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/*
* Copyright (C) 2011 The Guava Authors
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package com.google.common.cache;
import static com.google.common.base.Preconditions.checkNotNull;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Function;
import com.google.common.base.Supplier;
import java.io.Serializable;
import java.util.Map;
/**
* Computes or retrieves values, based on a key, for use in populating a {@code Cache}.
*
* <p>Most implementations will only need to implement {@link #load}. Other methods may be
* overridden as desired.
*
* @author Charles Fry
* @since 10.0
*/
@GwtCompatible(emulated = true)
public abstract class CacheLoader<K, V> {
/**
* Constructor for use by subclasses.
*/
protected CacheLoader() {}
/**
* Computes or retrieves the value corresponding to {@code key}.
*
* @param key the non-null key whose value should be loaded
* @return the value associated with {@code key}; <b>must not be null</b>
*/
public abstract V load(K key) throws Exception;
/**
* Computes or retrieves the values corresponding to {@code keys}. This method is called by
* {@link Cache#getAll}.
*
* <p>If the returned map doesn't contain all requested {@code keys} then the entries it does
* contain will be cached, but {@code getAll} will throw an exception. If the returned map
* contains extra keys not present in {@code keys} then all returned entries will be cached,
* but only the entries for {@code keys} will be returned from {@code getAll}.
*
* <p>This method should be overriden when bulk retrieval is significantly more efficient than
* many individual lookups. Note that {@link Cache#getAll} will defer to individual calls to
* {@link Cache#get} if this method is not overriden.
*
* @param keys the unique, non-null keys whose values should be loaded
* @return a map from each key in {@code keys} to the value associated with that key;
* <b>may not contain null values</b>
* @since 11.0
*/
public Map<K, V> loadAll(Iterable<? extends K> keys) throws Exception {
// This will be caught by getAll(), causing it to fall back to multiple calls to Cache.get
throw new UnsupportedLoadingOperationException();
}
/**
* Returns a {@code CacheLoader} which creates values by applying a {@code Function} to the key.
*/
public static <K, V> CacheLoader<K, V> from(Function<K, V> function) {
return new FunctionToCacheLoader<K, V>(function);
}
private static final class FunctionToCacheLoader<K, V>
extends CacheLoader<K, V> implements Serializable {
private final Function<K, V> computingFunction;
public FunctionToCacheLoader(Function<K, V> computingFunction) {
this.computingFunction = checkNotNull(computingFunction);
}
@Override
public V load(K key) {
return computingFunction.apply(key);
}
private static final long serialVersionUID = 0;
}
/**
* Returns a {@code CacheLoader} which obtains values from a {@code Supplier} (independent of the
* key).
*/
public static <V> CacheLoader<Object, V> from(Supplier<V> supplier) {
return new SupplierToCacheLoader<V>(supplier);
}
private static final class SupplierToCacheLoader<V>
extends CacheLoader<Object, V> implements Serializable {
private final Supplier<V> computingSupplier;
public SupplierToCacheLoader(Supplier<V> computingSupplier) {
this.computingSupplier = checkNotNull(computingSupplier);
}
@Override
public V load(Object key) {
return computingSupplier.get();
}
private static final long serialVersionUID = 0;
}
static final class UnsupportedLoadingOperationException extends UnsupportedOperationException {}
/**
* Thrown to indicate that an invalid response was returned from a call to {@link CacheLoader}.
*
* @since 11.0
*/
public static final class InvalidCacheLoadException extends RuntimeException {
public InvalidCacheLoadException(String message) {
super(message);
}
}
}
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