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.CanIgnoreReturnValue;
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 javax.annotation.CheckForNull;
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 * @param <K> the type of the cache's keys, which are not permitted to be null
039 * @param <V> the type of the cache's values, which are not permitted to be null
040 * @author Charles Fry
041 * @since 10.0
042 */
043@DoNotMock("Use CacheBuilder.newBuilder().build()")
044@GwtCompatible
045public interface Cache<K, V> {
046
047  /**
048   * Returns the value associated with {@code key} in this cache, or {@code null} if there is no
049   * cached value for {@code key}.
050   *
051   * @since 11.0
052   */
053  @CheckForNull
054  @CanIgnoreReturnValue // TODO(b/27479612): consider removing this?
055  V getIfPresent(@CompatibleWith("K") Object key);
056
057  /**
058   * Returns the value associated with {@code key} in this cache, obtaining that value from {@code
059   * loader} if necessary. The method improves upon the conventional "if cached, return; otherwise
060   * create, cache and return" pattern. For further improvements, use {@link LoadingCache} and its
061   * {@link LoadingCache#get(Object) get(K)} method instead of this one.
062   *
063   * <p>Among the improvements that this method and {@code LoadingCache.get(K)} both provide are:
064   *
065   * <ul>
066   *   <li>{@linkplain LoadingCache#get(Object) awaiting the result of a pending load} rather than
067   *       starting a redundant one
068   *   <li>eliminating the error-prone caching boilerplate
069   *   <li>tracking load {@linkplain #stats statistics}
070   * </ul>
071   *
072   * <p>Among the further improvements that {@code LoadingCache} can provide but this method cannot:
073   *
074   * <ul>
075   *   <li>consolidation of the loader logic to {@linkplain CacheBuilder#build(CacheLoader) a single
076   *       authoritative location}
077   *   <li>{@linkplain LoadingCache#refresh refreshing of entries}, including {@linkplain
078   *       CacheBuilder#refreshAfterWrite automated refreshing}
079   *   <li>{@linkplain LoadingCache#getAll bulk loading requests}, including {@linkplain
080   *       CacheLoader#loadAll bulk loading implementations}
081   * </ul>
082   *
083   * <p><b>Warning:</b> For any given key, every {@code loader} used with it should compute the same
084   * value. Otherwise, a call that passes one {@code loader} may return the result of another call
085   * with a differently behaving {@code loader}. For example, a call that requests a short timeout
086   * for an RPC may wait for a similar call that requests a long timeout, or a call by an
087   * unprivileged user may return a resource accessible only to a privileged user making a similar
088   * call. To prevent this problem, create a key object that includes all values that affect the
089   * result of the query. Or use {@code LoadingCache.get(K)}, which lacks the ability to refer to
090   * state other than that in the key.
091   *
092   * <p><b>Warning:</b> as with {@link CacheLoader#load}, {@code loader} <b>must not</b> return
093   * {@code null}; it may either return a non-null value or throw an exception.
094   *
095   * <p>No observable state associated with this cache is modified until loading completes.
096   *
097   * @throws ExecutionException if a checked exception was thrown while loading the value
098   * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
099   *     value
100   * @throws ExecutionError if an error was thrown while loading the value
101   * @since 11.0
102   */
103  @CanIgnoreReturnValue // TODO(b/27479612): consider removing this
104  V get(K key, Callable<? extends V> loader) throws ExecutionException;
105
106  /**
107   * Returns a map of the values associated with {@code keys} in this cache. The returned map will
108   * only contain entries which are already present in the cache.
109   *
110   * @since 11.0
111   */
112  /*
113   * <? extends Object> is mostly the same as <?> to plain Java. But to nullness checkers, they
114   * differ: <? extends Object> means "non-null types," while <?> means "all types."
115   */
116  ImmutableMap<K, V> getAllPresent(Iterable<? extends Object> keys);
117
118  /**
119   * Associates {@code value} with {@code key} in this cache. If the cache previously contained a
120   * value associated with {@code key}, the old value is replaced by {@code value}.
121   *
122   * <p>Prefer {@link #get(Object, Callable)} when using the conventional "if cached, return;
123   * otherwise create, cache and return" pattern.
124   *
125   * @since 11.0
126   */
127  void put(K key, V value);
128
129  /**
130   * Copies all of the mappings from the specified map to the cache. The effect of this call is
131   * equivalent to that of calling {@code put(k, v)} on this map once for each mapping from key
132   * {@code k} to value {@code v} in the specified map. The behavior of this operation is undefined
133   * if the specified map is modified while the operation is in progress.
134   *
135   * @since 12.0
136   */
137  void putAll(Map<? extends K, ? extends V> m);
138
139  /** Discards any cached value for key {@code key}. */
140  void invalidate(@CompatibleWith("K") Object key);
141
142  /**
143   * Discards any cached values for keys {@code keys}.
144   *
145   * @since 11.0
146   */
147  // For discussion of <? extends Object>, see getAllPresent.
148  void invalidateAll(Iterable<? extends Object> keys);
149
150  /** Discards all entries in the cache. */
151  void invalidateAll();
152
153  /** Returns the approximate number of entries in this cache. */
154  long size();
155
156  /**
157   * Returns a current snapshot of this cache's cumulative statistics, or a set of default values if
158   * the cache is not recording statistics. All statistics begin at zero and never decrease over the
159   * lifetime of the cache.
160   *
161   * <p><b>Warning:</b> this cache may not be recording statistical data. For example, a cache
162   * created using {@link CacheBuilder} only does so if the {@link CacheBuilder#recordStats} method
163   * was called. If statistics are not being recorded, a {@code CacheStats} instance with zero for
164   * all values is returned.
165   *
166   */
167  CacheStats stats();
168
169  /**
170   * Returns a view of the entries stored in this cache as a thread-safe map. Modifications made to
171   * the map directly affect the cache.
172   *
173   * <p>Iterators from the returned map are at least <i>weakly consistent</i>: they are safe for
174   * concurrent use, but if the cache is modified (including by eviction) after the iterator is
175   * created, it is undefined which of the changes (if any) will be reflected in that iterator.
176   */
177  ConcurrentMap<K, V> asMap();
178
179  /**
180   * Performs any pending maintenance operations needed by the cache. Exactly which activities are
181   * performed -- if any -- is implementation-dependent.
182   */
183  void cleanUp();
184}