Java tutorial
/* * 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.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}. * * @author Charles Fry * @since 11.0 */ @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="https://github.com/google/guava/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="https://github.com/google/guava/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(); }