LoadingCache.java revision 7dd252788645e940eada959bdde927426e2531c9 
1/*
2 * Copyright (C) 2011 The Guava Authors
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17package com.google.common.cache;
18
19import com.google.common.annotations.Beta;
20import com.google.common.annotations.GwtCompatible;
21import com.google.common.base.Function;
22import com.google.common.collect.ImmutableMap;
23import com.google.common.util.concurrent.ExecutionError;
24import com.google.common.util.concurrent.UncheckedExecutionException;
25
26import java.util.concurrent.ConcurrentMap;
27import java.util.concurrent.ExecutionException;
28
29/**
30 * A semi-persistent mapping from keys to values. Values are automatically loaded by the cache,
31 * and are stored in the cache until either evicted or manually invalidated.
32 *
33 * <p>Implementations of this interface are expected to be thread-safe, and can be safely accessed
34 * by multiple concurrent threads.
35 *
36 * <p>When evaluated as a {@link Function}, a cache yields the same result as invoking
37 * {@link #getUnchecked}.
38 *
39 * <p>Note that while this class is still annotated as {@link Beta}, the API is frozen from a
40 * consumer's standpoint. In other words existing methods are all considered {@code non-Beta} and
41 * won't be changed without going through an 18 month deprecation cycle; however new methods may be
42 * added at any time.
43 *
44 * @author Charles Fry
45 * @since 11.0
46 */
47@Beta
48@GwtCompatible
49public interface LoadingCache<K, V> extends Cache<K, V>, Function<K, V> {
50
51  /**
52   * Returns the value associated with {@code key} in this cache, first loading that value if
53   * necessary. No observable state associated with this cache is modified until loading completes.
54   *
55   * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
56   * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
57   * multiple threads can concurrently load values for distinct keys.
58   *
59   * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
60   * into the cache. Newly loaded values are added to the cache using
61   * {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
62   * with {@code key} while the new value was loading then a removal notification will be sent for
63   * the new value.
64   *
65   * <p>If the cache loader associated with this cache is known not to throw checked
66   * exceptions, then prefer {@link #getUnchecked} over this method.
67   *
68   * @throws ExecutionException if a checked exception was thrown while loading the value
69   * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
70   *     value
71   * @throws ExecutionError if an error was thrown while loading the value
72   */
73  V get(K key) throws ExecutionException;
74
75  /**
76   * Returns the value associated with {@code key} in this cache, first loading that value if
77   * necessary. No observable state associated with this cache is modified until loading
78   * completes. Unlike {@link #get}, this method does not throw a checked exception, and thus should
79   * only be used in situations where checked exceptions are not thrown by the cache loader.
80   *
81   * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
82   * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
83   * multiple threads can concurrently load values for distinct keys.
84   *
85   * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
86   * into the cache. Newly loaded values are added to the cache using
87   * {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
88   * with {@code key} while the new value was loading then a removal notification will be sent for
89   * the new value.
90   *
91   * <p><b>Warning:</b> this method silently converts checked exceptions to unchecked exceptions,
92   * and should not be used with cache loaders which throw checked exceptions. In such cases use
93   * {@link #get} instead.
94   *
95   * @throws UncheckedExecutionException if an exception was thrown while loading the value,
96   *     regardless of whether the exception was checked or unchecked
97   * @throws ExecutionError if an error was thrown while loading the value
98   */
99  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
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
162  ConcurrentMap<K, V> asMap();
163}
164