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.Equivalence;
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 /**
055 * Returns a new thread-safe interner which retains a weak reference to each instance it has
056 * interned, and so does not prevent these instances from being garbage-collected. This most
057 * likely does not perform as well as {@link #newStrongInterner}, but is the best alternative
058 * when the memory usage of that implementation is unacceptable. Note that unlike {@link
059 * String#intern}, using this interner does not consume memory in the permanent generation.
060 */
061 @GwtIncompatible("java.lang.ref.WeakReference")
062 public static <E> Interner<E> newWeakInterner() {
063 return new WeakInterner<E>();
064 }
065
066 private static class WeakInterner<E> implements Interner<E> {
067 // MapMaker is our friend, we know about this type
068 private final MapMakerInternalMap<E, Dummy> map = new MapMaker()
069 .weakKeys()
070 .keyEquivalence(Equivalence.equals())
071 .makeCustomMap();
072
073 @Override public E intern(E sample) {
074 while (true) {
075 // trying to read the canonical...
076 ReferenceEntry<E, Dummy> entry = map.getEntry(sample);
077 if (entry != null) {
078 E canonical = entry.getKey();
079 if (canonical != null) { // only matters if weak/soft keys are used
080 return canonical;
081 }
082 }
083
084 // didn't see it, trying to put it instead...
085 Dummy sneaky = map.putIfAbsent(sample, Dummy.VALUE);
086 if (sneaky == null) {
087 return sample;
088 } else {
089 /* Someone beat us to it! Trying again...
090 *
091 * Technically this loop not guaranteed to terminate, so theoretically (extremely
092 * unlikely) this thread might starve, but even then, there is always going to be another
093 * thread doing progress here.
094 */
095 }
096 }
097 }
098
099 private enum Dummy { VALUE }
100 }
101
102 /**
103 * Returns a function that delegates to the {@link Interner#intern} method of the given interner.
104 *
105 * @since 8.0
106 */
107 public static <E> Function<E, E> asFunction(Interner<E> interner) {
108 return new InternerFunction<E>(checkNotNull(interner));
109 }
110
111 private static class InternerFunction<E> implements Function<E, E> {
112
113 private final Interner<E> interner;
114
115 public InternerFunction(Interner<E> interner) {
116 this.interner = interner;
117 }
118
119 @Override public E apply(E input) {
120 return interner.intern(input);
121 }
122
123 @Override public int hashCode() {
124 return interner.hashCode();
125 }
126
127 @Override public boolean equals(Object other) {
128 if (other instanceof InternerFunction) {
129 InternerFunction<?> that = (InternerFunction<?>) other;
130 return interner.equals(that.interner);
131 }
132
133 return false;
134 }
135 }
136 }