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 {@code Cache}.
034 *
035 * <p>Most implementations will only need to implement {@link #load}. Other methods may be
036 * overridden as desired.
037 *
038 * @author Charles Fry
039 * @since 10.0
040 */
041 @GwtCompatible(emulated = true)
042 public abstract class CacheLoader<K, V> {
043 /**
044 * Constructor for use by subclasses.
045 */
046 protected CacheLoader() {}
047
048 /**
049 * Computes or retrieves the value corresponding to {@code key}.
050 *
051 * @param key the non-null key whose value should be loaded
052 * @return the value associated with {@code key}; <b>must not be null</b>
053 * @throws Exception if unable to load the result
054 * @throws InterruptedException if this method is interrupted. {@code InterruptedException} is
055 * treated like any other {@code Exception} in all respects except that, when it is caught,
056 * the thread's interrupt status is set
057 */
058 public abstract V load(K key) throws Exception;
059
060 /**
061 * Computes or retrieves a replacement value corresponding to an already-cached {@code key}. This
062 * method is called when an existing cache entry is refreshed by
063 * {@link CacheBuilder#refreshAfterWrite}, or through a call to {@link Cache#refresh}.
064 *
065 * <p>This implementation synchronously delegates to {@link #load}. It is recommended that it be
066 * overridden with an asynchronous implementation when using
067 * {@link CacheBuilder#refreshAfterWrite}.
068 *
069 * <p><b>Note:</b> <i>all exceptions thrown by this method will be logged and then swallowed</i>.
070 *
071 * @param key the non-null key whose value should be loaded
072 * @param oldValue the non-null old value corresponding to {@code key}
073 * @return the future new value associated with {@code key};
074 * <b>must not be null, must not return null</b>
075 * @throws Exception if unable to reload the result
076 * @throws InterruptedException if this method is interrupted. {@code InterruptedException} is
077 * treated like any other {@code Exception} in all respects except that, when it is caught,
078 * the thread's interrupt status is set
079 * @since 11.0
080 */
081 @GwtIncompatible("Futures")
082 public ListenableFuture<V> reload(K key, V oldValue) throws Exception {
083 return Futures.immediateFuture(load(key));
084 }
085
086 /**
087 * Computes or retrieves the values corresponding to {@code keys}. This method is called by
088 * {@link Cache#getAll}.
089 *
090 * <p>If the returned map doesn't contain all requested {@code keys} then the entries it does
091 * contain will be cached, but {@code getAll} will throw an exception. If the returned map
092 * contains extra keys not present in {@code keys} then all returned entries will be cached,
093 * but only the entries for {@code keys} will be returned from {@code getAll}.
094 *
095 * <p>This method should be overriden when bulk retrieval is significantly more efficient than
096 * many individual lookups. Note that {@link Cache#getAll} will defer to individual calls to
097 * {@link Cache#get} if this method is not overriden.
098 *
099 * @param keys the unique, non-null keys whose values should be loaded
100 * @return a map from each key in {@code keys} to the value associated with that key;
101 * <b>may not contain null values</b>
102 * @throws Exception if unable to load the result
103 * @throws InterruptedException if this method is interrupted. {@code InterruptedException} is
104 * treated like any other {@code Exception} in all respects except that, when it is caught,
105 * the thread's interrupt status is set
106 * @since 11.0
107 */
108 public Map<K, V> loadAll(Iterable<? extends K> keys) throws Exception {
109 // This will be caught by getAll(), causing it to fall back to multiple calls to Cache.get
110 throw new UnsupportedLoadingOperationException();
111 }
112
113 /**
114 * Returns a cache loader based on an <i>existing</i> function instance. Note that there's no need
115 * to create a <i>new</i> function just to pass it in here; just subclass {@code CacheLoader} and
116 * implement {@link #load load} instead.
117 *
118 * @param function the function to be used for loading values; must never return {@code null}
119 * @return a cache loader that loads values by passing each key to {@code function}
120 */
121 @Beta
122 public static <K, V> CacheLoader<K, V> from(Function<K, V> function) {
123 return new FunctionToCacheLoader<K, V>(function);
124 }
125
126 private static final class FunctionToCacheLoader<K, V>
127 extends CacheLoader<K, V> implements Serializable {
128 private final Function<K, V> computingFunction;
129
130 public FunctionToCacheLoader(Function<K, V> computingFunction) {
131 this.computingFunction = checkNotNull(computingFunction);
132 }
133
134 @Override
135 public V load(K key) {
136 return computingFunction.apply(key);
137 }
138
139 private static final long serialVersionUID = 0;
140 }
141
142 /**
143 * Returns a cache loader based on an <i>existing</i> supplier instance. Note that there's no need
144 * to create a <i>new</i> supplier just to pass it in here; just subclass {@code CacheLoader} and
145 * implement {@link #load load} instead.
146 *
147 * @param supplier the supplier to be used for loading values; must never return {@code null}
148 * @return a cache loader that loads values by calling {@link Supplier#get}, irrespective of the
149 * key
150 */
151 @Beta
152 public static <V> CacheLoader<Object, V> from(Supplier<V> supplier) {
153 return new SupplierToCacheLoader<V>(supplier);
154 }
155
156 private static final class SupplierToCacheLoader<V>
157 extends CacheLoader<Object, V> implements Serializable {
158 private final Supplier<V> computingSupplier;
159
160 public SupplierToCacheLoader(Supplier<V> computingSupplier) {
161 this.computingSupplier = checkNotNull(computingSupplier);
162 }
163
164 @Override
165 public V load(Object key) {
166 return computingSupplier.get();
167 }
168
169 private static final long serialVersionUID = 0;
170 }
171
172 static final class UnsupportedLoadingOperationException extends UnsupportedOperationException {}
173
174 /**
175 * Thrown to indicate that an invalid response was returned from a call to {@link CacheLoader}.
176 *
177 * @since 11.0
178 */
179 public static final class InvalidCacheLoadException extends RuntimeException {
180 public InvalidCacheLoadException(String message) {
181 super(message);
182 }
183 }
184 }