1 /* |
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2 * Copyright (c) 2016, Oracle and/or its affiliates. All rights reserved. |
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3 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER. |
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4 * |
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5 * This code is free software; you can redistribute it and/or modify it |
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6 * under the terms of the GNU General Public License version 2 only, as |
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7 * published by the Free Software Foundation. Oracle designates this |
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8 * particular file as subject to the "Classpath" exception as provided |
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9 * by Oracle in the LICENSE file that accompanied this code. |
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10 * |
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11 * This code is distributed in the hope that it will be useful, but WITHOUT |
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12 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or |
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13 * FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License |
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14 * version 2 for more details (a copy is included in the LICENSE file that |
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15 * accompanied this code). |
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16 * |
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17 * You should have received a copy of the GNU General Public License version |
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18 * 2 along with this work; if not, write to the Free Software Foundation, |
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19 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA. |
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20 * |
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21 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA |
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22 * or visit www.oracle.com if you need additional information or have any |
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23 * questions. |
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24 */ |
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25 package java.lang.reflect; |
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26 |
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27 import java.lang.ref.Reference; |
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28 import java.lang.ref.ReferenceQueue; |
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29 import java.lang.ref.WeakReference; |
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30 import java.util.Collection; |
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31 import java.util.Objects; |
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32 import java.util.concurrent.ConcurrentHashMap; |
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33 import java.util.function.BiFunction; |
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34 |
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35 /** |
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36 * A WeakHashMap-like data structure that uses a pair of weakly-referenced keys |
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37 * with identity equality semantics to associate a strongly-referenced value. |
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38 * Unlike WeakHashMap, this data structure is thread-safe. |
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39 * |
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40 * @param <K1> the type of 1st key in key pair |
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41 * @param <K2> the type of 2nd key in key pair |
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42 * @param <V> the type of value |
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43 * @author Peter Levart |
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44 */ |
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45 final class WeakPairMap<K1, K2, V> { |
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46 |
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47 private final ConcurrentHashMap<Pair<K1, K2>, V> map = new ConcurrentHashMap<>(); |
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48 private final ReferenceQueue<Object> queue = new ReferenceQueue<>(); |
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49 |
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50 /** |
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51 * Tests if the specified pair of keys are associated with a value |
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52 * in the WeakPairMap. |
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53 * |
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54 * @param k1 the 1st of the pair of keys |
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55 * @param k2 the 2nd of the pair of keys |
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56 * @return true if and only if the specified key pair is in this WeakPairMap, |
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57 * as determined by the identity comparison; false otherwise |
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58 * @throws NullPointerException if any of the specified keys is null |
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59 */ |
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60 public boolean containsKeyPair(K1 k1, K2 k2) { |
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61 expungeStaleAssociations(); |
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62 return map.containsKey(Pair.lookup(k1, k2)); |
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63 } |
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64 |
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65 /** |
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66 * Returns the value to which the specified pair of keys is mapped, or null |
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67 * if this WeakPairMap contains no mapping for the key pair. |
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68 * <p>More formally, if this WeakPairMap contains a mapping from a key pair |
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69 * {@code (_k1, _k2)} to a value {@code v} such that |
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70 * {@code k1 == _k1 && k2 == _k2}, then this method returns {@code v}; |
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71 * otherwise it returns {@code null}. |
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72 * (There can be at most one such mapping.) |
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73 * |
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74 * @param k1 the 1st of the pair of keys for which the mapped value is to |
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75 * be returned |
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76 * @param k2 the 2nd of the pair of keys for which the mapped value is to |
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77 * be returned |
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78 * @return the value to which the specified key pair is mapped, or null if |
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79 * this map contains no mapping for the key pair |
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80 * @throws NullPointerException if any of the specified keys is null |
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81 */ |
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82 public V get(K1 k1, K2 k2) { |
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83 expungeStaleAssociations(); |
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84 return map.get(Pair.lookup(k1, k2)); |
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85 } |
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86 |
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87 /** |
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88 * Maps the specified key pair to the specified value in this WeakPairMap. |
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89 * Neither the keys nor the value can be null. |
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90 * <p>The value can be retrieved by calling the {@link #get} method |
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91 * with the the same keys (compared by identity). |
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92 * |
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93 * @param k1 the 1st of the pair of keys with which the specified value is to |
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94 * be associated |
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95 * @param k2 the 2nd of the pair of keys with which the specified value is to |
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96 * be associated |
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97 * @param v value to be associated with the specified key pair |
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98 * @return the previous value associated with key pair, or {@code null} if |
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99 * there was no mapping for key pair |
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100 * @throws NullPointerException if any of the specified keys or value is null |
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101 */ |
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102 public V put(K1 k1, K2 k2, V v) { |
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103 expungeStaleAssociations(); |
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104 return map.put(Pair.weak(k1, k2, queue), v); |
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105 } |
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106 |
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107 /** |
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108 * If the specified key pair is not already associated with a value, |
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109 * associates it with the given value and returns {@code null}, else does |
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110 * nothing and returns the currently associated value. |
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111 * |
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112 * @param k1 the 1st of the pair of keys with which the specified value is to |
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113 * be associated |
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114 * @param k2 the 2nd of the pair of keys with which the specified value is to |
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115 * be associated |
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116 * @param v value to be associated with the specified key pair |
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117 * @return the previous value associated with key pair, or {@code null} if |
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118 * there was no mapping for key pair |
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119 * @throws NullPointerException if any of the specified keys or value is null |
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120 */ |
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121 public V putIfAbsent(K1 k1, K2 k2, V v) { |
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122 expungeStaleAssociations(); |
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123 return map.putIfAbsent(Pair.weak(k1, k2, queue), v); |
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124 } |
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125 |
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126 /** |
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127 * If the specified key pair is not already associated with a value, |
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128 * attempts to compute its value using the given mapping function |
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129 * and enters it into this WeakPairMap unless {@code null}. The entire |
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130 * method invocation is performed atomically, so the function is |
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131 * applied at most once per key pair. Some attempted update operations |
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132 * on this WeakPairMap by other threads may be blocked while computation |
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133 * is in progress, so the computation should be short and simple, |
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134 * and must not attempt to update any other mappings of this WeakPairMap. |
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135 * |
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136 * @param k1 the 1st of the pair of keys with which the |
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137 * computed value is to be associated |
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138 * @param k2 the 2nd of the pair of keys with which the |
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139 * computed value is to be associated |
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140 * @param mappingFunction the function to compute a value |
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141 * @return the current (existing or computed) value associated with |
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142 * the specified key pair, or null if the computed value is null |
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143 * @throws NullPointerException if any of the specified keys or |
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144 * mappingFunction is null |
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145 * @throws IllegalStateException if the computation detectably |
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146 * attempts a recursive update to this map |
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147 * that would otherwise never complete |
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148 * @throws RuntimeException or Error if the mappingFunction does so, in |
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149 * which case the mapping is left unestablished |
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150 */ |
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151 public V computeIfAbsent(K1 k1, K2 k2, |
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152 BiFunction<? super K1, ? super K2, ? extends V> |
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153 mappingFunction) { |
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154 expungeStaleAssociations(); |
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155 try { |
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156 return map.computeIfAbsent( |
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157 Pair.weak(k1, k2, queue), |
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158 pair -> mappingFunction.apply(pair.first(), pair.second())); |
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159 } finally { |
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160 Reference.reachabilityFence(k1); |
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161 Reference.reachabilityFence(k2); |
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162 } |
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163 } |
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164 |
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165 /** |
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166 * Returns a {@link Collection} view of the values contained in this |
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167 * WeakPairMap. The collection is backed by the WeakPairMap, so changes to |
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168 * the map are reflected in the collection, and vice-versa. The collection |
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169 * supports element removal, which removes the corresponding |
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170 * mapping from this map, via the {@code Iterator.remove}, |
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171 * {@code Collection.remove}, {@code removeAll}, |
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172 * {@code retainAll}, and {@code clear} operations. It does not |
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173 * support the {@code add} or {@code addAll} operations. |
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174 * |
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175 * @return the collection view |
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176 */ |
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177 public Collection<V> values() { |
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178 expungeStaleAssociations(); |
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179 return map.values(); |
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180 } |
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181 |
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182 /** |
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183 * Removes associations from this WeakPairMap for which at least one of the |
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184 * keys in key pair has been found weakly-reachable and corresponding |
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185 * WeakRefPeer(s) enqueued. Called as part of each public operation. |
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186 */ |
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187 private void expungeStaleAssociations() { |
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188 WeakRefPeer<?> peer; |
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189 while ((peer = (WeakRefPeer<?>) queue.poll()) != null) { |
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190 map.remove(peer.weakPair()); |
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191 } |
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192 } |
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193 |
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194 /** |
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195 * Common interface of both {@link Weak} and {@link Lookup} key pairs. |
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196 */ |
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197 private interface Pair<K1, K2> { |
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198 |
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199 static <K1, K2> Pair<K1, K2> weak(K1 k1, K2 k2, |
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200 ReferenceQueue<Object> queue) { |
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201 return new Weak<>(k1, k2, queue); |
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202 } |
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203 |
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204 static <K1, K2> Pair<K1, K2> lookup(K1 k1, K2 k2) { |
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205 return new Lookup<>(k1, k2); |
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206 } |
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207 |
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208 /** |
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209 * @return The 1st of the pair of keys (may be null for {@link Weak} |
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210 * when it gets cleared) |
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211 */ |
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212 K1 first(); |
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213 |
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214 /** |
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215 * @return The 2nd of the pair of keys (may be null for {@link Weak} |
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216 * when it gets cleared) |
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217 */ |
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218 K2 second(); |
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219 |
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220 static int hashCode(Object first, Object second) { |
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221 // assert first != null && second != null; |
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222 return System.identityHashCode(first) ^ |
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223 System.identityHashCode(second); |
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224 } |
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225 |
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226 static boolean equals(Object first, Object second, Pair<?, ?> p) { |
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227 return first != null && second != null && |
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228 first == p.first() && second == p.second(); |
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229 } |
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230 |
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231 /** |
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232 * A Pair where both keys are weakly-referenced. |
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233 * It is composed of two instances of {@link WeakRefPeer}s: |
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234 * <pre>{@code |
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235 * |
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236 * +-referent-> [K1] +-referent-> [K2] |
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237 * | | |
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238 * +----------------+ +----------------+ |
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239 * | Pair.Weak <: |-----peer----->| (anonymous) <: | |
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240 * | WeakRefPeer, | | WeakRefPeer | |
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241 * | Pair |<--weakPair()--| | |
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242 * +----------------+ +----------------+ |
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243 * | ^ |
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244 * | | |
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245 * +-weakPair()-+ |
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246 * |
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247 * }</pre> |
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248 * <p> |
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249 * Pair.Weak is used for CHM keys. Both peers are associated with the |
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250 * same {@link ReferenceQueue} so when either of their referents |
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251 * becomes weakly-reachable, the corresponding entries can be |
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252 * {@link #expungeStaleAssociations() expunged} from the map. |
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253 */ |
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254 final class Weak<K1, K2> extends WeakRefPeer<K1> implements Pair<K1, K2> { |
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255 |
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256 // saved hash so it can be retrieved after the reference is cleared |
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257 private final int hash; |
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258 // link to <K2> peer |
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259 private final WeakRefPeer<K2> peer; |
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260 |
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261 Weak(K1 k1, K2 k2, ReferenceQueue<Object> queue) { |
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262 super(k1, queue); |
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263 hash = Pair.hashCode(k1, k2); |
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264 peer = new WeakRefPeer<>(k2, queue) { |
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265 // link back to <K1> peer |
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266 @Override |
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267 Weak<?, ?> weakPair() { return Weak.this; } |
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268 }; |
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269 } |
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270 |
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271 @Override |
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272 Weak<?, ?> weakPair() { |
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273 return this; |
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274 } |
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275 |
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276 @Override |
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277 public K1 first() { |
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278 return get(); |
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279 } |
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280 |
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281 @Override |
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282 public K2 second() { |
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283 return peer.get(); |
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284 } |
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285 |
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286 @Override |
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287 public int hashCode() { |
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288 return hash; |
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289 } |
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290 |
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291 @Override |
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292 public boolean equals(Object obj) { |
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293 return this == obj || |
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294 (obj instanceof Pair && |
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295 Pair.equals(first(), second(), (Pair<?, ?>) obj)); |
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296 } |
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297 } |
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298 |
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299 /** |
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300 * Optimized lookup Pair, used as lookup key in methods like |
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301 * {@link java.util.Map#get(Object)} or |
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302 * {@link java.util.Map#containsKey(Object)}) where |
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303 * there is a great chance its allocation is eliminated |
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304 * by escape analysis when such lookups are inlined by JIT. |
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305 * All its methods are purposely designed so that 'this' is never |
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306 * passed to any other method or used as identity. |
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307 */ |
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308 final class Lookup<K1, K2> implements Pair<K1, K2> { |
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309 private final K1 k1; |
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310 private final K2 k2; |
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311 |
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312 Lookup(K1 k1, K2 k2) { |
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313 this.k1 = Objects.requireNonNull(k1); |
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314 this.k2 = Objects.requireNonNull(k2); |
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315 } |
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316 |
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317 @Override |
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318 public K1 first() { |
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319 return k1; |
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320 } |
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321 |
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322 @Override |
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323 public K2 second() { |
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324 return k2; |
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325 } |
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326 |
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327 @Override |
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328 public int hashCode() { |
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329 return Pair.hashCode(k1, k2); |
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330 } |
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331 |
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332 @Override |
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333 public boolean equals(Object obj) { |
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334 return obj instanceof Pair && |
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335 Pair.equals(k1, k2, (Pair<?, ?>) obj); |
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336 } |
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337 } |
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338 } |
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339 |
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340 /** |
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341 * Common abstract supertype of a pair of WeakReference peers. |
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342 */ |
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343 private static abstract class WeakRefPeer<K> extends WeakReference<K> { |
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344 |
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345 WeakRefPeer(K k, ReferenceQueue<Object> queue) { |
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346 super(Objects.requireNonNull(k), queue); |
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347 } |
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348 |
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349 /** |
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350 * @return the {@link Pair.Weak} side of the pair of peers. |
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351 */ |
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352 abstract Pair.Weak<?, ?> weakPair(); |
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353 } |
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354 } |
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