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