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/*
* 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.GwtIncompatible;
import com.google.common.annotations.J2ktIncompatible;
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;
import javax.annotation.CheckForNull;
/**
* Contains static methods pertaining to instances of {@link Interner}.
*
* @author Kevin Bourrillion
* @since 3.0
*/
@J2ktIncompatible
@GwtIncompatible
@ElementTypesAreNonnullByDefault
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 Interner build() {
if (!strong) {
mapMaker.weakKeys();
}
return new InternerImpl<>(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 Interner 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 Interner newWeakInterner() {
return newBuilder().weak().build();
}
@VisibleForTesting
static final class InternerImpl implements Interner {
// MapMaker is our friend, we know about this type
@VisibleForTesting final MapMakerInternalMap 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...
@SuppressWarnings("rawtypes") // using raw types to avoid a bug in our nullness checker :(
InternalEntry entry = map.getEntry(sample);
if (entry != null) {
Object canonical = entry.getKey();
if (canonical != null) { // only matters if weak/soft keys are used
// The compiler would know this is safe if not for our use of raw types (see above).
@SuppressWarnings("unchecked")
E result = (E) canonical;
return result;
}
}
// 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 Function asFunction(Interner interner) {
return new InternerFunction<>(checkNotNull(interner));
}
private static class InternerFunction implements Function {
private final Interner interner;
public InternerFunction(Interner 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(@CheckForNull Object other) {
if (other instanceof InternerFunction) {
InternerFunction that = (InternerFunction) other;
return interner.equals(that.interner);
}
return false;
}
}
}