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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 com.google.common.annotations.Beta;
import com.google.common.annotations.GwtCompatible;
import com.google.common.base.Function;
import com.google.common.collect.ImmutableMap;
import com.google.common.util.concurrent.ExecutionError;
import com.google.common.util.concurrent.UncheckedExecutionException;
import java.util.concurrent.ConcurrentMap;
import java.util.concurrent.ExecutionException;
/**
* 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.
*
* <p>Implementations of this interface are expected to be thread-safe, and can be safely accessed
* by multiple concurrent threads.
*
* <p>When evaluated as a {@link Function}, a cache yields the same result as invoking
* {@link #getUnchecked}.
*
* <p>Note that while this class is still annotated as {@link Beta}, the API is frozen from a
* consumer's standpoint. In other words existing methods are all considered {@code non-Beta} and
* won't be changed without going through an 18 month deprecation cycle; however new methods may be
* added at any time.
*
* @author Charles Fry
* @since 11.0
*/
@Beta
@GwtCompatible
public interface LoadingCache<K, V> extends Cache<K, V>, Function<K, V> {
/**
* Returns the value associated with {@code key} in this cache, first loading that value if
* necessary. No observable state associated with this cache is modified until loading completes.
*
* <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
* {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
* multiple threads can concurrently load values for distinct keys.
*
* <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
* into the cache. Newly loaded values are added to the cache using
* {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
* with {@code key} while the new value was loading then a removal notification will be sent for
* the new value.
*
* <p>If the cache loader associated with this cache is known not to throw checked
* exceptions, then prefer {@link #getUnchecked} over this method.
*
* @throws ExecutionException if a checked exception was thrown while loading the value
* @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
* value
* @throws ExecutionError if an error was thrown while loading the value
*/
V get(K key) throws ExecutionException;
/**
* Returns the value associated with {@code key} in this cache, first loading that value if
* necessary. No observable state associated with this cache is modified until loading
* completes. Unlike {@link #get}, this method does not throw a checked exception, and thus should
* only be used in situations where checked exceptions are not thrown by the cache loader.
*
* <p>If another call to {@link #get} or {@link #getUnchecked} is currently loading the value for
* {@code key}, simply waits for that thread to finish and returns its loaded value. Note that
* multiple threads can concurrently load values for distinct keys.
*
* <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#load} to load new values
* into the cache. Newly loaded values are added to the cache using
* {@code Cache.asMap().putIfAbsent} after loading has completed; if another value was associated
* with {@code key} while the new value was loading then a removal notification will be sent for
* the new value.
*
* <p><b>Warning:</b> this method silently converts checked exceptions to unchecked exceptions,
* and should not be used with cache loaders which throw checked exceptions. In such cases use
* {@link #get} instead.
*
* @throws UncheckedExecutionException if an exception was thrown while loading the value,
* regardless of whether the exception was checked or unchecked
* @throws ExecutionError if an error was thrown while loading the value
*/
V getUnchecked(K key);
/**
* Returns a map of the values associated with {@code keys}, creating or retrieving those values
* if necessary. The returned map contains entries that were already cached, combined with newly
* loaded entries; it will never contain null keys or values.
*
* <p>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. This method will throw an exception if
* {@link CacheLoader#loadAll} returns {@code null}, returns a map containing null keys or values,
* or fails to return an entry for each requested key.
*
* <p>Note that duplicate elements in {@code keys}, as determined by {@link Object#equals}, will
* be ignored.
*
* @throws ExecutionException if a checked exception was thrown while loading the values
* @throws UncheckedExecutionException if an unchecked exception was thrown while loading the
* values
* @throws ExecutionError if an error was thrown while loading the values
* @since 11.0
*/
ImmutableMap<K, V> getAll(Iterable<? extends K> keys) throws ExecutionException;
/**
* Discouraged. Provided to satisfy the {@code Function} interface; use {@link #get} or
* {@link #getUnchecked} instead.
*
* @throws UncheckedExecutionException if an exception was thrown while loading the value,
* regardless of whether the exception was checked or unchecked
* @throws ExecutionError if an error was thrown while loading the value
*/
@Override
V apply(K key);
/**
* Loads a new value for key {@code key}, possibly 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, <i>and the
* exception will be logged (using {@link java.util.logging.Logger}) and swallowed</i>.
*
* <p>Caches loaded by a {@link CacheLoader} will call {@link CacheLoader#reload} if the
* cache currently contains a value for {@code key}, and {@link CacheLoader#load} otherwise.
* Loading is asynchronous only if {@link CacheLoader#reload} was overridden with an
* asynchronous implementation.
*
* <p>Returns without doing anything if another thread is currently loading the value for
* {@code key}. If the cache loader associated with this cache performs refresh asynchronously
* then this method may return before refresh completes.
*
* @since 11.0
*/
void refresh(K key);
/**
* {@inheritDoc}
*
* <p><b>Note that although the view <i>is</i> modifiable, no method on the returned map will ever
* cause entries to be automatically loaded.</b>
*/
@Override
ConcurrentMap<K, V> asMap();
}
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