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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.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.annotations.GwtIncompatible;
import com.google.common.base.Function;
import com.google.common.base.Supplier;
import com.google.common.util.concurrent.Futures;
import com.google.common.util.concurrent.ListenableFuture;
import java.io.Serializable;
import java.util.Map;
/**
* Computes or retrieves values, based on a key, for use in populating a {@code Cache}.
*
* Most implementations will only need to implement {@link #load}. Other methods may be
* overridden as desired.
*
* @author Charles Fry
* @since 10.0
*/
@Beta
@GwtCompatible(emulated = true)
public abstract class CacheLoader {
/**
* 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}; must not be null
*/
public abstract V load(K key) throws Exception;
/**
* Computes or retrieves a replacement value corresponding to an already-cached {@code key}. This
* method is called when an existing cache entry is refreshed by
* {@link CacheBuilder#refreshAfterWrite}, or through a call to {@link Cache#refresh}.
*
* This implementation synchronously delegates to {@link #load}. It is recommended that it be
* overridden with an asynchronous implementation when using
* {@link CacheBuilder#refreshAfterWrite}.
*
*
Note: all exceptions thrown by this method will be logged and then swallowed.
*
* @param key the non-null key whose value should be loaded
* @param oldValue the non-null old value corresponding to {@code key}
* @return the future new value associated with {@code key};
* must not be null, must not return null
* @since 11.0
*/
@GwtIncompatible("Futures")
public ListenableFuture reload(K key, V oldValue) throws Exception {
return Futures.immediateFuture(load(key));
}
/**
* Computes or retrieves the values corresponding to {@code keys}. This method is called by
* {@link Cache#getAll}.
*
* 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}.
*
*
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;
* may not contain null values
* @since 11.0
*/
public Map loadAll(Iterable 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 CacheLoader from(Function function) {
return new FunctionToCacheLoader(function);
}
private static final class FunctionToCacheLoader
extends CacheLoader implements Serializable {
private final Function computingFunction;
public FunctionToCacheLoader(Function 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 CacheLoader