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/*
* Copyright 2014 Ben Manes. All Rights Reserved.
*
* 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.github.benmanes.caffeine.cache;
import java.util.Map;
import java.util.concurrent.CompletionException;
import org.checkerframework.checker.nullness.qual.NonNull;
import org.checkerframework.checker.nullness.qual.Nullable;
/**
* A semi-persistent mapping from keys to values. Values are automatically loaded by the cache,
* and are stored in the cache until either evicted or manually invalidated.
*
* Implementations of this interface are expected to be thread-safe, and can be safely accessed
* by multiple concurrent threads.
*
* @author [email protected] (Ben Manes)
* @param the type of keys maintained by this cache
* @param the type of mapped values
*/
public interface LoadingCache extends Cache {
/**
* Returns the value associated with the {@code key} in this cache, obtaining that value from
* {@link CacheLoader#load(Object)} if necessary.
*
* If another call to {@link #get} is currently loading the value for the {@code key}, this thread
* simply waits for that thread to finish and returns its loaded value. Note that multiple threads
* can concurrently load values for distinct keys.
*
* If the specified key is not already associated with a value, attempts to compute its value and
* enters it into this cache unless {@code null}. The entire method invocation is performed
* atomically, so the function is applied at most once per key. Some attempted update operations
* on this cache by other threads may be blocked while the computation is in progress, so the
* computation should be short and simple, and must not attempt to update any other mappings of
* this cache.
*
* @param key key with which the specified value is to be associated
* @return the current (existing or computed) value associated with the specified key, or null if
* the computed value is null
* @throws NullPointerException if the specified key is null
* @throws IllegalStateException if the computation detectably attempts a recursive update to this
* cache that would otherwise never complete
* @throws CompletionException if a checked exception was thrown while loading the value
* @throws RuntimeException or Error if the {@link CacheLoader} does so, in which case the mapping
* is left unestablished
*/
@Nullable
V get(@NonNull K key);
/**
* Returns a map of the values associated with the {@code keys}, creating or retrieving those
* values if necessary. The returned map contains entries that were already cached, combined with
* the newly loaded entries; it will never contain null keys or values.
*
* Caches loaded by a {@link CacheLoader} will issue a single request to
* {@link CacheLoader#loadAll} for all keys which are not already present in the cache. All
* entries returned by {@link CacheLoader#loadAll} will be stored in the cache, over-writing any
* previously cached values. If another call to {@link #get} tries to load the value for a key in
* {@code keys}, implementations may either have that thread load the entry or simply wait for
* this thread to finish and returns the loaded value. In the case of overlapping non-blocking
* loads, the last load to complete will replace the existing entry. Note that multiple threads
* can concurrently load values for distinct keys.
*
* Note that duplicate elements in {@code keys}, as determined by {@link Object#equals}, will be
* ignored.
*
* @param keys the keys whose associated values are to be returned
* @return the unmodifiable mapping of keys to values for the specified keys in this cache
* @throws NullPointerException if the specified collection is null or contains a null element
* @throws CompletionException if a checked exception was thrown while loading the value
* @throws RuntimeException or Error if the {@link CacheLoader} does so, if
* {@link CacheLoader#loadAll} returns {@code null}, or returns a map containing null keys
* or values. In all cases, the mapping is left unestablished.
*/
@NonNull
Map<@NonNull K, @NonNull V> getAll(@NonNull Iterable keys);
/**
* Loads a new value for the {@code key}, asynchronously. While the new value is loading the
* previous value (if any) will continue to be returned by {@code get(key)} unless it is evicted.
* If the new value is loaded successfully it will replace the previous value in the cache; if an
* exception is thrown while refreshing the previous value will remain, and the exception will
* be logged (using {@link java.util.logging.Logger}) and swallowed.
*
* Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#reload} if the cache
* currently contains a value for the {@code key}, and {@link CacheLoader#load} otherwise. Loading
* is asynchronous by delegating to the default executor.
*
* @param key key with which a value may be associated
* @throws NullPointerException if the specified key is null
*/
void refresh(@NonNull K key);
}