com.google.common.collect.Interners Maven / Gradle / Ivy
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This artifact provides a single jar that contains all classes required to use remote EJB and JMS, including
all dependencies. It is intended for use by those not using maven, maven users should just import the EJB and
JMS BOM's instead (shaded JAR's cause lots of problems with maven, as it is very easy to inadvertently end up
with different versions on classes on the class path).
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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;
}
}
}