ClassValueMap for this ClassValue.
242 * Since ClassValue(s) are usually assigned to static final fields, it makes
243 * sense to promote the key to constant too.
244 */
245 @Stable
246 private final Key key = new Key(this);
247
248 /**
249 * Key of a ClassValue and a tracker of the ClassValue at the same time.
250 */
251 private static final class Key extends PhantomReference<ClassValue<?>> {
252
253 /** Value stream for hash. See similar structure in ThreadLocal. */
254 private static final AtomicInteger nextHash = new AtomicInteger();
255
256 /** Good for power-of-two tables. See similar structure in ThreadLocal. */
257 private static final int HASH_INCREMENT = 0x61c88647;
258
259 /** Single global queue for all stale keys to be enqueued */
260 static final ReferenceQueue<ClassValue<?>> queue = new ReferenceQueue<>();
261
262 @Stable
263 private final int hash;
264
265 Key(ClassValue<?> cv) {
266 super(cv, queue);
267 hash = nextHash.getAndAdd(HASH_INCREMENT);
268 }
269
270 @Override
271 public int hashCode() {
272 return hash;
273 }
274
275 // equals is identity
276 }
277
278 /** A masked null value. */
279 private static final Object NULL_MASK = new Object();
280
281 /** Removed value(s) */
282 private static final class Removed {}
283
284 /** Return the backing map associated with this type, possibly constructing it. */
285 private static ClassValueMap getMap(Class<?> type) {
286 ClassValueMap map = type.classValueMap;
287 return (map != null) ? map : initializeMap(type);
288 }
289
290 private static ClassValueMap initializeMap(Class<?> type) {
291 ClassValueMap map = ClassValueMap.create();
292 return U.compareAndSwapObject(type, CLASS_VALUE_MAP_OFFSET, null, map)
293 ? map
294 : (ClassValueMap) U.getObjectVolatile(type, CLASS_VALUE_MAP_OFFSET);
295 }
296
297 /** A backing map for all ClassValues in a particular Class.
298 */
299 static final class ClassValueMap extends LinearProbeHashtable<Key, Object> {
300 // eh, needed to silent the gods
301 private static final long serialVersionUID = 1L;
302
303 // a node in a linked list of WeakReference(s) to ClassValueMap(s)
304 private static final class WeakNode extends WeakReference<ClassValueMap> {
305 WeakNode next;
306 WeakNode(ClassValueMap map) { super(map); }
307 }
308
309 // the head of the linked list of WeakReference(s) to ClassValueMap(s)
310 private static final AtomicReference<WeakNode> mapsList = new AtomicReference<>();
311
312 // the lock used to guard expunging logic
313 private static final AtomicBoolean expungeInProgress = new AtomicBoolean();
314
315 private ClassValueMap() {
316 super(1);
317 }
318
319 static ClassValueMap create() {
320 ClassValueMap map = new ClassValueMap();
321 WeakNode node = new WeakNode(map);
322 // register the weakly referenced map into the list
323 do {
324 node.next = mapsList.get();
325 } while (!mapsList.compareAndSet(node.next, node));
326 return map;
327 }
328
329 static void expungeStaleEntries() {
330 // make sure only one thread at a time is expunging stale entries;
331 // this is important for correctness so that no stale enqueued key
332 // is missed from being removed from any map that might possibly
333 // hold it.
334 if (!expungeInProgress.get() &&
335 expungeInProgress.compareAndSet(false, true)) try {
336 expungeStaleEntriesImpl();
337 } finally {
338 expungeInProgress.set(false);
339 }
340 }
341
342 private static void expungeStaleEntriesImpl() {
343 // start with an empty private list
344 WeakNode first = null, last = null;
345 // iterate Key(s) of GCed ClassValue(s)
346 Key key;
347 while ((key = (Key) Key.queue.poll()) != null) {
348 // steal any (newly) registered map(s) from the shared 'mapsList'
349 // as they might already be holding the stale 'key' that has just
350 // been removed from the queue. The following guarantees that
351 // no (stale-key, live-map) combination is missed:
352 // - a map is 1st registered into the mapsList and then non-stale
353 // keys are inserted into it
354 // - a non-stale key becomes stale when a ClassValue that it
355 // phantomly references is GCed; the key is then enqueued
356 // - a stale key is 1st removed from the queue and then mapsList
357 // is checked for any (newly) registered map(s)
358 if (mapsList.get() != null) {
359 WeakNode stolen = mapsList.getAndSet(null);
360 // add them to the end of current private list
361 if (last != null) {
362 last.next = stolen;
363 } else {
364 // assert first == null;
365 first = stolen;
366 }
367 }
368 // iterate the accumulated private list of maps
369 last = null;
370 for (WeakNode node = first; node != null; ) {
371 ClassValueMap map = node.get();
372 if (map == null) {
373 // Class and its ClassValueMap have already been GCed
374 // -> remove the node from the list
375 if (last == null) {
376 first = node = node.next;
377 } else {
378 last.next = node = node.next;
379 }
380 } else {
381 // ClassValueMap is still alive
382 // -> attempt to remove the key from it
383 map.remove(key);
384 // navigate to next in list
385 last = node;
386 node = node.next;
387 }
388 }
389 // at this time, 'last' points to the last WeakNode in the private
390 // list (if any) and the following always holds:
391 // assert (first == null) == (last == null);
392 }
393 // re-insert the stolen and expunged list of map(s) back into shared
394 // mapsList
395 if (last != null) {
396 do {
397 last.next = mapsList.get();
398 } while (!mapsList.compareAndSet(last.next, first));
399 }
400 }
401 }
402
403 /** We need to cas */
404 private static final Unsafe U = Unsafe.getUnsafe();
405 private static final long CLASS_VALUE_MAP_OFFSET;
406
407 static {
408 try {
409 CLASS_VALUE_MAP_OFFSET = U.objectFieldOffset(
410 Class.class.getDeclaredField("classValueMap"));
411 } catch (Exception e) {
412 throw new Error(e);
413 }
414 }
415 }