1 /*
2 * Copyright (c) 2010, 2013, 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
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23 * questions.
24 */
25
26 package java.lang;
27
28 import java.util.WeakHashMap;
29 import java.lang.ref.WeakReference;
30 import java.util.concurrent.atomic.AtomicInteger;
31
32 import static java.lang.ClassValue.ClassValueMap.probeHomeLocation;
33 import static java.lang.ClassValue.ClassValueMap.probeBackupLocations;
34
35 /**
36 * Lazily associate a computed value with (potentially) every type.
37 * For example, if a dynamic language needs to construct a message dispatch
38 * table for each class encountered at a message send call site,
39 * it can use a {@code ClassValue} to cache information needed to
40 * perform the message send quickly, for each class encountered.
41 * @author John Rose, JSR 292 EG
42 * @since 1.7
43 */
44 public abstract class ClassValue<T> {
45 /**
46 * Sole constructor. (For invocation by subclass constructors, typically
47 * implicit.)
48 */
49 protected ClassValue() {
50 }
51
52 /**
53 * Computes the given class's derived value for this {@code ClassValue}.
54 * <p>
55 * This method will be invoked within the first thread that accesses
56 * the value with the {@link #get get} method.
57 * <p>
58 * Normally, this method is invoked at most once per class,
59 * but it may be invoked again if there has been a call to
60 * {@link #remove remove}.
61 * <p>
62 * If this method throws an exception, the corresponding call to {@code get}
63 * will terminate abnormally with that exception, and no class value will be recorded.
64 *
65 * @param type the type whose class value must be computed
66 * @return the newly computed value associated with this {@code ClassValue}, for the given class or interface
67 * @see #get
68 * @see #remove
69 */
70 protected abstract T computeValue(Class<?> type);
71
72 /**
73 * Returns the value for the given class.
74 * If no value has yet been computed, it is obtained by
75 * an invocation of the {@link #computeValue computeValue} method.
76 * <p>
77 * The actual installation of the value on the class
78 * is performed atomically.
79 * At that point, if several racing threads have
80 * computed values, one is chosen, and returned to
81 * all the racing threads.
82 * <p>
83 * The {@code type} parameter is typically a class, but it may be any type,
84 * such as an interface, a primitive type (like {@code int.class}), or {@code void.class}.
85 * <p>
86 * In the absence of {@code remove} calls, a class value has a simple
87 * state diagram: uninitialized and initialized.
88 * When {@code remove} calls are made,
89 * the rules for value observation are more complex.
90 * See the documentation for {@link #remove remove} for more information.
91 *
92 * @param type the type whose class value must be computed or retrieved
93 * @return the current value associated with this {@code ClassValue}, for the given class or interface
94 * @throws NullPointerException if the argument is null
95 * @see #remove
96 * @see #computeValue
97 */
98 public T get(Class<?> type) {
99 // non-racing this.hashCodeForCache : final int
100 Entry<?>[] cache;
101 Entry<T> e = probeHomeLocation(cache = getCacheCarefully(type), this);
102 // racing e : current value <=> stale value from current cache or from stale cache
103 // invariant: e is null or an Entry with readable Entry.version and Entry.value
104 if (match(e))
105 // invariant: No false positive matches. False negatives are OK if rare.
106 // The key fact that makes this work: if this.version == e.version,
107 // then this thread has a right to observe (final) e.value.
108 return e.value();
109 // The fast path can fail for any of these reasons:
110 // 1. no entry has been computed yet
111 // 2. hash code collision (before or after reduction mod cache.length)
112 // 3. an entry has been removed (either on this type or another)
113 // 4. the GC has somehow managed to delete e.version and clear the reference
114 return getFromBackup(cache, type);
115 }
116
117 /**
118 * Removes the associated value for the given class.
119 * If this value is subsequently {@linkplain #get read} for the same class,
120 * its value will be reinitialized by invoking its {@link #computeValue computeValue} method.
121 * This may result in an additional invocation of the
122 * {@code computeValue} method for the given class.
123 * <p>
124 * In order to explain the interaction between {@code get} and {@code remove} calls,
125 * we must model the state transitions of a class value to take into account
126 * the alternation between uninitialized and initialized states.
127 * To do this, number these states sequentially from zero, and note that
128 * uninitialized (or removed) states are numbered with even numbers,
129 * while initialized (or re-initialized) states have odd numbers.
130 * <p>
131 * When a thread {@code T} removes a class value in state {@code 2N},
132 * nothing happens, since the class value is already uninitialized.
133 * Otherwise, the state is advanced atomically to {@code 2N+1}.
134 * <p>
135 * When a thread {@code T} queries a class value in state {@code 2N},
136 * the thread first attempts to initialize the class value to state {@code 2N+1}
137 * by invoking {@code computeValue} and installing the resulting value.
138 * <p>
139 * When {@code T} attempts to install the newly computed value,
140 * if the state is still at {@code 2N}, the class value will be initialized
141 * with the computed value, advancing it to state {@code 2N+1}.
142 * <p>
143 * Otherwise, whether the new state is even or odd,
144 * {@code T} will discard the newly computed value
145 * and retry the {@code get} operation.
146 * <p>
147 * Discarding and retrying is an important proviso,
148 * since otherwise {@code T} could potentially install
149 * a disastrously stale value. For example:
150 * <ul>
151 * <li>{@code T} calls {@code CV.get(C)} and sees state {@code 2N}
152 * <li>{@code T} quickly computes a time-dependent value {@code V0} and gets ready to install it
153 * <li>{@code T} is hit by an unlucky paging or scheduling event, and goes to sleep for a long time
154 * <li>...meanwhile, {@code T2} also calls {@code CV.get(C)} and sees state {@code 2N}
155 * <li>{@code T2} quickly computes a similar time-dependent value {@code V1} and installs it on {@code CV.get(C)}
156 * <li>{@code T2} (or a third thread) then calls {@code CV.remove(C)}, undoing {@code T2}'s work
157 * <li> the previous actions of {@code T2} are repeated several times
158 * <li> also, the relevant computed values change over time: {@code V1}, {@code V2}, ...
159 * <li>...meanwhile, {@code T} wakes up and attempts to install {@code V0}; <em>this must fail</em>
160 * </ul>
161 * We can assume in the above scenario that {@code CV.computeValue} uses locks to properly
162 * observe the time-dependent states as it computes {@code V1}, etc.
163 * This does not remove the threat of a stale value, since there is a window of time
164 * between the return of {@code computeValue} in {@code T} and the installation
165 * of the new value. No user synchronization is possible during this time.
166 *
167 * @param type the type whose class value must be removed
168 * @throws NullPointerException if the argument is null
169 */
170 public void remove(Class<?> type) {
171 ClassValueMap map = getMap(type);
172 map.removeEntry(this);
173 }
174
175 // Possible functionality for JSR 292 MR 1
176 /*public*/ void put(Class<?> type, T value) {
177 ClassValueMap map = getMap(type);
178 map.changeEntry(this, value);
179 }
180
181 /// --------
182 /// Implementation...
183 /// --------
184
185 /** Return the cache, if it exists, else a dummy empty cache. */
186 private static Entry<?>[] getCacheCarefully(Class<?> type) {
187 // racing type.classValueMap{.cacheArray} : null => new Entry[X] <=> new Entry[Y]
188 ClassValueMap map = type.classValueMap;
189 if (map == null) return EMPTY_CACHE;
190 Entry<?>[] cache = map.getCache();
191 return cache;
192 // invariant: returned value is safe to dereference and check for an Entry
193 }
194
195 /** Initial, one-element, empty cache used by all Class instances. Must never be filled. */
196 private static final Entry<?>[] EMPTY_CACHE = { null };
197
198 /**
199 * Slow tail of ClassValue.get to retry at nearby locations in the cache,
200 * or take a slow lock and check the hash table.
201 * Called only if the first probe was empty or a collision.
202 * This is a separate method, so compilers can process it independently.
203 */
204 private T getFromBackup(Entry<?>[] cache, Class<?> type) {
205 Entry<T> e = probeBackupLocations(cache, this);
206 if (e != null)
207 return e.value();
208 return getFromHashMap(type);
209 }
210
211 // Hack to suppress warnings on the (T) cast, which is a no-op.
212 @SuppressWarnings("unchecked")
213 Entry<T> castEntry(Entry<?> e) { return (Entry<T>) e; }
214
215 /** Called when the fast path of get fails, and cache reprobe also fails.
216 */
217 private T getFromHashMap(Class<?> type) {
218 // The fail-safe recovery is to fall back to the underlying classValueMap.
219 ClassValueMap map = getMap(type);
220 for (;;) {
221 Entry<T> e = map.startEntry(this);
222 if (!e.isPromise())
223 return e.value();
224 try {
225 // Try to make a real entry for the promised version.
226 e = makeEntry(e.version(), computeValue(type));
227 } finally {
228 // Whether computeValue throws or returns normally,
229 // be sure to remove the empty entry.
230 e = map.finishEntry(this, e);
231 }
232 if (e != null)
233 return e.value();
234 // else try again, in case a racing thread called remove (so e == null)
235 }
236 }
237
238 /** Check that e is non-null, matches this ClassValue, and is live. */
239 boolean match(Entry<?> e) {
240 // racing e.version : null (blank) => unique Version token => null (GC-ed version)
241 // non-racing this.version : v1 => v2 => ... (updates are read faithfully from volatile)
242 return (e != null && e.get() == this.version);
243 // invariant: No false positives on version match. Null is OK for false negative.
244 // invariant: If version matches, then e.value is readable (final set in Entry.<init>)
245 }
246
247 /** Internal hash code for accessing Class.classValueMap.cacheArray. */
248 final int hashCodeForCache = nextHashCode.getAndAdd(HASH_INCREMENT) & HASH_MASK;
249
250 /** Value stream for hashCodeForCache. See similar structure in ThreadLocal. */
251 private static final AtomicInteger nextHashCode = new AtomicInteger();
252
253 /** Good for power-of-two tables. See similar structure in ThreadLocal. */
254 private static final int HASH_INCREMENT = 0x61c88647;
255
256 /** Mask a hash code to be positive but not too large, to prevent wraparound. */
257 static final int HASH_MASK = (-1 >>> 2);
258
259 /**
260 * Private key for retrieval of this object from ClassValueMap.
261 */
262 static class Identity {
263 }
264 /**
265 * This ClassValue's identity, expressed as an opaque object.
266 * The main object {@code ClassValue.this} is incorrect since
267 * subclasses may override {@code ClassValue.equals}, which
268 * could confuse keys in the ClassValueMap.
269 */
270 final Identity identity = new Identity();
271
272 /**
273 * Current version for retrieving this class value from the cache.
274 * Any number of computeValue calls can be cached in association with one version.
275 * But the version changes when a remove (on any type) is executed.
276 * A version change invalidates all cache entries for the affected ClassValue,
277 * by marking them as stale. Stale cache entries do not force another call
278 * to computeValue, but they do require a synchronized visit to a backing map.
279 * <p>
280 * All user-visible state changes on the ClassValue take place under
281 * a lock inside the synchronized methods of ClassValueMap.
282 * Readers (of ClassValue.get) are notified of such state changes
283 * when this.version is bumped to a new token.
284 * This variable must be volatile so that an unsynchronized reader
285 * will receive the notification without delay.
286 * <p>
287 * If version were not volatile, one thread T1 could persistently hold onto
288 * a stale value this.value == V1, while another thread T2 advances
289 * (under a lock) to this.value == V2. This will typically be harmless,
290 * but if T1 and T2 interact causally via some other channel, such that
291 * T1's further actions are constrained (in the JMM) to happen after
292 * the V2 event, then T1's observation of V1 will be an error.
293 * <p>
294 * The practical effect of making this.version be volatile is that it cannot
295 * be hoisted out of a loop (by an optimizing JIT) or otherwise cached.
296 * Some machines may also require a barrier instruction to execute
297 * before this.version.
298 */
299 private volatile Version<T> version = new Version<>(this);
300 Version<T> version() { return version; }
301 void bumpVersion() { version = new Version<>(this); }
302 static class Version<T> {
303 private final ClassValue<T> classValue;
304 private final Entry<T> promise = new Entry<>(this);
305 Version(ClassValue<T> classValue) { this.classValue = classValue; }
306 ClassValue<T> classValue() { return classValue; }
307 Entry<T> promise() { return promise; }
308 boolean isLive() { return classValue.version() == this; }
309 }
310
311 /** One binding of a value to a class via a ClassValue.
312 * States are:<ul>
313 * <li> promise if value == Entry.this
314 * <li> else dead if version == null
315 * <li> else stale if version != classValue.version
316 * <li> else live </ul>
317 * Promises are never put into the cache; they only live in the
318 * backing map while a computeValue call is in flight.
319 * Once an entry goes stale, it can be reset at any time
320 * into the dead state.
321 */
322 static class Entry<T> extends WeakReference<Version<T>> {
323 final Object value; // usually of type T, but sometimes (Entry)this
324 Entry(Version<T> version, T value) {
325 super(version);
326 this.value = value; // for a regular entry, value is of type T
327 }
328 private void assertNotPromise() { assert(!isPromise()); }
329 /** For creating a promise. */
330 Entry(Version<T> version) {
331 super(version);
332 this.value = this; // for a promise, value is not of type T, but Entry!
333 }
334 /** Fetch the value. This entry must not be a promise. */
335 @SuppressWarnings("unchecked") // if !isPromise, type is T
336 T value() { assertNotPromise(); return (T) value; }
337 boolean isPromise() { return value == this; }
338 Version<T> version() { return get(); }
339 ClassValue<T> classValueOrNull() {
340 Version<T> v = version();
341 return (v == null) ? null : v.classValue();
342 }
343 boolean isLive() {
344 Version<T> v = version();
345 if (v == null) return false;
346 if (v.isLive()) return true;
347 clear();
348 return false;
349 }
350 Entry<T> refreshVersion(Version<T> v2) {
351 assertNotPromise();
352 @SuppressWarnings("unchecked") // if !isPromise, type is T
353 Entry<T> e2 = new Entry<>(v2, (T) value);
354 clear();
355 // value = null -- caller must drop
356 return e2;
357 }
358 static final Entry<?> DEAD_ENTRY = new Entry<>(null, null);
359 }
360
361 /** Return the backing map associated with this type. */
362 private static ClassValueMap getMap(Class<?> type) {
363 // racing type.classValueMap : null (blank) => unique ClassValueMap
364 // if a null is observed, a map is created (lazily, synchronously, uniquely)
365 // all further access to that map is synchronized
366 ClassValueMap map = type.classValueMap;
367 if (map != null) return map;
368 return initializeMap(type);
369 }
370
371 private static final Object CRITICAL_SECTION = new Object();
372 private static ClassValueMap initializeMap(Class<?> type) {
373 ClassValueMap map;
374 synchronized (CRITICAL_SECTION) { // private object to avoid deadlocks
375 // happens about once per type
376 if ((map = type.classValueMap) == null)
377 type.classValueMap = map = new ClassValueMap();
378 }
379 return map;
380 }
381
382 static <T> Entry<T> makeEntry(Version<T> explicitVersion, T value) {
383 // Note that explicitVersion might be different from this.version.
384 return new Entry<>(explicitVersion, value);
385
386 // As soon as the Entry is put into the cache, the value will be
387 // reachable via a data race (as defined by the Java Memory Model).
388 // This race is benign, assuming the value object itself can be
389 // read safely by multiple threads. This is up to the user.
390 //
391 // The entry and version fields themselves can be safely read via
392 // a race because they are either final or have controlled states.
393 // If the pointer from the entry to the version is still null,
394 // or if the version goes immediately dead and is nulled out,
395 // the reader will take the slow path and retry under a lock.
396 }
397
398 // The following class could also be top level and non-public:
399
400 /** A backing map for all ClassValues.
401 * Gives a fully serialized "true state" for each pair (ClassValue cv, Class type).
402 * Also manages an unserialized fast-path cache.
403 */
404 static class ClassValueMap extends WeakHashMap<ClassValue.Identity, Entry<?>> {
405 private Entry<?>[] cacheArray;
406 private int cacheLoad, cacheLoadLimit;
407
408 /** Number of entries initially allocated to each type when first used with any ClassValue.
409 * It would be pointless to make this much smaller than the Class and ClassValueMap objects themselves.
410 * Must be a power of 2.
411 */
412 private static final int INITIAL_ENTRIES = 32;
413
414 /** Build a backing map for ClassValues.
415 * Also, create an empty cache array and install it on the class.
416 */
417 ClassValueMap() {
418 sizeCache(INITIAL_ENTRIES);
419 }
420
421 Entry<?>[] getCache() { return cacheArray; }
422
423 /** Initiate a query. Store a promise (placeholder) if there is no value yet. */
424 synchronized
425 <T> Entry<T> startEntry(ClassValue<T> classValue) {
426 @SuppressWarnings("unchecked") // one map has entries for all value types <T>
427 Entry<T> e = (Entry<T>) get(classValue.identity);
428 Version<T> v = classValue.version();
429 if (e == null) {
430 e = v.promise();
431 // The presence of a promise means that a value is pending for v.
432 // Eventually, finishEntry will overwrite the promise.
433 put(classValue.identity, e);
434 // Note that the promise is never entered into the cache!
435 return e;
436 } else if (e.isPromise()) {
437 // Somebody else has asked the same question.
438 // Let the races begin!
439 if (e.version() != v) {
440 e = v.promise();
441 put(classValue.identity, e);
442 }
443 return e;
444 } else {
445 // there is already a completed entry here; report it
446 if (e.version() != v) {
447 // There is a stale but valid entry here; make it fresh again.
448 // Once an entry is in the hash table, we don't care what its version is.
449 e = e.refreshVersion(v);
450 put(classValue.identity, e);
451 }
452 // Add to the cache, to enable the fast path, next time.
453 checkCacheLoad();
454 addToCache(classValue, e);
455 return e;
456 }
457 }
458
459 /** Finish a query. Overwrite a matching placeholder. Drop stale incoming values. */
460 synchronized
461 <T> Entry<T> finishEntry(ClassValue<T> classValue, Entry<T> e) {
462 @SuppressWarnings("unchecked") // one map has entries for all value types <T>
463 Entry<T> e0 = (Entry<T>) get(classValue.identity);
464 if (e == e0) {
465 // We can get here during exception processing, unwinding from computeValue.
466 assert(e.isPromise());
467 remove(classValue.identity);
468 return null;
469 } else if (e0 != null && e0.isPromise() && e0.version() == e.version()) {
470 // If e0 matches the intended entry, there has not been a remove call
471 // between the previous startEntry and now. So now overwrite e0.
472 Version<T> v = classValue.version();
473 if (e.version() != v)
474 e = e.refreshVersion(v);
475 put(classValue.identity, e);
476 // Add to the cache, to enable the fast path, next time.
477 checkCacheLoad();
478 addToCache(classValue, e);
479 return e;
480 } else {
481 // Some sort of mismatch; caller must try again.
482 return null;
483 }
484 }
485
486 /** Remove an entry. */
487 synchronized
488 void removeEntry(ClassValue<?> classValue) {
489 Entry<?> e = remove(classValue.identity);
490 if (e == null) {
491 // Uninitialized, and no pending calls to computeValue. No change.
492 } else if (e.isPromise()) {
493 // State is uninitialized, with a pending call to finishEntry.
494 // Since remove is a no-op in such a state, keep the promise
495 // by putting it back into the map.
496 put(classValue.identity, e);
497 } else {
498 // In an initialized state. Bump forward, and de-initialize.
499 classValue.bumpVersion();
500 // Make all cache elements for this guy go stale.
501 removeStaleEntries(classValue);
502 }
503 }
504
505 /** Change the value for an entry. */
506 synchronized
507 <T> void changeEntry(ClassValue<T> classValue, T value) {
508 @SuppressWarnings("unchecked") // one map has entries for all value types <T>
509 Entry<T> e0 = (Entry<T>) get(classValue.identity);
510 Version<T> version = classValue.version();
511 if (e0 != null) {
512 if (e0.version() == version && e0.value() == value)
513 // no value change => no version change needed
514 return;
515 classValue.bumpVersion();
516 removeStaleEntries(classValue);
517 }
518 Entry<T> e = makeEntry(version, value);
519 put(classValue.identity, e);
520 // Add to the cache, to enable the fast path, next time.
521 checkCacheLoad();
522 addToCache(classValue, e);
523 }
524
525 /// --------
526 /// Cache management.
527 /// --------
528
529 // Statics do not need synchronization.
530
531 /** Load the cache entry at the given (hashed) location. */
532 static Entry<?> loadFromCache(Entry<?>[] cache, int i) {
533 // non-racing cache.length : constant
534 // racing cache[i & (mask)] : null <=> Entry
535 return cache[i & (cache.length-1)];
536 // invariant: returned value is null or well-constructed (ready to match)
537 }
538
539 /** Look in the cache, at the home location for the given ClassValue. */
540 static <T> Entry<T> probeHomeLocation(Entry<?>[] cache, ClassValue<T> classValue) {
541 return classValue.castEntry(loadFromCache(cache, classValue.hashCodeForCache));
542 }
543
544 /** Given that first probe was a collision, retry at nearby locations. */
545 static <T> Entry<T> probeBackupLocations(Entry<?>[] cache, ClassValue<T> classValue) {
546 if (PROBE_LIMIT <= 0) return null;
547 // Probe the cache carefully, in a range of slots.
548 int mask = (cache.length-1);
549 int home = (classValue.hashCodeForCache & mask);
550 Entry<?> e2 = cache[home]; // victim, if we find the real guy
551 if (e2 == null) {
552 return null; // if nobody is at home, no need to search nearby
553 }
554 // assume !classValue.match(e2), but do not assert, because of races
555 int pos2 = -1;
556 for (int i = home + 1; i < home + PROBE_LIMIT; i++) {
557 Entry<?> e = cache[i & mask];
558 if (e == null) {
559 break; // only search within non-null runs
560 }
561 if (classValue.match(e)) {
562 // relocate colliding entry e2 (from cache[home]) to first empty slot
563 cache[home] = e;
564 if (pos2 >= 0) {
565 cache[i & mask] = Entry.DEAD_ENTRY;
566 } else {
567 pos2 = i;
568 }
569 cache[pos2 & mask] = ((entryDislocation(cache, pos2, e2) < PROBE_LIMIT)
570 ? e2 // put e2 here if it fits
571 : Entry.DEAD_ENTRY);
572 return classValue.castEntry(e);
573 }
574 // Remember first empty slot, if any:
575 if (!e.isLive() && pos2 < 0) pos2 = i;
576 }
577 return null;
578 }
579
580 /** How far out of place is e? */
581 private static int entryDislocation(Entry<?>[] cache, int pos, Entry<?> e) {
582 ClassValue<?> cv = e.classValueOrNull();
583 if (cv == null) return 0; // entry is not live!
584 int mask = (cache.length-1);
585 return (pos - cv.hashCodeForCache) & mask;
586 }
587
588 /// --------
589 /// Below this line all functions are private, and assume synchronized access.
590 /// --------
591
592 private void sizeCache(int length) {
593 assert((length & (length-1)) == 0); // must be power of 2
594 cacheLoad = 0;
595 cacheLoadLimit = (int) ((double) length * CACHE_LOAD_LIMIT / 100);
596 cacheArray = new Entry<?>[length];
597 }
598
599 /** Make sure the cache load stays below its limit, if possible. */
600 private void checkCacheLoad() {
601 if (cacheLoad >= cacheLoadLimit) {
602 reduceCacheLoad();
603 }
604 }
605 private void reduceCacheLoad() {
606 removeStaleEntries();
607 if (cacheLoad < cacheLoadLimit)
608 return; // win
609 Entry<?>[] oldCache = getCache();
610 if (oldCache.length > HASH_MASK)
611 return; // lose
612 sizeCache(oldCache.length * 2);
613 for (Entry<?> e : oldCache) {
614 if (e != null && e.isLive()) {
615 addToCache(e);
616 }
617 }
618 }
619
620 /** Remove stale entries in the given range.
621 * Should be executed under a Map lock.
622 */
623 private void removeStaleEntries(Entry<?>[] cache, int begin, int count) {
624 if (PROBE_LIMIT <= 0) return;
625 int mask = (cache.length-1);
626 int removed = 0;
627 for (int i = begin; i < begin + count; i++) {
628 Entry<?> e = cache[i & mask];
629 if (e == null || e.isLive())
630 continue; // skip null and live entries
631 Entry<?> replacement = null;
632 if (PROBE_LIMIT > 1) {
633 // avoid breaking up a non-null run
634 replacement = findReplacement(cache, i);
635 }
636 cache[i & mask] = replacement;
637 if (replacement == null) removed += 1;
638 }
639 cacheLoad = Math.max(0, cacheLoad - removed);
640 }
641
642 /** Clearing a cache slot risks disconnecting following entries
643 * from the head of a non-null run, which would allow them
644 * to be found via reprobes. Find an entry after cache[begin]
645 * to plug into the hole, or return null if none is needed.
646 */
647 private Entry<?> findReplacement(Entry<?>[] cache, int home1) {
648 Entry<?> replacement = null;
649 int haveReplacement = -1, replacementPos = 0;
650 int mask = (cache.length-1);
651 for (int i2 = home1 + 1; i2 < home1 + PROBE_LIMIT; i2++) {
652 Entry<?> e2 = cache[i2 & mask];
653 if (e2 == null) break; // End of non-null run.
654 if (!e2.isLive()) continue; // Doomed anyway.
655 int dis2 = entryDislocation(cache, i2, e2);
656 if (dis2 == 0) continue; // e2 already optimally placed
657 int home2 = i2 - dis2;
658 if (home2 <= home1) {
659 // e2 can replace entry at cache[home1]
660 if (home2 == home1) {
661 // Put e2 exactly where he belongs.
662 haveReplacement = 1;
663 replacementPos = i2;
664 replacement = e2;
665 } else if (haveReplacement <= 0) {
666 haveReplacement = 0;
667 replacementPos = i2;
668 replacement = e2;
669 }
670 // And keep going, so we can favor larger dislocations.
671 }
672 }
673 if (haveReplacement >= 0) {
674 if (cache[(replacementPos+1) & mask] != null) {
675 // Be conservative, to avoid breaking up a non-null run.
676 cache[replacementPos & mask] = (Entry<?>) Entry.DEAD_ENTRY;
677 } else {
678 cache[replacementPos & mask] = null;
679 cacheLoad -= 1;
680 }
681 }
682 return replacement;
683 }
684
685 /** Remove stale entries in the range near classValue. */
686 private void removeStaleEntries(ClassValue<?> classValue) {
687 removeStaleEntries(getCache(), classValue.hashCodeForCache, PROBE_LIMIT);
688 }
689
690 /** Remove all stale entries, everywhere. */
691 private void removeStaleEntries() {
692 Entry<?>[] cache = getCache();
693 removeStaleEntries(cache, 0, cache.length + PROBE_LIMIT - 1);
694 }
695
696 /** Add the given entry to the cache, in its home location, unless it is out of date. */
697 private <T> void addToCache(Entry<T> e) {
698 ClassValue<T> classValue = e.classValueOrNull();
699 if (classValue != null)
700 addToCache(classValue, e);
701 }
702
703 /** Add the given entry to the cache, in its home location. */
704 private <T> void addToCache(ClassValue<T> classValue, Entry<T> e) {
705 if (PROBE_LIMIT <= 0) return; // do not fill cache
706 // Add e to the cache.
707 Entry<?>[] cache = getCache();
708 int mask = (cache.length-1);
709 int home = classValue.hashCodeForCache & mask;
710 Entry<?> e2 = placeInCache(cache, home, e, false);
711 if (e2 == null) return; // done
712 if (PROBE_LIMIT > 1) {
713 // try to move e2 somewhere else in his probe range
714 int dis2 = entryDislocation(cache, home, e2);
715 int home2 = home - dis2;
716 for (int i2 = home2; i2 < home2 + PROBE_LIMIT; i2++) {
717 if (placeInCache(cache, i2 & mask, e2, true) == null) {
718 return;
719 }
720 }
721 }
722 // Note: At this point, e2 is just dropped from the cache.
723 }
724
725 /** Store the given entry. Update cacheLoad, and return any live victim.
726 * 'Gently' means return self rather than dislocating a live victim.
727 */
728 private Entry<?> placeInCache(Entry<?>[] cache, int pos, Entry<?> e, boolean gently) {
729 Entry<?> e2 = overwrittenEntry(cache[pos]);
730 if (gently && e2 != null) {
731 // do not overwrite a live entry
732 return e;
733 } else {
734 cache[pos] = e;
735 return e2;
736 }
737 }
738
739 /** Note an entry that is about to be overwritten.
740 * If it is not live, quietly replace it by null.
741 * If it is an actual null, increment cacheLoad,
742 * because the caller is going to store something
743 * in its place.
744 */
745 private <T> Entry<T> overwrittenEntry(Entry<T> e2) {
746 if (e2 == null) cacheLoad += 1;
747 else if (e2.isLive()) return e2;
748 return null;
749 }
750
751 /** Percent loading of cache before resize. */
752 private static final int CACHE_LOAD_LIMIT = 67; // 0..100
753 /** Maximum number of probes to attempt. */
754 private static final int PROBE_LIMIT = 6; // 1..
755 // N.B. Set PROBE_LIMIT=0 to disable all fast paths.
756 }
757 }
758