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.*;
33 import java.util.concurrent.ConcurrentHashMap;
34 import java.util.function.Supplier;
35 import java.util.function.UnaryOperator;
36
37 /**
38 * This class provides management of {@link Set set} where it is desirable to
39 * remove elements automatically when the element is garbage collected. This is
40 * accomplished by using a backing map where the keys and values are either a
41 * {@link WeakReference} or a {@link SoftReference}.
42 * <p>
43 * To create a {@link ReferencedKeySet} the user must provide a {@link Supplier}
44 * of the backing map and whether {@link WeakReference} or
45 * {@link SoftReference} is to be used.
46 * {@snippet :
47 * Set<Long> set;
48 *
49 * set = ReferencedKeySet.create(false, HashMap::new);
50 * set.add(30_000_000L);
51 * set.add(30_000_001L);
52 * set.add(30_000_002L);
53 * set.add(30_000_003L);
54 * set.add(30_000_004L);
55 *
56 * set = ReferencedKeySet.create(true, ConcurrentHashMap::new);
57 * set.add(40_000_000L);
58 * set.add(40_000_001L);
59 * set.add(40_000_002L);
60 * set.add(40_000_003L);
61 * set.add(40_000_004L);
62 * }
63 *
64 * @implNote Care must be given that the backing map does replacement by
65 * replacing the value in the map entry instead of deleting the old entry and
66 * adding a new entry, otherwise replaced entries may end up with a strongly
67 * referenced key. {@link HashMap} and {@link ConcurrentHashMap} are known
68 * to be safe.
69 *
70 * @param <T> the type of elements maintained by this set
71 */
72 public final class ReferencedKeySet<T> extends AbstractSet<T> {
73 /**
74 * Backing {@link ReferencedKeySet} map.
75 */
76 final ReferencedKeyMap<T, ReferenceKey<T>> map;
77
78 /**
79 * @return a supplier to create a {@code ConcurrentHashMap} appropriate for use in the
80 * create methods.
81 * @param <E> the type of elements maintained by this set
82 */
83 public static <E> Supplier<Map<ReferenceKey<E>, ReferenceKey<E>>> concurrentHashMapSupplier() {
84 return ReferencedKeyMap.concurrentHashMapSupplier();
85 }
86
87 /**
88 * Private constructor.
89 *
90 * @param map backing map
91 */
92 private ReferencedKeySet(ReferencedKeyMap<T, ReferenceKey<T>> map) {
93 this.map = map;
94 }
95
96 /**
97 * Create a new {@link ReferencedKeySet} elements.
98 *
99 * @param isSoft true if {@link SoftReference} elements are to
100 * be used, {@link WeakReference} otherwise.
101 * @param supplier {@link Supplier} of the backing map
102 *
103 * @return a new set with {@link Reference} elements
104 *
105 * @param <E> the type of elements maintained by this set
106 */
107 public static <E> ReferencedKeySet<E>
108 create(boolean isSoft, Supplier<Map<ReferenceKey<E>, ReferenceKey<E>>> supplier) {
109 return new ReferencedKeySet<>(ReferencedKeyMap.create(isSoft, supplier));
110 }
111
112 /**
113 * Removes enqueued weak references from set.
114 */
115 public void removeStaleReferences() {
116 map.removeStaleReferences();
117 }
118
119 @Override
120 public Iterator<T> iterator() {
121 return map.keySet().iterator();
122 }
123
124 @Override
125 public int size() {
126 return map.size();
127 }
128
129 @Override
130 public boolean isEmpty() {
131 return map.isEmpty();
132 }
133
134 @Override
135 @SuppressWarnings("unchecked")
136 public boolean contains(Object o) {
137 return map.containsKey((T)o);
138 }
139
140 @Override
141 public boolean add(T e) {
142 return ReferencedKeyMap.internAddKey(map, e);
143 }
144
145 @Override
146 public boolean remove(Object o) {
147 return map.remove(o) != null;
148 }
149
150 @Override
151 public void clear() {
152 map.clear();
153 }
154
155 /**
156 * Gets an existing element from the set, returning null if not present or
157 * the old element if previously added.
158 *
159 * @param e element to get
160 *
161 * @return the old element if present, otherwise, null
162 */
163 public T get(T e) {
164 ReferenceKey<T> key = map.get(e);
165
166 return key == null ? null : key.get();
167 }
168
169 /**
170 * Intern an element to the set, returning the element if newly added or the
171 * old element if previously added.
172 *
173 * @param e element to add
174 *
175 * @return the old element if present, otherwise, the new element
176 */
177 public T intern(T e) {
178 return ReferencedKeyMap.intern(map, e);
179 }
180
181 /**
182 * Intern an element to the set, returning the element if newly added or the
183 * old element if previously added.
184 *
185 * @param e element to add
186 * @param interner operation to apply to key before adding to set
187 *
188 * @return the old element if present, otherwise, the new element
189 *
190 * @implNote This version of intern should not be called during phase1
191 * using a lambda. Use an UnaryOperator instance instead.
192 */
193 public T intern(T e, UnaryOperator<T> interner) {
194 return ReferencedKeyMap.intern(map, e, interner);
195 }
196 }