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.ExecutionException;
027
028 /**
029 * A semi-persistent mapping from keys to values. Values are automatically loaded by the cache,
030 * and are stored in the cache until either evicted or manually invalidated.
031 *
032 * <p>Implementations of this interface are expected to be thread-safe, and can be safely accessed
033 * by multiple concurrent threads.
034 *
035 * <p>All methods other than {@link #get} and {@link #getUnchecked} are optional.
036 *
037 * <p>When evaluated as a {@link Function}, a cache yields the same result as invoking
038 * {@link #getUnchecked}.
039 *
040 * @author Charles Fry
041 * @since 11.0
042 */
043 @Beta
044 @GwtCompatible
045 public interface LoadingCache<K, V> extends Cache<K, V>, Function<K, V> {
046
047 /**
048 * Returns the value associated with {@code key} in this cache, first loading that value if
049 * necessary. No observable state associated with this cache is modified until loading completes.
050 *
051 * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
052 * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
053 * multiple threads can concurrently load values for distinct keys.
054 *
055 * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
056 * into the cache. Newly loaded values are added to the cache using
057 * {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
058 * with {@code key} while the new value was loading then a removal notification will be sent for
059 * the new value.
060 *
061 * <p>If the cache loader associated with this cache is known not to throw checked
062 * exceptions, then prefer {@link #getUnchecked} over this method.
063 *
064 * @throws ExecutionException if a checked exception was thrown while loading the value
065 * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
066 * value
067 * @throws ExecutionError if an error was thrown while loading the value
068 */
069 V get(K key) throws ExecutionException;
070
071 /**
072 * Returns the value associated with {@code key} in this cache, first loading that value if
073 * necessary. No observable state associated with this cache is modified until loading
074 * completes. Unlike {@link #get}, this method does not throw a checked exception, and thus should
075 * only be used in situations where checked exceptions are not thrown by the cache loader.
076 *
077 * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
078 * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
079 * multiple threads can concurrently load values for distinct keys.
080 *
081 * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
082 * into the cache. Newly loaded values are added to the cache using
083 * {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
084 * with {@code key} while the new value was loading then a removal notification will be sent for
085 * the new value.
086 *
087 * <p><b>Warning:</b> this method silently converts checked exceptions to unchecked exceptions,
088 * and should not be used with cache loaders which throw checked exceptions. In such cases use
089 * {@link #get} instead.
090 *
091 * @throws UncheckedExecutionException if an exception was thrown while loading the value,
092 * regardless of whether the exception was checked or unchecked
093 * @throws ExecutionError if an error was thrown while loading the value
094 */
095 V getUnchecked(K key);
096
097 /**
098 * Returns a map of the values associated with {@code keys}, creating or retrieving those values
099 * if necessary. The returned map contains entries that were already cached, combined with newly
100 * loaded entries; it will never contain null keys or values.
101 *
102 * <p>Caches loaded by a {@link CacheLoader} will issue a single request to
103 * {@link CacheLoader#loadAll} for all keys which are not already present in the cache. All
104 * entries returned by {@link CacheLoader#loadAll} will be stored in the cache, over-writing
105 * any previously cached values. This method will throw an exception if
106 * {@link CacheLoader#loadAll} returns {@code null}, returns a map containing null keys or values,
107 * or fails to return an entry for each requested key.
108 *
109 * <p>Note that duplicate elements in {@code keys}, as determined by {@link Object#equals}, will
110 * be ignored.
111 *
112 * @throws ExecutionException if a checked exception was thrown while loading the values
113 * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
114 * values
115 * @throws ExecutionError if an error was thrown while loading the values
116 * @since 11.0
117 */
118 ImmutableMap<K, V> getAll(Iterable<? extends K> keys) throws ExecutionException;
119
120 /**
121 * Discouraged. Provided to satisfy the {@code Function} interface; use {@link #get} or
122 * {@link #getUnchecked} instead.
123 *
124 * @throws UncheckedExecutionException if an exception was thrown while loading the value,
125 * regardless of whether the exception was checked or unchecked
126 * @throws ExecutionError if an error was thrown while loading the value
127 */
128 @Override
129 V apply(K key);
130
131 /**
132 * Loads a new value for key {@code key}, possibly asynchronously. While the new value is loading
133 * the previous value (if any) will continue to be returned by {@code get(key)} unless it is
134 * evicted. If the new value is loaded successfully it will replace the previous value in the
135 * cache; if an exception is thrown while refreshing the previous value will remain, <i>and the
136 * exception will be logged (using {@link java.util.logging.Logger}) and swallowed</i>.
137 *
138 * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#reload} if the
139 * cache currently contains a value for {@code key}, and {@link CacheLoader#load} otherwise.
140 *
141 * <p>Returns without doing anything if another thread is currently loading the value for
142 * {@code key}. If the cache loader associated with this cache performs refresh asynchronously
143 * then this method may return before refresh completes.
144 *
145 * @since 11.0
146 */
147 void refresh(K key);
148 }