The MapMaker.java Java example source code
/*
* Copyright (C) 2009 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.checkArgument;
import static com.google.common.base.Preconditions.checkNotNull;
import static com.google.common.base.Preconditions.checkState;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Ascii;
import com.google.common.base.Equivalence;
import com.google.common.base.Function;
import com.google.common.base.MoreObjects;
import com.google.common.base.Throwables;
import com.google.common.collect.MapMakerInternalMap.Strength;
import com.google.errorprone.annotations.CanIgnoreReturnValue;
import java.io.Serializable;
import java.lang.ref.WeakReference;
import java.util.ConcurrentModificationException;
import java.util.Map;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
/**
* <p>A builder of {@link ConcurrentMap} instances that can have keys or values automatically
* wrapped in {@linkplain WeakReference weak} references.
*
* <p>Usage example: {@code
*
* ConcurrentMap<Request, Stopwatch> timers = new MapMaker()
* .concurrencyLevel(4)
* .weakKeys()
* .makeMap();}</pre>
*
* <p>These features are all optional; {@code new MapMaker().makeMap()} returns a valid concurrent
* map that behaves similarly to a {@link ConcurrentHashMap}.
*
* <p>The returned map is implemented as a hash table with similar performance characteristics to
* {@link ConcurrentHashMap}. It supports all optional operations of the {@code ConcurrentMap}
* interface. It does not permit null keys or values.
*
* <p>Note: by default, the returned map uses equality comparisons (the {@link Object#equals
* equals} method) to determine equality for keys or values. However, if {@link #weakKeys} was
* specified, the map uses identity ({@code ==}) comparisons instead for keys. Likewise, if
* {@link #weakValues} was specified, the map uses identity comparisons for values.
*
* <p>The view collections of the returned map have weakly consistent iterators. This means
* that they are safe for concurrent use, but if other threads modify the map after the iterator is
* created, it is undefined which of these changes, if any, are reflected in that iterator. These
* iterators never throw {@link ConcurrentModificationException}.
*
* <p>If {@link #weakKeys} or {@link #weakValues} are requested, it is possible for a key or value
* present in the map to be reclaimed by the
* garbage collector. Entries with reclaimed keys or values may be removed from the map on each map
* modification or on occasional map accesses; such entries may be counted by {@link Map#size}, but
* will never be visible to read or write operations. A partially-reclaimed entry is never exposed
* to the user. Any {@link java.util.Map.Entry} instance retrieved from the map's
* {@linkplain Map#entrySet entry set} is a snapshot of that entry's state at the time of retrieval;
* such entries do, however, support {@link java.util.Map.Entry#setValue}, which simply calls
* {@link Map#put} on the entry's key.
*
* <p>The maps produced by {@code MapMaker} are serializable, and the deserialized maps retain all
* the configuration properties of the original map. During deserialization, if the original map had
* used weak references, the entries are reconstructed as they were, but it's not unlikely they'll
* be quickly garbage-collected before they are ever accessed.
*
* <p>{@code new MapMaker().weakKeys().makeMap()} is a recommended replacement for
* {@link java.util.WeakHashMap}, but note that it compares keys using object identity whereas
* {@code WeakHashMap} uses {@link Object#equals}.
*
* @author Bob Lee
* @author Charles Fry
* @author Kevin Bourrillion
* @since 2.0
*/
@GwtCompatible(emulated = true)
public final class MapMaker {
private static final int DEFAULT_INITIAL_CAPACITY = 16;
private static final int DEFAULT_CONCURRENCY_LEVEL = 4;
static final int UNSET_INT = -1;
// TODO(kevinb): dispense with this after benchmarking
boolean useCustomMap;
int initialCapacity = UNSET_INT;
int concurrencyLevel = UNSET_INT;
Strength keyStrength;
Strength valueStrength;
Equivalence<Object> keyEquivalence;
/**
* Constructs a new {@code MapMaker} instance with default settings, including strong keys, strong
* values, and no automatic eviction of any kind.
*/
public MapMaker() {}
/**
* Sets a custom {@code Equivalence} strategy for comparing keys.
*
* <p>By default, the map uses {@link Equivalence#identity} to determine key equality when
* {@link #weakKeys} is specified, and {@link Equivalence#equals()} otherwise. The only place this
* is used is in {@link Interners.WeakInterner}.
*/
@CanIgnoreReturnValue
@GwtIncompatible // To be supported
MapMaker keyEquivalence(Equivalence<Object> equivalence) {
checkState(keyEquivalence == null, "key equivalence was already set to %s", keyEquivalence);
keyEquivalence = checkNotNull(equivalence);
this.useCustomMap = true;
return this;
}
Equivalence<Object> getKeyEquivalence() {
return MoreObjects.firstNonNull(keyEquivalence, getKeyStrength().defaultEquivalence());
}
/**
* Sets the minimum total size for the internal hash tables. For example, if the initial capacity
* is {@code 60}, and the concurrency level is {@code 8}, then eight segments are created, each
* having a hash table of size eight. Providing a large enough estimate at construction time
* avoids the need for expensive resizing operations later, but setting this value unnecessarily
* high wastes memory.
*
* @throws IllegalArgumentException if {@code initialCapacity} is negative
* @throws IllegalStateException if an initial capacity was already set
*/
@CanIgnoreReturnValue
public MapMaker initialCapacity(int initialCapacity) {
checkState(
this.initialCapacity == UNSET_INT,
"initial capacity was already set to %s",
this.initialCapacity);
checkArgument(initialCapacity >= 0);
this.initialCapacity = initialCapacity;
return this;
}
int getInitialCapacity() {
return (initialCapacity == UNSET_INT) ? DEFAULT_INITIAL_CAPACITY : initialCapacity;
}
/**
* Guides the allowed concurrency among update operations. Used as a hint for internal sizing. The
* table is internally partitioned to try to permit the indicated number of concurrent updates
* without contention. Because assignment of entries to these partitions is not necessarily
* uniform, the actual concurrency observed may vary. Ideally, you should choose a value to
* accommodate as many threads as will ever concurrently modify the table. Using a significantly
* higher value than you need can waste space and time, and a significantly lower value can lead
* to thread contention. But overestimates and underestimates within an order of magnitude do not
* usually have much noticeable impact. A value of one permits only one thread to modify the map
* at a time, but since read operations can proceed concurrently, this still yields higher
* concurrency than full synchronization. Defaults to 4.
*
* <p>Note: Prior to Guava release 9.0, the default was 16. It is possible the default will
* change again in the future. If you care about this value, you should always choose it
* explicitly.
*
* @throws IllegalArgumentException if {@code concurrencyLevel} is nonpositive
* @throws IllegalStateException if a concurrency level was already set
*/
@CanIgnoreReturnValue
public MapMaker concurrencyLevel(int concurrencyLevel) {
checkState(
this.concurrencyLevel == UNSET_INT,
"concurrency level was already set to %s",
this.concurrencyLevel);
checkArgument(concurrencyLevel > 0);
this.concurrencyLevel = concurrencyLevel;
return this;
}
int getConcurrencyLevel() {
return (concurrencyLevel == UNSET_INT) ? DEFAULT_CONCURRENCY_LEVEL : concurrencyLevel;
}
/**
* Specifies that each key (not value) stored in the map should be wrapped in a
* {@link WeakReference} (by default, strong references are used).
*
* <p>Warning: when this method is used, the resulting map will use identity ({@code ==})
* comparison to determine equality of keys, which is a technical violation of the {@link Map}
* specification, and may not be what you expect.
*
* @throws IllegalStateException if the key strength was already set
* @see WeakReference
*/
@CanIgnoreReturnValue
@GwtIncompatible // java.lang.ref.WeakReference
public MapMaker weakKeys() {
return setKeyStrength(Strength.WEAK);
}
MapMaker setKeyStrength(Strength strength) {
checkState(keyStrength == null, "Key strength was already set to %s", keyStrength);
keyStrength = checkNotNull(strength);
if (strength != Strength.STRONG) {
// STRONG could be used during deserialization.
useCustomMap = true;
}
return this;
}
Strength getKeyStrength() {
return MoreObjects.firstNonNull(keyStrength, Strength.STRONG);
}
/**
* Specifies that each value (not key) stored in the map should be wrapped in a
* {@link WeakReference} (by default, strong references are used).
*
* <p>Weak values will be garbage collected once they are weakly reachable. This makes them a poor
* candidate for caching.
*
* <p>Warning: when this method is used, the resulting map will use identity ({@code ==})
* comparison to determine equality of values. This technically violates the specifications of the
* methods {@link Map#containsValue containsValue}, {@link ConcurrentMap#remove(Object, Object)
* remove(Object, Object)} and {@link ConcurrentMap#replace(Object, Object, Object) replace(K, V,
* V)}, and may not be what you expect.
*
* @throws IllegalStateException if the value strength was already set
* @see WeakReference
*/
@CanIgnoreReturnValue
@GwtIncompatible // java.lang.ref.WeakReference
public MapMaker weakValues() {
return setValueStrength(Strength.WEAK);
}
MapMaker setValueStrength(Strength strength) {
checkState(valueStrength == null, "Value strength was already set to %s", valueStrength);
valueStrength = checkNotNull(strength);
if (strength != Strength.STRONG) {
// STRONG could be used during deserialization.
useCustomMap = true;
}
return this;
}
Strength getValueStrength() {
return MoreObjects.firstNonNull(valueStrength, Strength.STRONG);
}
/**
* Builds a thread-safe map. This method does not alter the state of this {@code MapMaker}
* instance, so it can be invoked again to create multiple independent maps.
*
* <p>The bulk operations {@code putAll}, {@code equals}, and {@code clear} are not guaranteed to
* be performed atomically on the returned map. Additionally, {@code size} and
* {@code containsValue} are implemented as bulk read operations, and thus may fail to observe
* concurrent writes.
*
* @return a serializable concurrent map having the requested features
*/
public <K, V> ConcurrentMap makeMap() {
if (!useCustomMap) {
return new ConcurrentHashMap<K, V>(getInitialCapacity(), 0.75f, getConcurrencyLevel());
}
return new MapMakerInternalMap<K, V>(this);
}
/**
* Returns a MapMakerInternalMap for the benefit of internal callers that use features of that
* class not exposed through ConcurrentMap.
*/
@GwtIncompatible // MapMakerInternalMap
<K, V> MapMakerInternalMap makeCustomMap() {
return new MapMakerInternalMap<K, V>(this);
}
/**
* Builds a map that supports atomic, on-demand computation of values. {@link Map#get} either
* returns an already-computed value for the given key, atomically computes it using the supplied
* function, or, if another thread is currently computing the value for this key, simply waits for
* that thread to finish and returns its computed value. Note that the function may be executed
* concurrently by multiple threads, but only for distinct keys.
*
* <p>New code should use {@link com.google.common.cache.CacheBuilder}, which supports
* {@linkplain com.google.common.cache.CacheStats statistics} collection, introduces the
* {@link com.google.common.cache.CacheLoader} interface for loading entries into the cache
* (allowing checked exceptions to be thrown in the process), and more cleanly separates
* computation from the cache's {@code Map} view.
*
* <p>If an entry's value has not finished computing yet, query methods besides {@code get} return
* immediately as if an entry doesn't exist. In other words, an entry isn't externally visible
* until the value's computation completes.
*
* <p>{@link Map#get} on the returned map will never return {@code null}. It may throw:
*
* <ul>
* <li>{@link NullPointerException} if the key is null or the computing function returns a null
* result
* <li>{@link ComputationException} if an exception was thrown by the computing function. If that
* exception is already of type {@link ComputationException} it is propagated directly; otherwise
* it is wrapped.
* </ul>
*
* <p>Note: Callers of {@code get} must ensure that the key argument is of type
* {@code K}. The {@code get} method accepts {@code Object}, so the key type is not checked at
* compile time. Passing an object of a type other than {@code K} can result in that object being
* unsafely passed to the computing function as type {@code K}, and unsafely stored in the map.
*
* <p>If {@link Map#put} is called before a computation completes, other threads waiting on the
* computation will wake up and return the stored value.
*
* <p>This method does not alter the state of this {@code MapMaker} instance, so it can be invoked
* again to create multiple independent maps.
*
* <p>Insertion, removal, update, and access operations on the returned map safely execute
* concurrently by multiple threads. Iterators on the returned map are weakly consistent,
* returning elements reflecting the state of the map at some point at or since the creation of
* the iterator. They do not throw {@link ConcurrentModificationException}, and may proceed
* concurrently with other operations.
*
* <p>The bulk operations {@code putAll}, {@code equals}, and {@code clear} are not guaranteed to
* be performed atomically on the returned map. Additionally, {@code size} and
* {@code containsValue} are implemented as bulk read operations, and thus may fail to observe
* concurrent writes.
*
* @param computingFunction the function used to compute new values
* @return a serializable concurrent map having the requested features
* @deprecated Caching functionality in {@code MapMaker} has been moved to
* {@link com.google.common.cache.CacheBuilder}, with {@link #makeComputingMap} being replaced
* by {@link com.google.common.cache.CacheBuilder#build}. See the
* <a href="https://github.com/google/guava/wiki/MapMakerMigration">MapMaker Migration
* Guide</a> for more details.
*/
@Deprecated
<K, V> ConcurrentMap makeComputingMap(Function computingFunction) {
return new MapMaker.ComputingMapAdapter<K, V>(this, computingFunction);
}
/**
* Returns a string representation for this MapMaker instance. The exact form of the returned
* string is not specificed.
*/
@Override
public String toString() {
MoreObjects.ToStringHelper s = MoreObjects.toStringHelper(this);
if (initialCapacity != UNSET_INT) {
s.add("initialCapacity", initialCapacity);
}
if (concurrencyLevel != UNSET_INT) {
s.add("concurrencyLevel", concurrencyLevel);
}
if (keyStrength != null) {
s.add("keyStrength", Ascii.toLowerCase(keyStrength.toString()));
}
if (valueStrength != null) {
s.add("valueStrength", Ascii.toLowerCase(valueStrength.toString()));
}
if (keyEquivalence != null) {
s.addValue("keyEquivalence");
}
return s.toString();
}
/**
* Overrides get() to compute on demand. Also throws an exception when {@code null} is returned
* from a computation.
*/
/*
* This might make more sense in ComputingConcurrentHashMap, but it causes a javac crash in some
* cases there: http://code.google.com/p/guava-libraries/issues/detail?id=950
*/
static final class ComputingMapAdapter<K, V> extends ComputingConcurrentHashMap
implements Serializable {
private static final long serialVersionUID = 0;
ComputingMapAdapter(MapMaker mapMaker, Function<? super K, ? extends V> computingFunction) {
super(mapMaker, computingFunction);
}
@SuppressWarnings("unchecked") // unsafe, which is one advantage of Cache over Map
@Override
public V get(Object key) {
V value;
try {
value = getOrCompute((K) key);
} catch (ExecutionException e) {
Throwable cause = e.getCause();
Throwables.propagateIfInstanceOf(cause, ComputationException.class);
throw new ComputationException(cause);
}
if (value == null) {
throw new NullPointerException(computingFunction + " returned null for key " + key + ".");
}
return value;
}
}
}
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