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