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
015package com.google.common.collect;
016
017import static com.google.common.base.Preconditions.checkNotNull;
018
019import com.google.common.annotations.Beta;
020import com.google.common.annotations.GwtIncompatible;
021import com.google.common.base.Equivalence;
022import com.google.common.base.Function;
023import com.google.common.collect.MapMakerInternalMap.ReferenceEntry;
024
025import 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
034public 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}