guava/src/com/google/common/cache/LoadingCache.java

/*
 * 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. ({@code
   *     ExecutionException} is thrown <a
   *     href="http://code.google.com/p/guava-libraries/wiki/CachesExplained#Interruption">even if
   *     computation was interrupted by an {@code InterruptedException}</a>.)
   * @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. (As
   *     explained in the last paragraph above, this should be an unchecked exception only.)
   * @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 value. ({@code
   *     ExecutionException} is thrown <a
   *     href="http://code.google.com/p/guava-libraries/wiki/CachesExplained#Interruption">even if
   *     computation was interrupted by an {@code InterruptedException}</a>.)
   * @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;

  /**
   * @deprecated 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. (As
   *     described in the documentation for {@link #getUnchecked}, {@code LoadingCache} should be
   *     used as a {@code Function} only with cache loaders that throw only unchecked exceptions.)
   */
  @Deprecated
  @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();
}