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 static com.google.common.base.Preconditions.checkNotNull;
018
019import com.google.common.annotations.GwtCompatible;
020import com.google.common.annotations.GwtIncompatible;
021import com.google.common.base.Function;
022import com.google.common.base.Supplier;
023import com.google.common.util.concurrent.Futures;
024import com.google.common.util.concurrent.ListenableFuture;
025import com.google.common.util.concurrent.ListenableFutureTask;
026import java.io.Serializable;
027import java.util.Map;
028import java.util.concurrent.Callable;
029import java.util.concurrent.Executor;
030
031/**
032 * Computes or retrieves values, based on a key, for use in populating a {@link LoadingCache}.
033 *
034 * <p>Most implementations will only need to implement {@link #load}. Other methods may be
035 * overridden as desired.
036 *
037 * <p>Usage example:
038 *
039 * <pre>{@code
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);
046 * }</pre>
047 *
048 * <p>Since this example doesn't support reloading or bulk loading, if you're able to use lambda
049 * expressions it can be specified even more easily:
050 *
051 * <pre>{@code
052 * CacheLoader<Key, Graph> loader = CacheLoader.from(key -> createExpensiveGraph(key));
053 * }</pre>
054 *
055 * @author Charles Fry
056 * @since 10.0
057 */
058@GwtCompatible(emulated = true)
059public abstract class CacheLoader<K, V> {
060  /**
061   * Constructor for use by subclasses.
062   */
063  protected CacheLoader() {}
064
065  /**
066   * Computes or retrieves the value corresponding to {@code key}.
067   *
068   * @param key the non-null key whose value should be loaded
069   * @return the value associated with {@code key}; <b>must not be null</b>
070   * @throws Exception if unable to load the result
071   * @throws InterruptedException if this method is interrupted. {@code InterruptedException} is
072   *     treated like any other {@code Exception} in all respects except that, when it is caught,
073   *     the thread's interrupt status is set
074   */
075  public abstract V load(K key) throws Exception;
076
077  /**
078   * Computes or retrieves a replacement value corresponding to an already-cached {@code key}. This
079   * method is called when an existing cache entry is refreshed by
080   * {@link CacheBuilder#refreshAfterWrite}, or through a call to {@link LoadingCache#refresh}.
081   *
082   * <p>This implementation synchronously delegates to {@link #load}. It is recommended that it be
083   * overridden with an asynchronous implementation when using
084   * {@link CacheBuilder#refreshAfterWrite}.
085   *
086   * <p><b>Note:</b> <i>all exceptions thrown by this method will be logged and then swallowed</i>.
087   *
088   * @param key the non-null key whose value should be loaded
089   * @param oldValue the non-null old value corresponding to {@code key}
090   * @return the future new value associated with {@code key}; <b>must not be null, must not return
091   *     null</b>
092   * @throws Exception if unable to reload the result
093   * @throws InterruptedException if this method is interrupted. {@code InterruptedException} is
094   *     treated like any other {@code Exception} in all respects except that, when it is caught,
095   *     the thread's interrupt status is set
096   * @since 11.0
097   */
098  @GwtIncompatible // Futures
099  public ListenableFuture<V> reload(K key, V oldValue) throws Exception {
100    checkNotNull(key);
101    checkNotNull(oldValue);
102    return Futures.immediateFuture(load(key));
103  }
104
105  /**
106   * Computes or retrieves the values corresponding to {@code keys}. This method is called by
107   * {@link LoadingCache#getAll}.
108   *
109   * <p>If the returned map doesn't contain all requested {@code keys} then the entries it does
110   * contain will be cached, but {@code getAll} will throw an exception. If the returned map
111   * contains extra keys not present in {@code keys} then all returned entries will be cached, but
112   * only the entries for {@code keys} will be returned from {@code getAll}.
113   *
114   * <p>This method should be overridden when bulk retrieval is significantly more efficient than
115   * many individual lookups. Note that {@link LoadingCache#getAll} will defer to individual calls
116   * to {@link LoadingCache#get} if this method is not overridden.
117   *
118   * @param keys the unique, non-null keys whose values should be loaded
119   * @return a map from each key in {@code keys} to the value associated with that key; <b>may not
120   *     contain null values</b>
121   * @throws Exception if unable to load the result
122   * @throws InterruptedException if this method is interrupted. {@code InterruptedException} is
123   *     treated like any other {@code Exception} in all respects except that, when it is caught,
124   *     the thread's interrupt status is set
125   * @since 11.0
126   */
127  public Map<K, V> loadAll(Iterable<? extends K> keys) throws Exception {
128    // This will be caught by getAll(), causing it to fall back to multiple calls to
129    // LoadingCache.get
130    throw new UnsupportedLoadingOperationException();
131  }
132
133  /**
134   * Returns a cache loader that uses {@code function} to load keys, and without supporting either
135   * reloading or bulk loading. This is most useful when you can pass a lambda expression. Otherwise
136   * it is useful mostly when you already have an existing function instance.
137   *
138   * @param function the function to be used for loading values; must never return {@code null}
139   * @return a cache loader that loads values by passing each key to {@code function}
140   */
141  public static <K, V> CacheLoader<K, V> from(Function<K, V> function) {
142    return new FunctionToCacheLoader<K, V>(function);
143  }
144
145  private static final class FunctionToCacheLoader<K, V> extends CacheLoader<K, V>
146      implements Serializable {
147    private final Function<K, V> computingFunction;
148
149    public FunctionToCacheLoader(Function<K, V> computingFunction) {
150      this.computingFunction = checkNotNull(computingFunction);
151    }
152
153    @Override
154    public V load(K key) {
155      return computingFunction.apply(checkNotNull(key));
156    }
157
158    private static final long serialVersionUID = 0;
159  }
160
161  /**
162   * Returns a cache loader based on an <i>existing</i> supplier instance. Note that there's no need
163   * to create a <i>new</i> supplier just to pass it in here; just subclass {@code CacheLoader} and
164   * implement {@link #load load} instead.
165   *
166   * @param supplier the supplier to be used for loading values; must never return {@code null}
167   * @return a cache loader that loads values by calling {@link Supplier#get}, irrespective of the
168   *     key
169   */
170  public static <V> CacheLoader<Object, V> from(Supplier<V> supplier) {
171    return new SupplierToCacheLoader<V>(supplier);
172  }
173
174  /**
175   * Returns a {@code CacheLoader} which wraps {@code loader}, executing calls to
176   * {@link CacheLoader#reload} using {@code executor}.
177   *
178   * <p>This method is useful only when {@code loader.reload} has a synchronous implementation, such
179   * as {@linkplain #reload the default implementation}.
180   *
181   * @since 17.0
182   */
183  @GwtIncompatible // Executor + Futures
184  public static <K, V> CacheLoader<K, V> asyncReloading(
185      final CacheLoader<K, V> loader, final Executor executor) {
186    checkNotNull(loader);
187    checkNotNull(executor);
188    return new CacheLoader<K, V>() {
189      @Override
190      public V load(K key) throws Exception {
191        return loader.load(key);
192      }
193
194      @Override
195      public ListenableFuture<V> reload(final K key, final V oldValue) throws Exception {
196        ListenableFutureTask<V> task =
197            ListenableFutureTask.create(
198                new Callable<V>() {
199                  @Override
200                  public V call() throws Exception {
201                    return loader.reload(key, oldValue).get();
202                  }
203                });
204        executor.execute(task);
205        return task;
206      }
207
208      @Override
209      public Map<K, V> loadAll(Iterable<? extends K> keys) throws Exception {
210        return loader.loadAll(keys);
211      }
212    };
213  }
214
215  private static final class SupplierToCacheLoader<V> extends CacheLoader<Object, V>
216      implements Serializable {
217    private final Supplier<V> computingSupplier;
218
219    public SupplierToCacheLoader(Supplier<V> computingSupplier) {
220      this.computingSupplier = checkNotNull(computingSupplier);
221    }
222
223    @Override
224    public V load(Object key) {
225      checkNotNull(key);
226      return computingSupplier.get();
227    }
228
229    private static final long serialVersionUID = 0;
230  }
231
232  /**
233   * Exception thrown by {@code loadAll()} to indicate that it is not supported.
234   *
235   * @since 19.0
236   */
237  public static final class UnsupportedLoadingOperationException
238      extends UnsupportedOperationException {
239    // Package-private because this should only be thrown by loadAll() when it is not overridden.
240    // Cache implementors may want to catch it but should not need to be able to throw it.
241    UnsupportedLoadingOperationException() {}
242  }
243
244  /**
245   * Thrown to indicate that an invalid response was returned from a call to {@link CacheLoader}.
246   *
247   * @since 11.0
248   */
249  public static final class InvalidCacheLoadException extends RuntimeException {
250    public InvalidCacheLoadException(String message) {
251      super(message);
252    }
253  }
254}