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 17 package com.google.common.cache; 18 19 import com.google.common.annotations.Beta; 20 import com.google.common.annotations.GwtCompatible; 21 import com.google.common.base.Function; 22 import com.google.common.collect.ImmutableMap; 23 import com.google.common.util.concurrent.ExecutionError; 24 import com.google.common.util.concurrent.UncheckedExecutionException; 25 26 import java.util.concurrent.ConcurrentMap; 27 import 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 49 public 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. ({@code 69 * ExecutionException} is thrown <a 70 * href="http://code.google.com/p/guava-libraries/wiki/CachesExplained#Interruption">even if 71 * computation was interrupted by an {@code InterruptedException}</a>.) 72 * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the 73 * value 74 * @throws ExecutionError if an error was thrown while loading the value 75 */ get(K key)76 V get(K key) throws ExecutionException; 77 78 /** 79 * Returns the value associated with {@code key} in this cache, first loading that value if 80 * necessary. No observable state associated with this cache is modified until loading 81 * completes. Unlike {@link #get}, this method does not throw a checked exception, and thus should 82 * only be used in situations where checked exceptions are not thrown by the cache loader. 83 * 84 * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for 85 * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that 86 * multiple threads can concurrently load values for distinct keys. 87 * 88 * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values 89 * into the cache. Newly loaded values are added to the cache using 90 * {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated 91 * with {@code key} while the new value was loading then a removal notification will be sent for 92 * the new value. 93 * 94 * <p><b>Warning:</b> this method silently converts checked exceptions to unchecked exceptions, 95 * and should not be used with cache loaders which throw checked exceptions. In such cases use 96 * {@link #get} instead. 97 * 98 * @throws UncheckedExecutionException if an exception was thrown while loading the value. (As 99 * explained in the last paragraph above, this should be an unchecked exception only.) 100 * @throws ExecutionError if an error was thrown while loading the value 101 */ getUnchecked(K key)102 V getUnchecked(K key); 103 104 /** 105 * Returns a map of the values associated with {@code keys}, creating or retrieving those values 106 * if necessary. The returned map contains entries that were already cached, combined with newly 107 * loaded entries; it will never contain null keys or values. 108 * 109 * <p>Caches loaded by a {@link CacheLoader} will issue a single request to 110 * {@link CacheLoader#loadAll} for all keys which are not already present in the cache. All 111 * entries returned by {@link CacheLoader#loadAll} will be stored in the cache, over-writing 112 * any previously cached values. This method will throw an exception if 113 * {@link CacheLoader#loadAll} returns {@code null}, returns a map containing null keys or values, 114 * or fails to return an entry for each requested key. 115 * 116 * <p>Note that duplicate elements in {@code keys}, as determined by {@link Object#equals}, will 117 * be ignored. 118 * 119 * @throws ExecutionException if a checked exception was thrown while loading the value. ({@code 120 * ExecutionException} is thrown <a 121 * href="http://code.google.com/p/guava-libraries/wiki/CachesExplained#Interruption">even if 122 * computation was interrupted by an {@code InterruptedException}</a>.) 123 * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the 124 * values 125 * @throws ExecutionError if an error was thrown while loading the values 126 * @since 11.0 127 */ getAll(Iterable<? extends K> keys)128 ImmutableMap<K, V> getAll(Iterable<? extends K> keys) throws ExecutionException; 129 130 /** 131 * @deprecated Provided to satisfy the {@code Function} interface; use {@link #get} or 132 * {@link #getUnchecked} instead. 133 * @throws UncheckedExecutionException if an exception was thrown while loading the value. (As 134 * described in the documentation for {@link #getUnchecked}, {@code LoadingCache} should be 135 * used as a {@code Function} only with cache loaders that throw only unchecked exceptions.) 136 */ 137 @Deprecated 138 @Override apply(K key)139 V apply(K key); 140 141 /** 142 * Loads a new value for key {@code key}, possibly asynchronously. While the new value is loading 143 * the previous value (if any) will continue to be returned by {@code get(key)} unless it is 144 * evicted. If the new value is loaded successfully it will replace the previous value in the 145 * cache; if an exception is thrown while refreshing the previous value will remain, <i>and the 146 * exception will be logged (using {@link java.util.logging.Logger}) and swallowed</i>. 147 * 148 * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#reload} if the 149 * cache currently contains a value for {@code key}, and {@link CacheLoader#load} otherwise. 150 * Loading is asynchronous only if {@link CacheLoader#reload} was overridden with an 151 * asynchronous implementation. 152 * 153 * <p>Returns without doing anything if another thread is currently loading the value for 154 * {@code key}. If the cache loader associated with this cache performs refresh asynchronously 155 * then this method may return before refresh completes. 156 * 157 * @since 11.0 158 */ refresh(K key)159 void refresh(K key); 160 161 /** 162 * {@inheritDoc} 163 * 164 * <p><b>Note that although the view <i>is</i> modifiable, no method on the returned map will ever 165 * cause entries to be automatically loaded.</b> 166 */ 167 @Override asMap()168 ConcurrentMap<K, V> asMap(); 169 } 170