001 /*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017 package com.google.common.cache;
018
019 import static com.google.common.base.Preconditions.checkNotNull;
020
021 import com.google.common.annotations.Beta;
022 import com.google.common.annotations.GwtCompatible;
023 import com.google.common.annotations.GwtIncompatible;
024 import com.google.common.base.Function;
025 import com.google.common.base.Supplier;
026 import com.google.common.util.concurrent.Futures;
027 import com.google.common.util.concurrent.ListenableFuture;
028
029 import java.io.Serializable;
030 import java.util.Map;
031
032 /**
033 * Computes or retrieves values, based on a key, for use in populating a {@link LoadingCache}.
034 *
035 * <p>Most implementations will only need to implement {@link #load}. Other methods may be
036 * overridden as desired.
037 *
038 * <p>Usage example: <pre> {@code
039 *
040 * CacheLoader<Key, Graph> loader = new CacheLoader<Key, Graph>() {
041 * public Graph load(Key key) throws AnyException {
042 * return createExpensiveGraph(key);
043 * }
044 * };
045 * LoadingCache<Key, Graph> cache = CacheBuilder.newBuilder().build(loader);}</pre>
046 *
047 * @author Charles Fry
048 * @since 10.0
049 */
050 @GwtCompatible(emulated = true)
051 public abstract class CacheLoader<K, V> {
052 /**
053 * Constructor for use by subclasses.
054 */
055 protected CacheLoader() {}
056
057 /**
058 * Computes or retrieves the value corresponding to {@code key}.
059 *
060 * @param key the non-null key whose value should be loaded
061 * @return the value associated with {@code key}; <b>must not be null</b>
062 * @throws Exception if unable to load the result
063 * @throws InterruptedException if this method is interrupted. {@code InterruptedException} is
064 * treated like any other {@code Exception} in all respects except that, when it is caught,
065 * the thread's interrupt status is set
066 */
067 public abstract V load(K key) throws Exception;
068
069 /**
070 * Computes or retrieves a replacement value corresponding to an already-cached {@code key}. This
071 * method is called when an existing cache entry is refreshed by
072 * {@link CacheBuilder#refreshAfterWrite}, or through a call to {@link LoadingCache#refresh}.
073 *
074 * <p>This implementation synchronously delegates to {@link #load}. It is recommended that it be
075 * overridden with an asynchronous implementation when using
076 * {@link CacheBuilder#refreshAfterWrite}.
077 *
078 * <p><b>Note:</b> <i>all exceptions thrown by this method will be logged and then swallowed</i>.
079 *
080 * @param key the non-null key whose value should be loaded
081 * @param oldValue the non-null old value corresponding to {@code key}
082 * @return the future new value associated with {@code key};
083 * <b>must not be null, must not return null</b>
084 * @throws Exception if unable to reload the result
085 * @throws InterruptedException if this method is interrupted. {@code InterruptedException} is
086 * treated like any other {@code Exception} in all respects except that, when it is caught,
087 * the thread's interrupt status is set
088 * @since 11.0
089 */
090 @GwtIncompatible("Futures")
091 public ListenableFuture<V> reload(K key, V oldValue) throws Exception {
092 return Futures.immediateFuture(load(key));
093 }
094
095 /**
096 * Computes or retrieves the values corresponding to {@code keys}. This method is called by
097 * {@link LoadingCache#getAll}.
098 *
099 * <p>If the returned map doesn't contain all requested {@code keys} then the entries it does
100 * contain will be cached, but {@code getAll} will throw an exception. If the returned map
101 * contains extra keys not present in {@code keys} then all returned entries will be cached,
102 * but only the entries for {@code keys} will be returned from {@code getAll}.
103 *
104 * <p>This method should be overriden when bulk retrieval is significantly more efficient than
105 * many individual lookups. Note that {@link LoadingCache#getAll} will defer to individual calls
106 * to {@link LoadingCache#get} if this method is not overriden.
107 *
108 * @param keys the unique, non-null keys whose values should be loaded
109 * @return a map from each key in {@code keys} to the value associated with that key;
110 * <b>may not contain null values</b>
111 * @throws Exception if unable to load the result
112 * @throws InterruptedException if this method is interrupted. {@code InterruptedException} is
113 * treated like any other {@code Exception} in all respects except that, when it is caught,
114 * the thread's interrupt status is set
115 * @since 11.0
116 */
117 public Map<K, V> loadAll(Iterable<? extends K> keys) throws Exception {
118 // This will be caught by getAll(), causing it to fall back to multiple calls to
119 // LoadingCache.get
120 throw new UnsupportedLoadingOperationException();
121 }
122
123 /**
124 * Returns a cache loader based on an <i>existing</i> function instance. Note that there's no need
125 * to create a <i>new</i> function just to pass it in here; just subclass {@code CacheLoader} and
126 * implement {@link #load load} instead.
127 *
128 * @param function the function to be used for loading values; must never return {@code null}
129 * @return a cache loader that loads values by passing each key to {@code function}
130 */
131 @Beta
132 public static <K, V> CacheLoader<K, V> from(Function<K, V> function) {
133 return new FunctionToCacheLoader<K, V>(function);
134 }
135
136 private static final class FunctionToCacheLoader<K, V>
137 extends CacheLoader<K, V> implements Serializable {
138 private final Function<K, V> computingFunction;
139
140 public FunctionToCacheLoader(Function<K, V> computingFunction) {
141 this.computingFunction = checkNotNull(computingFunction);
142 }
143
144 @Override
145 public V load(K key) {
146 return computingFunction.apply(key);
147 }
148
149 private static final long serialVersionUID = 0;
150 }
151
152 /**
153 * Returns a cache loader based on an <i>existing</i> supplier instance. Note that there's no need
154 * to create a <i>new</i> supplier just to pass it in here; just subclass {@code CacheLoader} and
155 * implement {@link #load load} instead.
156 *
157 * @param supplier the supplier to be used for loading values; must never return {@code null}
158 * @return a cache loader that loads values by calling {@link Supplier#get}, irrespective of the
159 * key
160 */
161 @Beta
162 public static <V> CacheLoader<Object, V> from(Supplier<V> supplier) {
163 return new SupplierToCacheLoader<V>(supplier);
164 }
165
166 private static final class SupplierToCacheLoader<V>
167 extends CacheLoader<Object, V> implements Serializable {
168 private final Supplier<V> computingSupplier;
169
170 public SupplierToCacheLoader(Supplier<V> computingSupplier) {
171 this.computingSupplier = checkNotNull(computingSupplier);
172 }
173
174 @Override
175 public V load(Object key) {
176 return computingSupplier.get();
177 }
178
179 private static final long serialVersionUID = 0;
180 }
181
182 static final class UnsupportedLoadingOperationException extends UnsupportedOperationException {}
183
184 /**
185 * Thrown to indicate that an invalid response was returned from a call to {@link CacheLoader}.
186 *
187 * @since 11.0
188 */
189 public static final class InvalidCacheLoadException extends RuntimeException {
190 public InvalidCacheLoadException(String message) {
191 super(message);
192 }
193 }
194 }