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 * @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 LoadingCache#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 LoadingCache#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 LoadingCache#getAll} will defer to individual calls
097 * to {@link LoadingCache#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
110 // LoadingCache.get
111 throw new UnsupportedLoadingOperationException();
112 }
113
114 /**
115 * Returns a cache loader based on an <i>existing</i> function instance. Note that there's no need
116 * to create a <i>new</i> function just to pass it in here; just subclass {@code CacheLoader} and
117 * implement {@link #load load} instead.
118 *
119 * @param function the function to be used for loading values; must never return {@code null}
120 * @return a cache loader that loads values by passing each key to {@code function}
121 */
122 @Beta
123 public static <K, V> CacheLoader<K, V> from(Function<K, V> function) {
124 return new FunctionToCacheLoader<K, V>(function);
125 }
126
127 private static final class FunctionToCacheLoader<K, V>
128 extends CacheLoader<K, V> implements Serializable {
129 private final Function<K, V> computingFunction;
130
131 public FunctionToCacheLoader(Function<K, V> computingFunction) {
132 this.computingFunction = checkNotNull(computingFunction);
133 }
134
135 @Override
136 public V load(K key) {
137 return computingFunction.apply(key);
138 }
139
140 private static final long serialVersionUID = 0;
141 }
142
143 /**
144 * Returns a cache loader based on an <i>existing</i> supplier instance. Note that there's no need
145 * to create a <i>new</i> supplier just to pass it in here; just subclass {@code CacheLoader} and
146 * implement {@link #load load} instead.
147 *
148 * @param supplier the supplier to be used for loading values; must never return {@code null}
149 * @return a cache loader that loads values by calling {@link Supplier#get}, irrespective of the
150 * key
151 */
152 @Beta
153 public static <V> CacheLoader<Object, V> from(Supplier<V> supplier) {
154 return new SupplierToCacheLoader<V>(supplier);
155 }
156
157 private static final class SupplierToCacheLoader<V>
158 extends CacheLoader<Object, V> implements Serializable {
159 private final Supplier<V> computingSupplier;
160
161 public SupplierToCacheLoader(Supplier<V> computingSupplier) {
162 this.computingSupplier = checkNotNull(computingSupplier);
163 }
164
165 @Override
166 public V load(Object key) {
167 return computingSupplier.get();
168 }
169
170 private static final long serialVersionUID = 0;
171 }
172
173 static final class UnsupportedLoadingOperationException extends UnsupportedOperationException {}
174
175 /**
176 * Thrown to indicate that an invalid response was returned from a call to {@link CacheLoader}.
177 *
178 * @since 11.0
179 */
180 public static final class InvalidCacheLoadException extends RuntimeException {
181 public InvalidCacheLoadException(String message) {
182 super(message);
183 }
184 }
185 }