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>All methods other than {@link #get} and {@link #getUnchecked} are optional.
37 *
38 * <p>When evaluated as a {@link Function}, a cache yields the same result as invoking
39 * {@link #getUnchecked}.
40 *
41 * @author Charles Fry
42 * @since 11.0
43 */
44@Beta
45@GwtCompatible
46public interface LoadingCache<K, V> extends Cache<K, V>, Function<K, V> {
47
48  /**
49   * Returns the value associated with {@code key} in this cache, first loading that value if
50   * necessary. No observable state associated with this cache is modified until loading completes.
51   *
52   * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
53   * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
54   * multiple threads can concurrently load values for distinct keys.
55   *
56   * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
57   * into the cache. Newly loaded values are added to the cache using
58   * {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
59   * with {@code key} while the new value was loading then a removal notification will be sent for
60   * the new value.
61   *
62   * <p>If the cache loader associated with this cache is known not to throw checked
63   * exceptions, then prefer {@link #getUnchecked} over this method.
64   *
65   * @throws ExecutionException if a checked exception was thrown while loading the value
66   * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
67   *     value
68   * @throws ExecutionError if an error was thrown while loading the value
69   */
70  V get(K key) throws ExecutionException;
71
72  /**
73   * Returns the value associated with {@code key} in this cache, first loading that value if
74   * necessary. No observable state associated with this cache is modified until loading
75   * completes. Unlike {@link #get}, this method does not throw a checked exception, and thus should
76   * only be used in situations where checked exceptions are not thrown by the cache loader.
77   *
78   * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
79   * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
80   * multiple threads can concurrently load values for distinct keys.
81   *
82   * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
83   * into the cache. Newly loaded values are added to the cache using
84   * {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
85   * with {@code key} while the new value was loading then a removal notification will be sent for
86   * the new value.
87   *
88   * <p><b>Warning:</b> this method silently converts checked exceptions to unchecked exceptions,
89   * and should not be used with cache loaders which throw checked exceptions. In such cases use
90   * {@link #get} instead.
91   *
92   * @throws UncheckedExecutionException if an exception was thrown while loading the value,
93   *     regardless of whether the exception was checked or unchecked
94   * @throws ExecutionError if an error was thrown while loading the value
95   */
96  V getUnchecked(K key);
97
98  /**
99   * 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}
159