| /* |
| * Copyright (C) 2007 The Guava Authors |
| * |
| * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except |
| * in compliance with the License. You may obtain a copy of the License at |
| * |
| * http://www.apache.org/licenses/LICENSE-2.0 |
| * |
| * Unless required by applicable law or agreed to in writing, software distributed under the License |
| * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express |
| * or implied. See the License for the specific language governing permissions and limitations under |
| * the License. |
| */ |
| |
| package com.google.common.collect; |
| |
| import static com.google.common.base.Preconditions.checkNotNull; |
| |
| import com.google.common.annotations.Beta; |
| import com.google.common.annotations.GwtIncompatible; |
| import com.google.common.annotations.VisibleForTesting; |
| import com.google.common.base.Equivalence; |
| import com.google.common.base.Function; |
| import com.google.common.collect.MapMaker.Dummy; |
| import com.google.common.collect.MapMakerInternalMap.InternalEntry; |
| |
| /** |
| * Contains static methods pertaining to instances of {@link Interner}. |
| * |
| * @author Kevin Bourrillion |
| * @since 3.0 |
| */ |
| @Beta |
| @GwtIncompatible |
| public final class Interners { |
| private Interners() {} |
| |
| /** |
| * Builder for {@link Interner} instances. |
| * |
| * @since 21.0 |
| */ |
| public static class InternerBuilder { |
| private final MapMaker mapMaker = new MapMaker(); |
| private boolean strong = true; |
| |
| private InternerBuilder() {} |
| |
| /** |
| * Instructs the {@link InternerBuilder} to build a strong interner. |
| * |
| * @see Interners#newStrongInterner() |
| */ |
| public InternerBuilder strong() { |
| this.strong = true; |
| return this; |
| } |
| |
| /** |
| * Instructs the {@link InternerBuilder} to build a weak interner. |
| * |
| * @see Interners#newWeakInterner() |
| */ |
| @GwtIncompatible("java.lang.ref.WeakReference") |
| public InternerBuilder weak() { |
| this.strong = false; |
| return this; |
| } |
| |
| /** |
| * Sets the concurrency level that will be used by the to-be-built {@link Interner}. |
| * |
| * @see MapMaker#concurrencyLevel(int) |
| */ |
| public InternerBuilder concurrencyLevel(int concurrencyLevel) { |
| this.mapMaker.concurrencyLevel(concurrencyLevel); |
| return this; |
| } |
| |
| public <E> Interner<E> build() { |
| if (!strong) { |
| mapMaker.weakKeys(); |
| } |
| return new InternerImpl<E>(mapMaker); |
| } |
| } |
| |
| /** Returns a fresh {@link InternerBuilder} instance. */ |
| public static InternerBuilder newBuilder() { |
| return new InternerBuilder(); |
| } |
| |
| /** |
| * Returns a new thread-safe interner which retains a strong reference to each instance it has |
| * interned, thus preventing these instances from being garbage-collected. If this retention is |
| * acceptable, this implementation may perform better than {@link #newWeakInterner}. |
| */ |
| public static <E> Interner<E> newStrongInterner() { |
| return newBuilder().strong().build(); |
| } |
| |
| /** |
| * Returns a new thread-safe interner which retains a weak reference to each instance it has |
| * interned, and so does not prevent these instances from being garbage-collected. This most |
| * likely does not perform as well as {@link #newStrongInterner}, but is the best alternative when |
| * the memory usage of that implementation is unacceptable. |
| */ |
| @GwtIncompatible("java.lang.ref.WeakReference") |
| public static <E> Interner<E> newWeakInterner() { |
| return newBuilder().weak().build(); |
| } |
| |
| @VisibleForTesting |
| static final class InternerImpl<E> implements Interner<E> { |
| // MapMaker is our friend, we know about this type |
| @VisibleForTesting final MapMakerInternalMap<E, Dummy, ?, ?> map; |
| |
| private InternerImpl(MapMaker mapMaker) { |
| this.map = |
| MapMakerInternalMap.createWithDummyValues(mapMaker.keyEquivalence(Equivalence.equals())); |
| } |
| |
| @Override |
| public E intern(E sample) { |
| while (true) { |
| // trying to read the canonical... |
| InternalEntry<E, Dummy, ?> entry = map.getEntry(sample); |
| if (entry != null) { |
| E canonical = entry.getKey(); |
| if (canonical != null) { // only matters if weak/soft keys are used |
| return canonical; |
| } |
| } |
| |
| // didn't see it, trying to put it instead... |
| Dummy sneaky = map.putIfAbsent(sample, Dummy.VALUE); |
| if (sneaky == null) { |
| return sample; |
| } else { |
| /* Someone beat us to it! Trying again... |
| * |
| * Technically this loop not guaranteed to terminate, so theoretically (extremely |
| * unlikely) this thread might starve, but even then, there is always going to be another |
| * thread doing progress here. |
| */ |
| } |
| } |
| } |
| } |
| |
| /** |
| * Returns a function that delegates to the {@link Interner#intern} method of the given interner. |
| * |
| * @since 8.0 |
| */ |
| public static <E> Function<E, E> asFunction(Interner<E> interner) { |
| return new InternerFunction<E>(checkNotNull(interner)); |
| } |
| |
| private static class InternerFunction<E> implements Function<E, E> { |
| |
| private final Interner<E> interner; |
| |
| public InternerFunction(Interner<E> interner) { |
| this.interner = interner; |
| } |
| |
| @Override |
| public E apply(E input) { |
| return interner.intern(input); |
| } |
| |
| @Override |
| public int hashCode() { |
| return interner.hashCode(); |
| } |
| |
| @Override |
| public boolean equals(Object other) { |
| if (other instanceof InternerFunction) { |
| InternerFunction<?> that = (InternerFunction<?>) other; |
| return interner.equals(that.interner); |
| } |
| |
| return false; |
| } |
| } |
| } |