001/*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
005 * in compliance with the License. You may obtain a copy of the License at
006 *
007 * http://www.apache.org/licenses/LICENSE-2.0
008 *
009 * Unless required by applicable law or agreed to in writing, software distributed under the License
010 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
011 * or implied. See the License for the specific language governing permissions and limitations under
012 * the License.
013 */
014
015package com.google.common.cache;
016
017import com.google.common.annotations.GwtCompatible;
018import com.google.common.collect.ImmutableMap;
019import com.google.common.util.concurrent.ExecutionError;
020import com.google.common.util.concurrent.UncheckedExecutionException;
021import com.google.errorprone.annotations.CheckReturnValue;
022import com.google.errorprone.annotations.CompatibleWith;
023import com.google.errorprone.annotations.DoNotMock;
024import java.util.Map;
025import java.util.concurrent.Callable;
026import java.util.concurrent.ConcurrentMap;
027import java.util.concurrent.ExecutionException;
028import org.checkerframework.checker.nullness.qual.Nullable;
029
030/**
031 * A semi-persistent mapping from keys to values. Cache entries are manually added using {@link
032 * #get(Object, Callable)} or {@link #put(Object, Object)}, and are stored in the cache until either
033 * evicted or manually invalidated. The common way to build instances is using {@link CacheBuilder}.
034 *
035 * <p>Implementations of this interface are expected to be thread-safe, and can be safely accessed
036 * by multiple concurrent threads.
037 *
038 * @author Charles Fry
039 * @since 10.0
040 */
041@DoNotMock("Use CacheBuilder.newBuilder().build()")
042@GwtCompatible
043public interface Cache<K, V> {
044
045  /**
046   * Returns the value associated with {@code key} in this cache, or {@code null} if there is no
047   * cached value for {@code key}.
048   *
049   * @since 11.0
050   */
051  @Nullable
052  V getIfPresent(@CompatibleWith("K") Object key);
053
054  /**
055   * Returns the value associated with {@code key} in this cache, obtaining that value from {@code
056   * loader} if necessary. The method improves upon the conventional "if cached, return; otherwise
057   * create, cache and return" pattern. For further improvements, use {@link LoadingCache} and its
058   * {@link LoadingCache#get(Object) get(K)} method instead of this one.
059   *
060   * <p>Among the improvements that this method and {@code LoadingCache.get(K)} both provide are:
061   *
062   * <ul>
063   *   <li>{@linkplain LoadingCache#get(Object) awaiting the result of a pending load} rather than
064   *       starting a redundant one
065   *   <li>eliminating the error-prone caching boilerplate
066   *   <li>tracking load {@linkplain #stats statistics}
067   * </ul>
068   *
069   * <p>Among the further improvements that {@code LoadingCache} can provide but this method cannot:
070   *
071   * <ul>
072   *   <li>consolidation of the loader logic to {@linkplain CacheBuilder#build(CacheLoader) a single
073   *       authoritative location}
074   *   <li>{@linkplain LoadingCache#refresh refreshing of entries}, including {@linkplain
075   *       CacheBuilder#refreshAfterWrite automated refreshing}
076   *   <li>{@linkplain LoadingCache#getAll bulk loading requests}, including {@linkplain
077   *       CacheLoader#loadAll bulk loading implementations}
078   * </ul>
079   *
080   * <p><b>Warning:</b> For any given key, every {@code loader} used with it should compute the same
081   * value. Otherwise, a call that passes one {@code loader} may return the result of another call
082   * with a differently behaving {@code loader}. For example, a call that requests a short timeout
083   * for an RPC may wait for a similar call that requests a long timeout, or a call by an
084   * unprivileged user may return a resource accessible only to a privileged user making a similar
085   * call. To prevent this problem, create a key object that includes all values that affect the
086   * result of the query. Or use {@code LoadingCache.get(K)}, which lacks the ability to refer to
087   * state other than that in the key.
088   *
089   * <p><b>Warning:</b> as with {@link CacheLoader#load}, {@code loader} <b>must not</b> return
090   * {@code null}; it may either return a non-null value or throw an exception.
091   *
092   * <p>No observable state associated with this cache is modified until loading completes.
093   *
094   * @throws ExecutionException if a checked exception was thrown while loading the value
095   * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
096   *     value
097   * @throws ExecutionError if an error was thrown while loading the value
098   * @since 11.0
099   */
100  V get(K key, Callable<? extends V> loader) throws ExecutionException;
101
102  /**
103   * Returns a map of the values associated with {@code keys} in this cache. The returned map will
104   * only contain entries which are already present in the cache.
105   *
106   * @since 11.0
107   */
108  ImmutableMap<K, V> getAllPresent(Iterable<?> keys);
109
110  /**
111   * Associates {@code value} with {@code key} in this cache. If the cache previously contained a
112   * value associated with {@code key}, the old value is replaced by {@code value}.
113   *
114   * <p>Prefer {@link #get(Object, Callable)} when using the conventional "if cached, return;
115   * otherwise create, cache and return" pattern.
116   *
117   * @since 11.0
118   */
119  void put(K key, V value);
120
121  /**
122   * Copies all of the mappings from the specified map to the cache. The effect of this call is
123   * equivalent to that of calling {@code put(k, v)} on this map once for each mapping from key
124   * {@code k} to value {@code v} in the specified map. The behavior of this operation is undefined
125   * if the specified map is modified while the operation is in progress.
126   *
127   * @since 12.0
128   */
129  void putAll(Map<? extends K, ? extends V> m);
130
131  /** Discards any cached value for key {@code key}. */
132  void invalidate(@CompatibleWith("K") Object key);
133
134  /**
135   * Discards any cached values for keys {@code keys}.
136   *
137   * @since 11.0
138   */
139  void invalidateAll(Iterable<?> keys);
140
141  /** Discards all entries in the cache. */
142  void invalidateAll();
143
144  /** Returns the approximate number of entries in this cache. */
145  @CheckReturnValue
146  long size();
147
148  /**
149   * Returns a current snapshot of this cache's cumulative statistics, or a set of default values if
150   * the cache is not recording statistics. All statistics begin at zero and never decrease over the
151   * lifetime of the cache.
152   *
153   * <p><b>Warning:</b> this cache may not be recording statistical data. For example, a cache
154   * created using {@link CacheBuilder} only does so if the {@link CacheBuilder#recordStats} method
155   * was called. If statistics are not being recorded, a {@code CacheStats} instance with zero for
156   * all values is returned.
157   *
158   */
159  @CheckReturnValue
160  CacheStats stats();
161
162  /**
163   * Returns a view of the entries stored in this cache as a thread-safe map. Modifications made to
164   * the map directly affect the cache.
165   *
166   * <p>Iterators from the returned map are at least <i>weakly consistent</i>: they are safe for
167   * concurrent use, but if the cache is modified (including by eviction) after the iterator is
168   * created, it is undefined which of the changes (if any) will be reflected in that iterator.
169   */
170  @CheckReturnValue
171  ConcurrentMap<K, V> asMap();
172
173  /**
174   * Performs any pending maintenance operations needed by the cache. Exactly which activities are
175   * performed -- if any -- is implementation-dependent.
176   */
177  void cleanUp();
178}