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/*
* Copyright (C) 2011 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.cache;
import com.google.common.annotations.GwtCompatible;
import com.google.common.collect.ImmutableMap;
import com.google.common.util.concurrent.ExecutionError;
import com.google.common.util.concurrent.UncheckedExecutionException;
import com.google.errorprone.annotations.CompatibleWith;
import java.util.Map;
import java.util.concurrent.Callable;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
import org.checkerframework.checker.nullness.qual.Nullable;
/**
* A semi-persistent mapping from keys to values. Cache entries are manually added using {@link
* #get(Object, Callable)} or {@link #put(Object, Object)}, and are stored in the cache until either
* evicted or manually invalidated. The common way to build instances is using {@link CacheBuilder}.
*
* <p>Implementations of this interface are expected to be thread-safe, and can be safely accessed
* by multiple concurrent threads.
*
* @author Charles Fry
* @since 10.0
*/
@GwtCompatible
public interface Cache<K, V> {
/**
* Returns the value associated with {@code key} in this cache, or {@code null} if there is no
* cached value for {@code key}.
*
* @since 11.0
*/
@Nullable
V getIfPresent(@CompatibleWith("K") Object key);
/**
* Returns the value associated with {@code key} in this cache, obtaining that value from {@code
* loader} if necessary. The method improves upon the conventional "if cached, return; otherwise
* create, cache and return" pattern. For further improvements, use {@link LoadingCache} and its
* {@link LoadingCache#get(Object) get(K)} method instead of this one.
*
* <p>Among the improvements that this method and {@code LoadingCache.get(K)} both provide are:
*
* <ul>
* <li>{@linkplain LoadingCache#get(Object) awaiting the result of a pending load} rather than
* starting a redundant one
* <li>eliminating the error-prone caching boilerplate
* <li>tracking load {@linkplain #stats statistics}
* </ul>
*
* <p>Among the further improvements that {@code LoadingCache} can provide but this method cannot:
*
* <ul>
* <li>consolidation of the loader logic to {@linkplain CacheBuilder#build(CacheLoader) a single
* authoritative location}
* <li>{@linkplain LoadingCache#refresh refreshing of entries}, including {@linkplain
* CacheBuilder#refreshAfterWrite automated refreshing}
* <li>{@linkplain LoadingCache#getAll bulk loading requests}, including {@linkplain
* CacheLoader#loadAll bulk loading implementations}
* </ul>
*
* <p><b>Warning:</b> For any given key, every {@code loader} used with it should compute the same
* value. Otherwise, a call that passes one {@code loader} may return the result of another call
* with a differently behaving {@code loader}. For example, a call that requests a short timeout
* for an RPC may wait for a similar call that requests a long timeout, or a call by an
* unprivileged user may return a resource accessible only to a privileged user making a similar
* call. To prevent this problem, create a key object that includes all values that affect the
* result of the query. Or use {@code LoadingCache.get(K)}, which lacks the ability to refer to
* state other than that in the key.
*
* <p><b>Warning:</b> as with {@link CacheLoader#load}, {@code loader} <b>must not</b> return
* {@code null}; it may either return a non-null value or throw an exception.
*
* <p>No observable state associated with this cache is modified until loading completes.
*
* @throws ExecutionException if a checked exception was thrown while loading the value
* @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
* value
* @throws ExecutionError if an error was thrown while loading the value
* @since 11.0
*/
V get(K key, Callable<? extends V> loader) throws ExecutionException;
/**
* Returns a map of the values associated with {@code keys} in this cache. The returned map will
* only contain entries which are already present in the cache.
*
* @since 11.0
*/
ImmutableMap<K, V> getAllPresent(Iterable<?> keys);
/**
* Associates {@code value} with {@code key} in this cache. If the cache previously contained a
* value associated with {@code key}, the old value is replaced by {@code value}.
*
* <p>Prefer {@link #get(Object, Callable)} when using the conventional "if cached, return;
* otherwise create, cache and return" pattern.
*
* @since 11.0
*/
void put(K key, V value);
/**
* Copies all of the mappings from the specified map to the cache. The effect of this call is
* equivalent to that of calling {@code put(k, v)} on this map once for each mapping from key
* {@code k} to value {@code v} in the specified map. The behavior of this operation is undefined
* if the specified map is modified while the operation is in progress.
*
* @since 12.0
*/
void putAll(Map<? extends K, ? extends V> m);
/** Discards any cached value for key {@code key}. */
void invalidate(@CompatibleWith("K") Object key);
/**
* Discards any cached values for keys {@code keys}.
*
* @since 11.0
*/
void invalidateAll(Iterable<?> keys);
/** Discards all entries in the cache. */
void invalidateAll();
/** Returns the approximate number of entries in this cache. */
long size();
/**
* Returns a current snapshot of this cache's cumulative statistics, or a set of default values if
* the cache is not recording statistics. All statistics begin at zero and never decrease over the
* lifetime of the cache.
*
* <p><b>Warning:</b> this cache may not be recording statistical data. For example, a cache
* created using {@link CacheBuilder} only does so if the {@link CacheBuilder#recordStats} method
* was called. If statistics are not being recorded, a {@code CacheStats} instance with zero for
* all values is returned.
*
*/
CacheStats stats();
/**
* Returns a view of the entries stored in this cache as a thread-safe map. Modifications made to
* the map directly affect the cache.
*
* <p>Iterators from the returned map are at least <i>weakly consistent</i>: they are safe for
* concurrent use, but if the cache is modified (including by eviction) after the iterator is
* created, it is undefined which of the changes (if any) will be reflected in that iterator.
*/
ConcurrentMap<K, V> asMap();
/**
* Performs any pending maintenance operations needed by the cache. Exactly which activities are
* performed -- if any -- is implementation-dependent.
*/
void cleanUp();
}
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