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