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 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 * @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  @CanIgnoreReturnValue // TODO(b/27479612): consider removing this?
054  @Nullable V getIfPresent(@CompatibleWith("K") Object key);
055
056  /**
057   * Returns the value associated with {@code key} in this cache, obtaining that value from {@code
058   * loader} if necessary. The method improves upon the conventional "if cached, return; otherwise
059   * create, cache and return" pattern. For further improvements, use {@link LoadingCache} and its
060   * {@link LoadingCache#get(Object) get(K)} method instead of this one.
061   *
062   * <p>Among the improvements that this method and {@code LoadingCache.get(K)} both provide are:
063   *
064   * <ul>
065   *   <li>{@linkplain LoadingCache#get(Object) awaiting the result of a pending load} rather than
066   *       starting a redundant one
067   *   <li>eliminating the error-prone caching boilerplate
068   *   <li>tracking load {@linkplain #stats statistics}
069   * </ul>
070   *
071   * <p>Among the further improvements that {@code LoadingCache} can provide but this method cannot:
072   *
073   * <ul>
074   *   <li>consolidation of the loader logic to {@linkplain CacheBuilder#build(CacheLoader) a single
075   *       authoritative location}
076   *   <li>{@linkplain LoadingCache#refresh refreshing of entries}, including {@linkplain
077   *       CacheBuilder#refreshAfterWrite automated refreshing}
078   *   <li>{@linkplain LoadingCache#getAll bulk loading requests}, including {@linkplain
079   *       CacheLoader#loadAll bulk loading implementations}
080   * </ul>
081   *
082   * <p><b>Warning:</b> For any given key, every {@code loader} used with it should compute the same
083   * value. Otherwise, a call that passes one {@code loader} may return the result of another call
084   * with a differently behaving {@code loader}. For example, a call that requests a short timeout
085   * for an RPC may wait for a similar call that requests a long timeout, or a call by an
086   * unprivileged user may return a resource accessible only to a privileged user making a similar
087   * call. To prevent this problem, create a key object that includes all values that affect the
088   * result of the query. Or use {@code LoadingCache.get(K)}, which lacks the ability to refer to
089   * state other than that in the key.
090   *
091   * <p><b>Warning:</b> as with {@link CacheLoader#load}, {@code loader} <b>must not</b> return
092   * {@code null}; it may either return a non-null value or throw an exception.
093   *
094   * <p>No observable state associated with this cache is modified until loading completes.
095   *
096   * @throws ExecutionException if a checked exception was thrown while loading the value
097   * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
098   *     value
099   * @throws ExecutionError if an error was thrown while loading the value
100   * @since 11.0
101   */
102  @CanIgnoreReturnValue // TODO(b/27479612): consider removing this
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  long size();
154
155  /**
156   * Returns a current snapshot of this cache's cumulative statistics, or a set of default values if
157   * the cache is not recording statistics. All statistics begin at zero and never decrease over the
158   * lifetime of the cache.
159   *
160   * <p><b>Warning:</b> this cache may not be recording statistical data. For example, a cache
161   * created using {@link CacheBuilder} only does so if the {@link CacheBuilder#recordStats} method
162   * was called. If statistics are not being recorded, a {@code CacheStats} instance with zero for
163   * all values is returned.
164   *
165   */
166  CacheStats stats();
167
168  /**
169   * Returns a view of the entries stored in this cache as a thread-safe map. Modifications made to
170   * the map directly affect the cache.
171   *
172   * <p>Iterators from the returned map are at least <i>weakly consistent</i>: they are safe for
173   * concurrent use, but if the cache is modified (including by eviction) after the iterator is
174   * created, it is undefined which of the changes (if any) will be reflected in that iterator.
175   *
176   * <p><b>Warning to users of Java 8+:</b> do not call any of the new <i>default methods</i> that
177   * have been newly added to {@link ConcurrentMap}! These are marked with "Since: 1.8" in the
178   * {@code ConcurrentMap} documentation. They will not function correctly and it is impossible for
179   * Guava to fix them until Guava is ready to <i>require</i> Java 8 for all users.
180   */
181  ConcurrentMap<K, V> asMap();
182
183  /**
184   * Performs any pending maintenance operations needed by the cache. Exactly which activities are
185   * performed -- if any -- is implementation-dependent.
186   */
187  void cleanUp();
188}