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>All methods other than {@link #get} and {@link #getUnchecked} are optional.
037     *
038     * <p>When evaluated as a {@link Function}, a cache yields the same result as invoking
039     * {@link #getUnchecked}.
040     *
041     * @author Charles Fry
042     * @since 11.0
043     */
044    @Beta
045    @GwtCompatible
046    public interface LoadingCache<K, V> extends Cache<K, V>, Function<K, V> {
047    
048      /**
049       * Returns the value associated with {@code key} in this cache, first loading that value if
050       * necessary. No observable state associated with this cache is modified until loading completes.
051       *
052       * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
053       * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
054       * multiple threads can concurrently load values for distinct keys.
055       *
056       * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
057       * into the cache. Newly loaded values are added to the cache using
058       * {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
059       * with {@code key} while the new value was loading then a removal notification will be sent for
060       * the new value.
061       *
062       * <p>If the cache loader associated with this cache is known not to throw checked
063       * exceptions, then prefer {@link #getUnchecked} over this method.
064       *
065       * @throws ExecutionException if a checked exception was thrown while loading the value
066       * @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
067       *     value
068       * @throws ExecutionError if an error was thrown while loading the value
069       */
070      V get(K key) throws ExecutionException;
071    
072      /**
073       * Returns the value associated with {@code key} in this cache, first loading that value if
074       * necessary. No observable state associated with this cache is modified until loading
075       * completes. Unlike {@link #get}, this method does not throw a checked exception, and thus should
076       * only be used in situations where checked exceptions are not thrown by the cache loader.
077       *
078       * <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
079       * {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
080       * multiple threads can concurrently load values for distinct keys.
081       *
082       * <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
083       * into the cache. Newly loaded values are added to the cache using
084       * {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
085       * with {@code key} while the new value was loading then a removal notification will be sent for
086       * the new value.
087       *
088       * <p><b>Warning:</b> this method silently converts checked exceptions to unchecked exceptions,
089       * and should not be used with cache loaders which throw checked exceptions. In such cases use
090       * {@link #get} instead.
091       *
092       * @throws UncheckedExecutionException if an exception was thrown while loading the value,
093       *     regardless of whether the exception was checked or unchecked
094       * @throws ExecutionError if an error was thrown while loading the value
095       */
096      V getUnchecked(K key);
097    
098      /**
099       * 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    }