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