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 static com.google.common.base.Preconditions.checkNotNull;
020    
021    import com.google.common.annotations.Beta;
022    import com.google.common.base.Function;
023    import com.google.common.base.Supplier;
024    
025    import java.io.Serializable;
026    
027    /**
028     * Computes or retrieves values, based on a key, for use in populating a {@code Cache}.
029     *
030     * @author Charles Fry
031     * @since 10.0
032     */
033    @Beta
034    public abstract class CacheLoader<K, V> {
035    
036      /**
037       * Returns a {@code CacheLoader} which creates values by applying a {@code Function} to the key.
038       */
039      public static <K, V> CacheLoader<K, V> from(Function<K, V> function) {
040        return new FunctionToCacheLoader<K, V>(function);
041      }
042    
043      private static final class FunctionToCacheLoader<K, V>
044          extends CacheLoader<K, V> implements Serializable {
045        private final Function<K, V> computingFunction;
046    
047        public FunctionToCacheLoader(Function<K, V> computingFunction) {
048          this.computingFunction = checkNotNull(computingFunction);
049        }
050    
051        @Override
052        public V load(K key) {
053          return computingFunction.apply(key);
054        }
055    
056        private static final long serialVersionUID = 0;
057      }
058    
059      /**
060       * Returns a {@code CacheLoader} which obtains values from a {@code Supplier} (independent of the
061       * key).
062       */
063      public static <V> CacheLoader<Object, V> from(Supplier<V> supplier) {
064        return new SupplierToCacheLoader<V>(supplier);
065      }
066    
067      private static final class SupplierToCacheLoader<V>
068          extends CacheLoader<Object, V> implements Serializable {
069        private final Supplier<V> computingSupplier;
070    
071        public SupplierToCacheLoader(Supplier<V> computingSupplier) {
072          this.computingSupplier = checkNotNull(computingSupplier);
073        }
074    
075        @Override
076        public V load(Object key) {
077          return computingSupplier.get();
078        }
079    
080        private static final long serialVersionUID = 0;
081      }
082    
083      /**
084       * Computes or retrieves the value corresponding to {@code key}.
085       *
086       * @param key the key whose value should be loaded; will never be null
087       * @return the value associated with {@code key}; <b>may not be null</b>
088       */
089      public abstract V load(K key) throws Exception;
090    
091      // TODO(fry): loadAll
092    
093    }