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
017package com.google.common.cache;
018
019import com.google.common.annotations.Beta;
020import com.google.common.annotations.GwtCompatible;
021import com.google.common.base.Function;
022import com.google.common.collect.ImmutableMap;
023import com.google.common.util.concurrent.ExecutionError;
024import com.google.common.util.concurrent.UncheckedExecutionException;
025
026import java.util.concurrent.ConcurrentMap;
027import java.util.concurrent.ExecutionException;
028
029/**
030 * A semi-persistent mapping from keys to values. Values are automatically loaded by the cache,
031 * and are stored in the cache until either evicted or manually invalidated.
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 * <p>When evaluated as a {@link Function}, a cache yields the same result as invoking
037 * {@link #getUnchecked}.
038 *
039 * <p>Note that while this class is still annotated as {@link Beta}, the API is frozen from a
040 * consumer's standpoint. In other words existing methods are all considered {@code non-Beta} and
041 * won't be changed without going through an 18 month deprecation cycle; however new methods may be
042 * added at any time.
043 *
044 * @author Charles Fry
045 * @since 11.0
046 */
047@Beta
048@GwtCompatible
049public interface LoadingCache<K, V> extends Cache<K, V>, Function<K, V> {
050
051  /**
052   * Returns the value associated with {@code key} in this cache, first loading that value if
053   * necessary. No observable state associated with this cache is modified until loading completes.
054   *
055   * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
056   * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
057   * multiple threads can concurrently load values for distinct keys.
058   *
059   * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
060   * into the cache. Newly loaded values are added to the cache using
061   * {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
062   * with {@code key} while the new value was loading then a removal notification will be sent for
063   * the new value.
064   *
065   * <p>If the cache loader associated with this cache is known not to throw checked
066   * exceptions, then prefer {@link #getUnchecked} over this method.
067   *
068   * @throws ExecutionException if a checked exception was thrown while loading the value. ({@code
069   *     ExecutionException} is thrown <a
070   *     href="http://code.google.com/p/guava-libraries/wiki/CachesExplained#Interruption">even if
071   *     computation was interrupted by an {@code InterruptedException}</a>.)
072   * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
073   *     value
074   * @throws ExecutionError if an error was thrown while loading the value
075   */
076  V get(K key) throws ExecutionException;
077
078  /**
079   * Returns the value associated with {@code key} in this cache, first loading that value if
080   * necessary. No observable state associated with this cache is modified until loading
081   * completes. Unlike {@link #get}, this method does not throw a checked exception, and thus should
082   * only be used in situations where checked exceptions are not thrown by the cache loader.
083   *
084   * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
085   * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
086   * multiple threads can concurrently load values for distinct keys.
087   *
088   * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
089   * into the cache. Newly loaded values are added to the cache using
090   * {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
091   * with {@code key} while the new value was loading then a removal notification will be sent for
092   * the new value.
093   *
094   * <p><b>Warning:</b> this method silently converts checked exceptions to unchecked exceptions,
095   * and should not be used with cache loaders which throw checked exceptions. In such cases use
096   * {@link #get} instead.
097   *
098   * @throws UncheckedExecutionException if an exception was thrown while loading the value. (As
099   *     explained in the last paragraph above, this should be an unchecked exception only.)
100   * @throws ExecutionError if an error was thrown while loading the value
101   */
102  V getUnchecked(K key);
103
104  /**
105   * Returns a map of the values associated with {@code keys}, creating or retrieving those values
106   * if necessary. The returned map contains entries that were already cached, combined with newly
107   * loaded entries; it will never contain null keys or values.
108   *
109   * <p>Caches loaded by a {@link CacheLoader} will issue a single request to
110   * {@link CacheLoader#loadAll} for all keys which are not already present in the cache. All
111   * entries returned by {@link CacheLoader#loadAll} will be stored in the cache, over-writing
112   * any previously cached values. This method will throw an exception if
113   * {@link CacheLoader#loadAll} returns {@code null}, returns a map containing null keys or values,
114   * or fails to return an entry for each requested key.
115   *
116   * <p>Note that duplicate elements in {@code keys}, as determined by {@link Object#equals}, will
117   * be ignored.
118   *
119   * @throws ExecutionException if a checked exception was thrown while loading the value. ({@code
120   *     ExecutionException} is thrown <a
121   *     href="http://code.google.com/p/guava-libraries/wiki/CachesExplained#Interruption">even if
122   *     computation was interrupted by an {@code InterruptedException}</a>.)
123   * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
124   *     values
125   * @throws ExecutionError if an error was thrown while loading the values
126   * @since 11.0
127   */
128  ImmutableMap<K, V> getAll(Iterable<? extends K> keys) throws ExecutionException;
129
130  /**
131   * Discouraged. Provided to satisfy the {@code Function} interface; use {@link #get} or
132   * {@link #getUnchecked} instead.
133   *
134   * @throws UncheckedExecutionException if an exception was thrown while loading the value. (As
135   *     described in the documentation for {@link #getUnchecked}, {@code LoadingCache} should be
136   *     used as a {@code Function} only with cache loaders that throw only unchecked exceptions.)
137   */
138  @Override
139  V apply(K key);
140
141  /**
142   * Loads a new value for key {@code key}, possibly asynchronously. While the new value is loading
143   * the previous value (if any) will continue to be returned by {@code get(key)} unless it is
144   * evicted. If the new value is loaded successfully it will replace the previous value in the
145   * cache; if an exception is thrown while refreshing the previous value will remain, <i>and the
146   * exception will be logged (using {@link java.util.logging.Logger}) and swallowed</i>.
147   *
148   * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#reload} if the
149   * cache currently contains a value for {@code key}, and {@link CacheLoader#load} otherwise.
150   * Loading is asynchronous only if {@link CacheLoader#reload} was overridden with an
151   * asynchronous implementation.
152   *
153   * <p>Returns without doing anything if another thread is currently loading the value for
154   * {@code key}. If the cache loader associated with this cache performs refresh asynchronously
155   * then this method may return before refresh completes.
156   *
157   * @since 11.0
158   */
159  void refresh(K key);
160
161  /**
162   * {@inheritDoc}
163   *
164   * <p><b>Note that although the view <i>is</i> modifiable, no method on the returned map will ever
165   * cause entries to be automatically loaded.</b>
166   */
167  @Override
168  ConcurrentMap<K, V> asMap();
169}