Coverage Summary for Class: MapRetrievalCache (com.google.common.graph)

1 /*
2  * Copyright (C) 2016 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.graph;
18 
19 import static com.google.common.base.Preconditions.checkNotNull;
20 
21 import java.util.Map;
22 import javax.annotation.CheckForNull;
23 
24 /**
25  * A {@link MapIteratorCache} that adds additional caching. In addition to the caching provided by
26  * {@link MapIteratorCache}, this structure caches values for the two most recently retrieved keys.
27  *
28  * @author James Sexton
29  */
30 @ElementTypesAreNonnullByDefault
31 final class MapRetrievalCache<K, V> extends MapIteratorCache<K, V> {
32   // See the note about volatile in the superclass.
33   @CheckForNull private transient volatile CacheEntry<K, V> cacheEntry1;
34   @CheckForNull private transient volatile CacheEntry<K, V> cacheEntry2;
35 
36   MapRetrievalCache(Map<K, V> backingMap) {
37     super(backingMap);
38   }
39 
40   @SuppressWarnings("unchecked") // Safe because we only cast if key is found in map.
41   @Override
42   @CheckForNull
43   V get(Object key) {
44     checkNotNull(key);
45     V value = getIfCached(key);
46     if (value != null) {
47       return value;
48     }
49 
50     value = getWithoutCaching(key);
51     if (value != null) {
52       addToCache((K) key, value);
53     }
54     return value;
55   }
56 
57   // Internal methods (package-visible, but treat as only subclass-visible)
58 
59   @Override
60   @CheckForNull
61   V getIfCached(@CheckForNull Object key) {
62     V value = super.getIfCached(key);
63     if (value != null) {
64       return value;
65     }
66 
67     // Store a local reference to the cache entry. If the backing map is immutable, this,
68     // in combination with immutable cache entries, will ensure a thread-safe cache.
69     CacheEntry<K, V> entry;
70 
71     // Check cache. We use == on purpose because it's cheaper and a cache miss is ok.
72     entry = cacheEntry1;
73     if (entry != null && entry.key == key) {
74       return entry.value;
75     }
76     entry = cacheEntry2;
77     if (entry != null && entry.key == key) {
78       // Promote second cache entry to first so the access pattern
79       // [K1, K2, K1, K3, K1, K4...] still hits the cache half the time.
80       addToCache(entry);
81       return entry.value;
82     }
83     return null;
84   }
85 
86   @Override
87   void clearCache() {
88     super.clearCache();
89     cacheEntry1 = null;
90     cacheEntry2 = null;
91   }
92 
93   private void addToCache(K key, V value) {
94     addToCache(new CacheEntry<K, V>(key, value));
95   }
96 
97   private void addToCache(CacheEntry<K, V> entry) {
98     // Slide new entry into first cache position. Drop previous entry in second cache position.
99     cacheEntry2 = cacheEntry1;
100     cacheEntry1 = entry;
101   }
102 
103   private static final class CacheEntry<K, V> {
104     final K key;
105     final V value;
106 
107     CacheEntry(K key, V value) {
108       this.key = key;
109       this.value = value;
110     }
111   }
112 }