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. The common way to build instances is using 031 * {@link CacheBuilder}. 032 * 033 * <p>Implementations of this interface are expected to be thread-safe, and can be safely accessed 034 * by multiple concurrent threads. 035 * 036 * @author Charles Fry 037 * @since 10.0 038 */ 039@GwtCompatible 040public interface Cache<K, V> { 041 042 /** 043 * Returns the value associated with {@code key} in this cache, or {@code null} if there is no 044 * cached value for {@code key}. 045 * 046 * @since 11.0 047 */ 048 @Nullable 049 V getIfPresent(Object key); 050 051 /** 052 * Returns the value associated with {@code key} in this cache, obtaining that value from {@code 053 * loader} if necessary. The method improves upon the conventional "if cached, return; otherwise 054 * create, cache and return" pattern. For further improvements, use {@link LoadingCache} and its 055 * {@link LoadingCache#get(Object) get(K)} method instead of this one. 056 * 057 * <p>Among the improvements that this method and {@code LoadingCache.get(K)} both provide are: 058 * 059 * <ul> 060 * <li>{@linkplain LoadingCache#get(Object) awaiting the result of a pending load} rather than 061 * starting a redundant one 062 * <li>eliminating the error-prone caching boilerplate 063 * <li>tracking load {@linkplain #stats statistics} 064 * </ul> 065 * 066 * <p>Among the further improvements that {@code LoadingCache} can provide but this method cannot: 067 * 068 * <ul> 069 * <li>consolidation of the loader logic to {@linkplain CacheBuilder#build(CacheLoader) a single 070 * authoritative location} 071 * <li>{@linkplain LoadingCache#refresh refreshing of entries}, including {@linkplain 072 * CacheBuilder#refreshAfterWrite automated refreshing} 073 * <li>{@linkplain LoadingCache#getAll bulk loading requests}, including {@linkplain 074 * CacheLoader#loadAll bulk loading implementations} 075 * </ul> 076 * 077 * <p><b>Warning:</b> For any given key, every {@code loader} used with it should compute the same 078 * value. Otherwise, a call that passes one {@code loader} may return the result of another call 079 * with a differently behaving {@code loader}. For example, a call that requests a short timeout 080 * for an RPC may wait for a similar call that requests a long timeout, or a call by an 081 * unprivileged user may return a resource accessible only to a privileged user making a similar 082 * call. To prevent this problem, create a key object that includes all values that affect the 083 * result of the query. Or use {@code LoadingCache.get(K)}, which lacks the ability to refer to 084 * state other than that in the key. 085 * 086 * <p><b>Warning:</b> as with {@link CacheLoader#load}, {@code loader} <b>must not</b> return 087 * {@code null}; it may either return a non-null value or throw an exception. 088 * 089 * <p>No observable state associated with this cache is modified until loading completes. 090 * 091 * @throws ExecutionException if a checked exception was thrown while loading the value 092 * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the 093 * value 094 * @throws ExecutionError if an error was thrown while loading the value 095 * 096 * @since 11.0 097 */ 098 V get(K key, Callable<? extends V> loader) throws ExecutionException; 099 100 /** 101 * Returns a map of the values associated with {@code keys} in this cache. The returned map will 102 * only contain entries which are already present in the cache. 103 * 104 * @since 11.0 105 */ 106 ImmutableMap<K, V> getAllPresent(Iterable<?> keys); 107 108 /** 109 * Associates {@code value} with {@code key} in this cache. If the cache previously contained a 110 * value associated with {@code key}, the old value is replaced by {@code value}. 111 * 112 * <p>Prefer {@link #get(Object, Callable)} when using the conventional "if cached, return; 113 * otherwise create, cache and return" pattern. 114 * 115 * @since 11.0 116 */ 117 void put(K key, V value); 118 119 /** 120 * Copies all of the mappings from the specified map to the cache. The effect of this call is 121 * equivalent to that of calling {@code put(k, v)} on this map once for each mapping from key 122 * {@code k} to value {@code v} in the specified map. The behavior of this operation is undefined 123 * if the specified map is modified while the operation is in progress. 124 * 125 * @since 12.0 126 */ 127 void putAll(Map<? extends K, ? extends V> m); 128 129 /** 130 * Discards any cached value for key {@code key}. 131 */ 132 void invalidate(Object key); 133 134 /** 135 * Discards any cached values for keys {@code keys}. 136 * 137 * @since 11.0 138 */ 139 void invalidateAll(Iterable<?> keys); 140 141 /** 142 * Discards all entries in the cache. 143 */ 144 void invalidateAll(); 145 146 /** 147 * Returns the approximate number of entries in this cache. 148 */ 149 long size(); 150 151 /** 152 * Returns a current snapshot of this cache's cumulative statistics, or a set of default values if 153 * the cache is not recording statistics. All statistics begin at zero and never decrease over the 154 * lifetime of the cache. 155 * 156 * <p><b>Warning:</b> this cache may not be recording statistical data. For example, a cache 157 * created using {@link CacheBuilder} only does so if the {@link CacheBuilder#recordStats} method 158 * was called. If statistics are not being recorded, a {@code CacheStats} instance with zero for 159 * all values is returned. 160 * 161 */ 162 CacheStats stats(); 163 164 /** 165 * Returns a view of the entries stored in this cache as a thread-safe map. Modifications made to 166 * the map directly affect the cache. 167 * 168 * <p>Iterators from the returned map are at least <i>weakly consistent</i>: they are safe for 169 * concurrent use, but if the cache is modified (including by eviction) after the iterator is 170 * created, it is undefined which of the changes (if any) will be reflected in that iterator. 171 */ 172 ConcurrentMap<K, V> asMap(); 173 174 /** 175 * Performs any pending maintenance operations needed by the cache. Exactly which activities are 176 * performed -- if any -- is implementation-dependent. 177 */ 178 void cleanUp(); 179}