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 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
045@ElementTypesAreNonnullByDefault
046public interface Cache<K, V> {
047
048  /**
049   * Returns the value associated with {@code key} in this cache, or {@code null} if there is no
050   * cached value for {@code key}.
051   *
052   * @since 11.0
053   */
054  @CheckForNull
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  V get(K key, Callable<? extends V> loader) throws ExecutionException;
104
105  /**
106   * Returns a map of the values associated with {@code keys} in this cache. The returned map will
107   * only contain entries which are already present in the cache.
108   *
109   * @since 11.0
110   */
111  /*
112   * <? extends Object> is mostly the same as <?> to plain Java. But to nullness checkers, they
113   * differ: <? extends Object> means "non-null types," while <?> means "all types."
114   */
115  ImmutableMap<K, V> getAllPresent(Iterable<? extends Object> keys);
116
117  /**
118   * Associates {@code value} with {@code key} in this cache. If the cache previously contained a
119   * value associated with {@code key}, the old value is replaced by {@code value}.
120   *
121   * <p>Prefer {@link #get(Object, Callable)} when using the conventional "if cached, return;
122   * otherwise create, cache and return" pattern.
123   *
124   * @since 11.0
125   */
126  void put(K key, V value);
127
128  /**
129   * Copies all of the mappings from the specified map to the cache. The effect of this call is
130   * equivalent to that of calling {@code put(k, v)} on this map once for each mapping from key
131   * {@code k} to value {@code v} in the specified map. The behavior of this operation is undefined
132   * if the specified map is modified while the operation is in progress.
133   *
134   * @since 12.0
135   */
136  void putAll(Map<? extends K, ? extends V> m);
137
138  /** Discards any cached value for key {@code key}. */
139  void invalidate(@CompatibleWith("K") Object key);
140
141  /**
142   * Discards any cached values for keys {@code keys}.
143   *
144   * @since 11.0
145   */
146  // For discussion of <? extends Object>, see getAllPresent.
147  void invalidateAll(Iterable<? extends Object> keys);
148
149  /** Discards all entries in the cache. */
150  void invalidateAll();
151
152  /** Returns the approximate number of entries in this cache. */
153  @CheckReturnValue
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  @CheckReturnValue
168  CacheStats stats();
169
170  /**
171   * Returns a view of the entries stored in this cache as a thread-safe map. Modifications made to
172   * the map directly affect the cache.
173   *
174   * <p>Iterators from the returned map are at least <i>weakly consistent</i>: they are safe for
175   * concurrent use, but if the cache is modified (including by eviction) after the iterator is
176   * created, it is undefined which of the changes (if any) will be reflected in that iterator.
177   *
178   * <p><b>Warning to users of Java 8+:</b> do not call any of the new <i>default methods</i> that
179   * have been newly added to {@link ConcurrentMap}! These are marked with "Since: 1.8" in the
180   * {@code ConcurrentMap} documentation. They will not function correctly and it is impossible for
181   * Guava to fix them until Guava is ready to <i>require</i> Java 8 for all users.
182   */
183  @CheckReturnValue
184  ConcurrentMap<K, V> asMap();
185
186  /**
187   * Performs any pending maintenance operations needed by the cache. Exactly which activities are
188   * performed -- if any -- is implementation-dependent.
189   */
190  void cleanUp();
191}