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 @CheckReturnValue 179 ConcurrentMap<K, V> asMap(); 180 181 /** 182 * Performs any pending maintenance operations needed by the cache. Exactly which activities are 183 * performed -- if any -- is implementation-dependent. 184 */ 185 void cleanUp(); 186}