1   /*
2    * Copyright (C) 2011 The Guava Authors
3    *
4    * Licensed under the Apache License, Version 2.0 (the "License");
5    * you may not use this file except in compliance with the License.
6    * You may obtain a copy of the License at
7    *
8    * http://www.apache.org/licenses/LICENSE-2.0
9    *
10   * Unless required by applicable law or agreed to in writing, software
11   * distributed under the License is distributed on an "AS IS" BASIS,
12   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13   * See the License for the specific language governing permissions and
14   * limitations under the License.
15   */
16  
17  package com.google.common.cache;
18  
19  import com.google.common.annotations.Beta;
20  import com.google.common.annotations.GwtCompatible;
21  
22  /**
23   * An object that can receive a notification when an entry is removed from a cache. The removal
24   * resulting in notification could have occured to an entry being manually removed or replaced, or
25   * due to eviction resulting from timed expiration, exceeding a maximum size, or garbage
26   * collection.
27   *
28   * <p>An instance may be called concurrently by multiple threads to process different entries.
29   * Implementations of this interface should avoid performing blocking calls or synchronizing on
30   * shared resources.
31   *
32   * @param <K> the most general type of keys this listener can listen for; for
33   *     example {@code Object} if any key is acceptable
34   * @param <V> the most general type of values this listener can listen for; for
35   *     example {@code Object} if any key is acceptable
36   * @author Charles Fry
37   * @since 10.0
38   */
39  @Beta
40  @GwtCompatible
41  public interface RemovalListener<K, V> {
42    /**
43     * Notifies the listener that a removal occurred at some point in the past.
44     */
45    // Technically should accept RemovalNotification<? extends K, ? extends V>, but because
46    // RemovalNotification is guaranteed covariant, let's make users' lives simpler.
47    void onRemoval(RemovalNotification<K, V> notification);
48  }