001 /*
002 * Copyright (C) 2007 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015 package com.google.common.collect;
016
017 import static com.google.common.base.Preconditions.checkNotNull;
018
019 import com.google.common.annotations.Beta;
020 import com.google.common.annotations.GwtIncompatible;
021 import com.google.common.base.Equivalences;
022 import com.google.common.base.Function;
023 import com.google.common.collect.MapMakerInternalMap.ReferenceEntry;
024
025 import java.util.concurrent.ConcurrentMap;
026
027 /**
028 * Contains static methods pertaining to instances of {@link Interner}.
029 *
030 * @author Kevin Bourrillion
031 * @since 3.0
032 */
033 @Beta
034 public final class Interners {
035 private Interners() {}
036
037 /**
038 * Returns a new thread-safe interner which retains a strong reference to each instance it has
039 * interned, thus preventing these instances from being garbage-collected. If this retention is
040 * acceptable, this implementation may perform better than {@link #newWeakInterner}. Note that
041 * unlike {@link String#intern}, using this interner does not consume memory in the permanent
042 * generation.
043 */
044 public static <E> Interner<E> newStrongInterner() {
045 final ConcurrentMap<E, E> map = new MapMaker().makeMap();
046 return new Interner<E>() {
047 @Override public E intern(E sample) {
048 E canonical = map.putIfAbsent(checkNotNull(sample), sample);
049 return (canonical == null) ? sample : canonical;
050 }
051 };
052 }
053
054 private static class CustomInterner<E> implements Interner<E> {
055 // MapMaker is our friend, we know about this type
056 private final MapMakerInternalMap<E, Dummy> map;
057
058 CustomInterner(GenericMapMaker<? super E, Object> mm) {
059 this.map = mm
060 .strongValues()
061 .keyEquivalence(Equivalences.equals())
062 .makeCustomMap();
063 }
064
065 @Override public E intern(E sample) {
066 while (true) {
067 // trying to read the canonical...
068 ReferenceEntry<E, Dummy> entry = map.getEntry(sample);
069 if (entry != null) {
070 E canonical = entry.getKey();
071 if (canonical != null) { // only matters if weak/soft keys are used
072 return canonical;
073 }
074 }
075
076 // didn't see it, trying to put it instead...
077 Dummy sneaky = map.putIfAbsent(sample, Dummy.VALUE);
078 if (sneaky == null) {
079 return sample;
080 } else {
081 /* Someone beat us to it! Trying again...
082 *
083 * Technically this loop not guaranteed to terminate, so theoretically (extremely
084 * unlikely) this thread might starve, but even then, there is always going to be another
085 * thread doing progress here.
086 */
087 }
088 }
089 }
090
091 private enum Dummy { VALUE }
092 }
093
094 /**
095 * Returns a new thread-safe interner which retains a weak reference to each instance it has
096 * interned, and so does not prevent these instances from being garbage-collected. This most
097 * likely does not perform as well as {@link #newStrongInterner}, but is the best alternative
098 * when the memory usage of that implementation is unacceptable. Note that unlike {@link
099 * String#intern}, using this interner does not consume memory in the permanent generation.
100 */
101 @GwtIncompatible("java.lang.ref.WeakReference")
102 public static <E> Interner<E> newWeakInterner() {
103 return new CustomInterner<E>(new MapMaker().weakKeys());
104 }
105
106 /**
107 * Returns a function that delegates to the {@link Interner#intern} method of the given interner.
108 *
109 * @since 8.0
110 */
111 public static <E> Function<E, E> asFunction(Interner<E> interner) {
112 return new InternerFunction<E>(checkNotNull(interner));
113 }
114
115 private static class InternerFunction<E> implements Function<E, E> {
116
117 private final Interner<E> interner;
118
119 public InternerFunction(Interner<E> interner) {
120 this.interner = interner;
121 }
122
123 @Override public E apply(E input) {
124 return interner.intern(input);
125 }
126
127 @Override public int hashCode() {
128 return interner.hashCode();
129 }
130
131 @Override public boolean equals(Object other) {
132 if (other instanceof InternerFunction<?>) {
133 InternerFunction<?> that = (InternerFunction<?>) other;
134 return interner.equals(that.interner);
135 }
136
137 return false;
138 }
139 }
140 }