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    }