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>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
049 public 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
069 * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
070 * value
071 * @throws ExecutionError if an error was thrown while loading the value
072 */
073 V get(K key) throws ExecutionException;
074
075 /**
076 * Returns the value associated with {@code key} in this cache, first loading that value if
077 * necessary. No observable state associated with this cache is modified until loading
078 * completes. Unlike {@link #get}, this method does not throw a checked exception, and thus should
079 * only be used in situations where checked exceptions are not thrown by the cache loader.
080 *
081 * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
082 * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
083 * multiple threads can concurrently load values for distinct keys.
084 *
085 * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
086 * into the cache. Newly loaded values are added to the cache using
087 * {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
088 * with {@code key} while the new value was loading then a removal notification will be sent for
089 * the new value.
090 *
091 * <p><b>Warning:</b> this method silently converts checked exceptions to unchecked exceptions,
092 * and should not be used with cache loaders which throw checked exceptions. In such cases use
093 * {@link #get} instead.
094 *
095 * @throws UncheckedExecutionException if an exception was thrown while loading the value,
096 * regardless of whether the exception was checked or unchecked
097 * @throws ExecutionError if an error was thrown while loading the value
098 */
099 V getUnchecked(K key);
100
101 /**
102 * Returns a map of the values associated with {@code keys}, creating or retrieving those values
103 * if necessary. The returned map contains entries that were already cached, combined with newly
104 * loaded entries; it will never contain null keys or values.
105 *
106 * <p>Caches loaded by a {@link CacheLoader} will issue a single request to
107 * {@link CacheLoader#loadAll} for all keys which are not already present in the cache. All
108 * entries returned by {@link CacheLoader#loadAll} will be stored in the cache, over-writing
109 * any previously cached values. This method will throw an exception if
110 * {@link CacheLoader#loadAll} returns {@code null}, returns a map containing null keys or values,
111 * or fails to return an entry for each requested key.
112 *
113 * <p>Note that duplicate elements in {@code keys}, as determined by {@link Object#equals}, will
114 * be ignored.
115 *
116 * @throws ExecutionException if a checked exception was thrown while loading the values
117 * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
118 * values
119 * @throws ExecutionError if an error was thrown while loading the values
120 * @since 11.0
121 */
122 ImmutableMap<K, V> getAll(Iterable<? extends K> keys) throws ExecutionException;
123
124 /**
125 * Discouraged. Provided to satisfy the {@code Function} interface; use {@link #get} or
126 * {@link #getUnchecked} instead.
127 *
128 * @throws UncheckedExecutionException if an exception was thrown while loading the value,
129 * regardless of whether the exception was checked or unchecked
130 * @throws ExecutionError if an error was thrown while loading the value
131 */
132 @Override
133 V apply(K key);
134
135 /**
136 * Loads a new value for key {@code key}, possibly asynchronously. While the new value is loading
137 * the previous value (if any) will continue to be returned by {@code get(key)} unless it is
138 * evicted. If the new value is loaded successfully it will replace the previous value in the
139 * cache; if an exception is thrown while refreshing the previous value will remain, <i>and the
140 * exception will be logged (using {@link java.util.logging.Logger}) and swallowed</i>.
141 *
142 * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#reload} if the
143 * cache currently contains a value for {@code key}, and {@link CacheLoader#load} otherwise.
144 * Loading is asynchronous only if {@link CacheLoader#reload} was overridden with an
145 * asynchronous implementation.
146 *
147 * <p>Returns without doing anything if another thread is currently loading the value for
148 * {@code key}. If the cache loader associated with this cache performs refresh asynchronously
149 * then this method may return before refresh completes.
150 *
151 * @since 11.0
152 */
153 void refresh(K key);
154
155 /**
156 * {@inheritDoc}
157 *
158 * <p><b>Note that although the view <i>is</i> modifiable, no method on the returned map will ever
159 * cause entries to be automatically loaded.</b>
160 */
161 @Override
162 ConcurrentMap<K, V> asMap();
163 }