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.CompatibleWith;
022import java.util.Map;
023import java.util.concurrent.Callable;
024import java.util.concurrent.ConcurrentMap;
025import java.util.concurrent.ExecutionException;
026import org.checkerframework.checker.nullness.qual.Nullable;
027
028/**
029 * A semi-persistent mapping from keys to values. Cache entries are manually added using {@link
030 * #get(Object, Callable)} or {@link #put(Object, Object)}, and are stored in the cache until either
031 * evicted or manually invalidated. The common way to build instances is using {@link CacheBuilder}.
032 *
033 * <p>Implementations of this interface are expected to be thread-safe, and can be safely accessed
034 * by multiple concurrent threads.
035 *
036 * @author Charles Fry
037 * @since 10.0
038 */
039@GwtCompatible
040public interface Cache<K, V> {
041
042  /**
043   * Returns the value associated with {@code key} in this cache, or {@code null} if there is no
044   * cached value for {@code key}.
045   *
046   * @since 11.0
047   */
048  @Nullable
049  V getIfPresent(@CompatibleWith("K") Object key);
050
051  /**
052   * Returns the value associated with {@code key} in this cache, obtaining that value from {@code
053   * loader} if necessary. The method improves upon the conventional "if cached, return; otherwise
054   * create, cache and return" pattern. For further improvements, use {@link LoadingCache} and its
055   * {@link LoadingCache#get(Object) get(K)} method instead of this one.
056   *
057   * <p>Among the improvements that this method and {@code LoadingCache.get(K)} both provide are:
058   *
059   * <ul>
060   *   <li>{@linkplain LoadingCache#get(Object) awaiting the result of a pending load} rather than
061   *       starting a redundant one
062   *   <li>eliminating the error-prone caching boilerplate
063   *   <li>tracking load {@linkplain #stats statistics}
064   * </ul>
065   *
066   * <p>Among the further improvements that {@code LoadingCache} can provide but this method cannot:
067   *
068   * <ul>
069   *   <li>consolidation of the loader logic to {@linkplain CacheBuilder#build(CacheLoader) a single
070   *       authoritative location}
071   *   <li>{@linkplain LoadingCache#refresh refreshing of entries}, including {@linkplain
072   *       CacheBuilder#refreshAfterWrite automated refreshing}
073   *   <li>{@linkplain LoadingCache#getAll bulk loading requests}, including {@linkplain
074   *       CacheLoader#loadAll bulk loading implementations}
075   * </ul>
076   *
077   * <p><b>Warning:</b> For any given key, every {@code loader} used with it should compute the same
078   * value. Otherwise, a call that passes one {@code loader} may return the result of another call
079   * with a differently behaving {@code loader}. For example, a call that requests a short timeout
080   * for an RPC may wait for a similar call that requests a long timeout, or a call by an
081   * unprivileged user may return a resource accessible only to a privileged user making a similar
082   * call. To prevent this problem, create a key object that includes all values that affect the
083   * result of the query. Or use {@code LoadingCache.get(K)}, which lacks the ability to refer to
084   * state other than that in the key.
085   *
086   * <p><b>Warning:</b> as with {@link CacheLoader#load}, {@code loader} <b>must not</b> return
087   * {@code null}; it may either return a non-null value or throw an exception.
088   *
089   * <p>No observable state associated with this cache is modified until loading completes.
090   *
091   * @throws ExecutionException if a checked exception was thrown while loading the value
092   * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
093   *     value
094   * @throws ExecutionError if an error was thrown while loading the value
095   * @since 11.0
096   */
097  V get(K key, Callable<? extends V> loader) throws ExecutionException;
098
099  /**
100   * Returns a map of the values associated with {@code keys} in this cache. The returned map will
101   * only contain entries which are already present in the cache.
102   *
103   * @since 11.0
104   */
105  ImmutableMap<K, V> getAllPresent(Iterable<?> keys);
106
107  /**
108   * Associates {@code value} with {@code key} in this cache. If the cache previously contained a
109   * value associated with {@code key}, the old value is replaced by {@code value}.
110   *
111   * <p>Prefer {@link #get(Object, Callable)} when using the conventional "if cached, return;
112   * otherwise create, cache and return" pattern.
113   *
114   * @since 11.0
115   */
116  void put(K key, V value);
117
118  /**
119   * Copies all of the mappings from the specified map to the cache. The effect of this call is
120   * equivalent to that of calling {@code put(k, v)} on this map once for each mapping from key
121   * {@code k} to value {@code v} in the specified map. The behavior of this operation is undefined
122   * if the specified map is modified while the operation is in progress.
123   *
124   * @since 12.0
125   */
126  void putAll(Map<? extends K, ? extends V> m);
127
128  /** Discards any cached value for key {@code key}. */
129  void invalidate(@CompatibleWith("K") Object key);
130
131  /**
132   * Discards any cached values for keys {@code keys}.
133   *
134   * @since 11.0
135   */
136  void invalidateAll(Iterable<?> keys);
137
138  /** Discards all entries in the cache. */
139  void invalidateAll();
140
141  /** Returns the approximate number of entries in this cache. */
142  long size();
143
144  /**
145   * Returns a current snapshot of this cache's cumulative statistics, or a set of default values if
146   * the cache is not recording statistics. All statistics begin at zero and never decrease over the
147   * lifetime of the cache.
148   *
149   * <p><b>Warning:</b> this cache may not be recording statistical data. For example, a cache
150   * created using {@link CacheBuilder} only does so if the {@link CacheBuilder#recordStats} method
151   * was called. If statistics are not being recorded, a {@code CacheStats} instance with zero for
152   * all values is returned.
153   *
154   */
155  CacheStats stats();
156
157  /**
158   * Returns a view of the entries stored in this cache as a thread-safe map. Modifications made to
159   * the map directly affect the cache.
160   *
161   * <p>Iterators from the returned map are at least <i>weakly consistent</i>: they are safe for
162   * concurrent use, but if the cache is modified (including by eviction) after the iterator is
163   * created, it is undefined which of the changes (if any) will be reflected in that iterator.
164   */
165  ConcurrentMap<K, V> asMap();
166
167  /**
168   * Performs any pending maintenance operations needed by the cache. Exactly which activities are
169   * performed -- if any -- is implementation-dependent.
170   */
171  void cleanUp();
172}