001/*
002 * Copyright (C) 2011 The Guava Authors
003 *
004 * Licensed under the Apache License, Version 2.0 (the "License");
005 * you may not use this file except in compliance with the License.
006 * You may obtain a copy of the License at
007 *
008 * http://www.apache.org/licenses/LICENSE-2.0
009 *
010 * Unless required by applicable law or agreed to in writing, software
011 * distributed under the License is distributed on an "AS IS" BASIS,
012 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
013 * See the License for the specific language governing permissions and
014 * limitations under the License.
015 */
016
017package com.google.common.cache;
018
019import com.google.common.annotations.GwtCompatible;
020
021/**
022 * An object that can receive a notification when an entry is removed from a cache. The removal
023 * resulting in notification could have occured to an entry being manually removed or replaced, or
024 * due to eviction resulting from timed expiration, exceeding a maximum size, or garbage
025 * collection.
026 *
027 * <p>An instance may be called concurrently by multiple threads to process different entries.
028 * Implementations of this interface should avoid performing blocking calls or synchronizing on
029 * shared resources.
030 *
031 * @param <K> the most general type of keys this listener can listen for; for
032 *     example {@code Object} if any key is acceptable
033 * @param <V> the most general type of values this listener can listen for; for
034 *     example {@code Object} if any key is acceptable
035 * @author Charles Fry
036 * @since 10.0
037 */
038@GwtCompatible
039public interface RemovalListener<K, V> {
040  /**
041   * Notifies the listener that a removal occurred at some point in the past.
042   *
043   * <p>This does not always signify that the key is now absent from the cache,
044   * as it may have already been re-added.
045   */
046  // Technically should accept RemovalNotification<? extends K, ? extends V>, but because
047  // RemovalNotification is guaranteed covariant, let's make users' lives simpler.
048  void onRemoval(RemovalNotification<K, V> notification);
049}