1 /*
2 * Copyright (c) 2023, 2024, 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 jdk.internal.util;
27
28 import java.lang.ref.Reference;
29 import java.lang.ref.ReferenceQueue;
30 import java.lang.ref.SoftReference;
31 import java.lang.ref.WeakReference;
32 import java.util.AbstractMap;
33 import java.util.Collection;
34 import java.util.HashMap;
35 import java.util.Objects;
36 import java.util.Map;
37 import java.util.Set;
38 import java.util.concurrent.ConcurrentHashMap;
39 import java.util.function.Supplier;
40 import java.util.function.UnaryOperator;
41 import java.util.stream.Collectors;
42 import java.util.stream.Stream;
43
44 import jdk.internal.access.SharedSecrets;
45
46 /**
47 * This class provides management of {@link Map maps} where it is desirable to
48 * remove entries automatically when the key is garbage collected. This is
49 * accomplished by using a backing map where the keys are either a
50 * {@link WeakReference} or a {@link SoftReference}.
51 * Keys must be {@linkplain Class#isIdentity() identity objects.}
52 * <p>
53 * To create a {@link ReferencedKeyMap} the user must provide a {@link Supplier}
54 * of the backing map and whether {@link WeakReference} or
55 * {@link SoftReference} is to be used.
56 *
57 * {@snippet :
58 * // Use HashMap and WeakReference
59 * Map<Long, String> map = ReferencedKeyMap.create(false, HashMap::new);
60 * map.put(10_000_000L, "a");
61 * map.put(10_000_001L, "b");
62 * map.put(10_000_002L, "c");
63 * map.put(10_000_003L, "d");
64 * map.put(10_000_004L, "e");
65 *
66 * // Use ConcurrentHashMap and SoftReference
67 * map = ReferencedKeyMap.create(true, ConcurrentHashMap::new);
68 * map.put(20_000_000L, "v");
69 * map.put(20_000_001L, "w");
70 * map.put(20_000_002L, "x");
71 * map.put(20_000_003L, "y");
72 * map.put(20_000_004L, "z");
73 * }
74 *
75 * @implNote Care must be given that the backing map does replacement by
76 * replacing the value in the map entry instead of deleting the old entry and
77 * adding a new entry, otherwise replaced entries may end up with a strongly
78 * referenced key. {@link HashMap} and {@link ConcurrentHashMap} are known
79 * to be safe.
80 *
81 * @param <K> the type of keys maintained by this map
82 * @param <V> the type of mapped values
83 *
84 * @since 21
85 */
86 public final class ReferencedKeyMap<K, V> implements Map<K, V> {
87 /**
88 * true if {@link SoftReference} keys are to be used,
89 * {@link WeakReference} otherwise.
90 */
91 private final boolean isSoft;
92
93 /**
94 * Backing {@link Map}.
95 */
96 private final Map<ReferenceKey<K>, V> map;
97
98 /**
99 * {@link ReferenceQueue} for cleaning up entries.
100 */
101 private final ReferenceQueue<K> stale;
102
103 /**
104 * Private constructor.
105 *
106 * @param isSoft true if {@link SoftReference} keys are to
107 * be used, {@link WeakReference} otherwise.
108 * @param map backing map
109 * @param stale {@link ReferenceQueue} for cleaning up entries
110 */
111 private ReferencedKeyMap(boolean isSoft, Map<ReferenceKey<K>, V> map, ReferenceQueue<K> stale) {
112 this.isSoft = isSoft;
113 this.map = map;
114 this.stale = stale;
115 }
116
117 /**
118 * Create a new {@link ReferencedKeyMap} map.
119 *
120 * @param isSoft true if {@link SoftReference} keys are to
121 * be used, {@link WeakReference} otherwise.
122 * @param supplier {@link Supplier} of the backing map
123 *
124 * @return a new map with {@link Reference} keys
125 *
126 * @param <K> the type of keys maintained by the new map
127 * @param <V> the type of mapped values
128 */
129 public static <K, V> ReferencedKeyMap<K, V>
130 create(boolean isSoft, Supplier<Map<ReferenceKey<K>, V>> supplier) {
131 return create(isSoft, false, supplier);
132 }
133
134 /**
135 * Create a new {@link ReferencedKeyMap} map.
136 *
137 * @param isSoft true if {@link SoftReference} keys are to
138 * be used, {@link WeakReference} otherwise.
139 * @param useNativeQueue true if uses NativeReferenceQueue
140 * otherwise use {@link ReferenceQueue}.
141 * @param supplier {@link Supplier} of the backing map
142 *
143 * @return a new map with {@link Reference} keys
144 *
145 * @param <K> the type of keys maintained by the new map
146 * @param <V> the type of mapped values
147 */
148 public static <K, V> ReferencedKeyMap<K, V>
149 create(boolean isSoft, boolean useNativeQueue, Supplier<Map<ReferenceKey<K>, V>> supplier) {
150 return new ReferencedKeyMap<K, V>(isSoft, supplier.get(),
151 useNativeQueue ? SharedSecrets.getJavaLangRefAccess().newNativeReferenceQueue()
152 : new ReferenceQueue<>()
153 );
154 }
155
156 /**
157 * {@return a key suitable for a map entry}
158 *
159 * @param key unwrapped key
160 */
161 @SuppressWarnings("unchecked")
162 private ReferenceKey<K> entryKey(Object key) {
163 if (isSoft) {
164 return new SoftReferenceKey<>((K)key, stale);
165 } else {
166 return new WeakReferenceKey<>((K)key, stale);
167 }
168 }
169
170 /**
171 * {@return a key suitable for lookup}
172 *
173 * @param key unwrapped key
174 */
175 @SuppressWarnings("unchecked")
176 private ReferenceKey<K> lookupKey(Object key) {
177 return new StrongReferenceKey<>((K)key);
178 }
179
180 @Override
181 public int size() {
182 removeStaleReferences();
183 return map.size();
184 }
185
186 @Override
187 public boolean isEmpty() {
188 removeStaleReferences();
189 return map.isEmpty();
190 }
191
192 @Override
193 public boolean containsKey(Object key) {
194 Objects.requireNonNull(key, "key must not be null");
195 removeStaleReferences();
196 return map.containsKey(lookupKey(key));
197 }
198
199 @Override
200 public boolean containsValue(Object value) {
201 Objects.requireNonNull(value, "value must not be null");
202 removeStaleReferences();
203 return map.containsValue(value);
204 }
205
206 @Override
207 public V get(Object key) {
208 Objects.requireNonNull(key, "key must not be null");
209 removeStaleReferences();
210 return map.get(lookupKey(key));
211 }
212
213 @Override
214 public V put(K key, V newValue) {
215 Objects.requireNonNull(key, "key must not be null");
216 Objects.requireNonNull(newValue, "value must not be null");
217 removeStaleReferences();
218 ReferenceKey<K> entryKey = entryKey(key);
219 // If {@code put} returns non-null then was actually a {@code replace}
220 // and older key was used. In that case the new key was not used and the
221 // reference marked stale.
222 V oldValue = map.put(entryKey, newValue);
223 if (oldValue != null) {
224 entryKey.unused();
225 }
226 return oldValue;
227 }
228
229 @Override
230 public V remove(Object key) {
231 // Rely on gc to clean up old key.
232 return map.remove(lookupKey(key));
233 }
234
235 @Override
236 public void putAll(Map<? extends K, ? extends V> m) {
237 removeStaleReferences();
238 for (Entry<? extends K, ? extends V> entry : m.entrySet()) {
239 K key = entry.getKey();
240 V value = entry.getValue();
241 put(key, value);
242 }
243 }
244
245 @Override
246 public void clear() {
247 removeStaleReferences();
248 // Rely on gc to clean up old keys.
249 map.clear();
250 }
251
252 /**
253 * Common routine for collecting the current set of keys.
254 *
255 * @return {@link Stream} of valid keys (unwrapped)
256 */
257 private Stream<K> filterKeySet() {
258 return map.keySet()
259 .stream()
260 .map(ReferenceKey::get)
261 .filter(Objects::nonNull);
262 }
263
264 @Override
265 public Set<K> keySet() {
266 removeStaleReferences();
267 return filterKeySet().collect(Collectors.toSet());
268 }
269
270 @Override
271 public Collection<V> values() {
272 removeStaleReferences();
273 return map.values();
274 }
275
276 @Override
277 public Set<Entry<K, V>> entrySet() {
278 removeStaleReferences();
279 return filterKeySet()
280 .map(k -> new AbstractMap.SimpleEntry<>(k, get(k)))
281 .collect(Collectors.toSet());
282 }
283
284 @Override
285 public V putIfAbsent(K key, V newValue) {
286 removeStaleReferences();
287 ReferenceKey<K> entryKey = entryKey(key);
288 // If {@code putIfAbsent} returns non-null then was actually a
289 // {@code replace} and older key was used. In that case the new key was
290 // not used and the reference marked stale.
291 V oldValue = map.putIfAbsent(entryKey, newValue);
292 if (oldValue != null) {
293 entryKey.unused();
294 }
295 return oldValue;
296 }
297
298 @Override
299 public boolean remove(Object key, Object value) {
300 // Rely on gc to clean up old key.
301 return map.remove(lookupKey(key), value);
302 }
303
304 @Override
305 public boolean replace(K key, V oldValue, V newValue) {
306 removeStaleReferences();
307 // If replace is successful then the older key will be used and the
308 // lookup key will suffice.
309 return map.replace(lookupKey(key), oldValue, newValue);
310 }
311
312 @Override
313 public V replace(K key, V value) {
314 removeStaleReferences();
315 // If replace is successful then the older key will be used and the
316 // lookup key will suffice.
317 return map.replace(lookupKey(key), value);
318 }
319
320 @Override
321 public String toString() {
322 removeStaleReferences();
323 return filterKeySet()
324 .map(k -> k + "=" + get(k))
325 .collect(Collectors.joining(", ", "{", "}"));
326 }
327
328 /**
329 * Removes enqueued weak references from map.
330 */
331 public void removeStaleReferences() {
332 while (true) {
333 Object key = stale.poll();
334 if (key == null) {
335 break;
336 }
337 map.remove(key);
338 }
339 }
340
341 /**
342 * Puts an entry where the key and the value are the same. Used for
343 * interning values in a set.
344 *
345 * @implNote Requires a {@link ReferencedKeyMap} whose {@code V} type
346 * is a {@code ReferenceKey<K>}. Otherwise, a {@link ClassCastException} will
347 * be thrown.
348 *
349 * @param setMap {@link ReferencedKeyMap} where interning takes place
350 * @param key key to add
351 *
352 * @param <T> type of key
353 *
354 * @return the old key instance if found otherwise the new key instance
355 *
356 * @throws ClassCastException if {@code V} is not {@code EntryKey<T>}
357 */
358 static <T> T intern(ReferencedKeyMap<T, ReferenceKey<T>> setMap, T key) {
359 T value = existingKey(setMap, key);
360 if (value != null) {
361 return value;
362 }
363 return internKey(setMap, key);
364 }
365
366 /**
367 * Puts an entry where the key and the value are the same. Used for
368 * interning values in a set.
369 *
370 * @implNote Requires a {@link ReferencedKeyMap} whose {@code V} type
371 * is a {@code ReferenceKey<K>}. Otherwise, a {@link ClassCastException} will
372 * be thrown.
373 *
374 * @param setMap {@link ReferencedKeyMap} where interning takes place
375 * @param key key to add
376 * @param interner operation to apply to key before adding to map
377 *
378 * @param <T> type of key
379 *
380 * @return the old key instance if found otherwise the new key instance
381 *
382 * @throws ClassCastException if {@code V} is not {@code EntryKey<T>}
383 *
384 * @implNote This version of intern should not be called during phase1
385 * using a lambda. Use an UnaryOperator instance instead.
386 */
387 static <T> T intern(ReferencedKeyMap<T, ReferenceKey<T>> setMap, T key, UnaryOperator<T> interner) {
388 T value = existingKey(setMap, key);
389 if (value != null) {
390 return value;
391 }
392 key = interner.apply(key);
393 return internKey(setMap, key);
394 }
395
396 /**
397 * Check if the key already exists in the map.
398 *
399 * @param setMap {@link ReferencedKeyMap} where interning takes place
400 * @param key key to test
401 *
402 * @param <T> type of key
403 *
404 * @return key if found otherwise null
405 */
406 private static <T> T existingKey(ReferencedKeyMap<T, ReferenceKey<T>> setMap, T key) {
407 setMap.removeStaleReferences();
408 ReferenceKey<T> entryKey = setMap.get(setMap.lookupKey(key));
409 return entryKey != null ? entryKey.get() : null;
410 }
411
412 /**
413 * Attempt to add key to map.
414 *
415 * @param setMap {@link ReferencedKeyMap} where interning takes place
416 * @param key key to add
417 *
418 * @param <T> type of key
419 *
420 * @return the old key instance if found otherwise the new key instance
421 */
422 private static <T> T internKey(ReferencedKeyMap<T, ReferenceKey<T>> setMap, T key) {
423 ReferenceKey<T> entryKey = setMap.entryKey(key);
424 T interned;
425 do {
426 setMap.removeStaleReferences();
427 ReferenceKey<T> existing = setMap.map.putIfAbsent(entryKey, entryKey);
428 if (existing == null) {
429 return key;
430 } else {
431 // If {@code putIfAbsent} returns non-null then was actually a
432 // {@code replace} and older key was used. In that case the new
433 // key was not used and the reference marked stale.
434 interned = existing.get();
435 if (interned != null) {
436 entryKey.unused();
437 }
438 }
439 } while (interned == null);
440 return interned;
441 }
442
443
444 /**
445 * Attempt to add key to map if absent.
446 *
447 * @param setMap {@link ReferencedKeyMap} where interning takes place
448 * @param key key to add
449 *
450 * @param <T> type of key
451 *
452 * @return true if the key was added
453 */
454 static <T> boolean internAddKey(ReferencedKeyMap<T, ReferenceKey<T>> setMap, T key) {
455 ReferenceKey<T> entryKey = setMap.entryKey(key);
456 setMap.removeStaleReferences();
457 ReferenceKey<T> existing = setMap.map.putIfAbsent(entryKey, entryKey);
458 if (existing == null) {
459 return true;
460 } else {
461 // If {@code putIfAbsent} returns non-null then was actually a
462 // {@code replace} and older key was used. In that case the new
463 // key was not used and the reference marked stale.
464 entryKey.unused();
465 return false;
466 }
467 }
468
469 }