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    }