1 package android.util;
2
3 import java.util.LinkedHashMap;
4 import java.util.Map;
5
6 /**
7 * A cache that holds strong references to a limited number of values. Each time
8 * a value is accessed, it is moved to the head of a queue. When a value is
9 * added to a full cache, the value at the end of that queue is evicted and may
10 * become eligible for garbage collection.
11 * Cache保存一个强引用来限制内容数量，每当Item被访问的时候，此Item就会移动到队列的头部。
12 * 当cache已满的时候加入新的item时，在队列尾部的item会被回收。
13 * <p>If your cached values hold resources that need to be explicitly released,
14 * override {@link #entryRemoved}.
15 * 如果你cache的某个值需要明确释放，重写entryRemoved()
16 * <p>If a cache miss should be computed on demand for the corresponding keys,
17 * override {@link #create}. This simplifies the calling code, allowing it to
18 * assume a value will always be returned, even when there's a cache miss.
19 * 如果key相对应的item丢掉啦，重写create().这简化了调用代码，即使丢失了也总会返回。
20 * <p>By default, the cache size is measured in the number of entries. Override
21 * {@link #sizeOf} to size the cache in different units. For example, this cache
22 * is limited to 4MiB of bitmaps: 默认cache大小是测量的item的数量，重写sizeof计算不同item的
23 * 大小。
24 * <pre> {@code
25 * int cacheSize = 4 * 1024 * 1024; // 4MiB
26 * LruCache<String, Bitmap> bitmapCache = new LruCache<String, Bitmap>(cacheSize) {
27 * protected int sizeOf(String key, Bitmap value) {
28 * return value.getByteCount();
29 * }
30 * }}</pre>
31 *
32 * <p>This class is thread-safe. Perform multiple cache operations atomically by
33 * synchronizing on the cache: <pre> {@code
34 * synchronized (cache) {
35 * if (cache.get(key) == null) {
36 * cache.put(key, value);
37 * }
38 * }}</pre>
39 *
40 * <p>This class does not allow null to be used as a key or value. A return
41 * value of null from {@link #get}, {@link #put} or {@link #remove} is
42 * unambiguous: the key was not in the cache.
43 * 不允许key或者value为null
44 * 当get（），put（），remove（）返回值为null时，key相应的项不在cache中
45 */
46 public class LruCache<K, V> {
47 private final LinkedHashMap<K, V> map;
48
49 /** Size of this cache in units. Not necessarily the number of elements. */
50 private int size; //已经存储的大小
51 private int maxSize; //规定的最大存储空间
52
53 private int putCount; //put的次数
54 private int createCount; //create的次数
55 private int evictionCount; //回收的次数
56 private int hitCount; //命中的次数
57 private int missCount; //丢失的次数
58
59 /**
60 * @param maxSize for caches that do not override {@link #sizeOf}, this is
61 * the maximum number of entries in the cache. For all other caches,
62 * this is the maximum sum of the sizes of the entries in this cache.
63 */
64 public LruCache(int maxSize) {
65 if (maxSize <= 0) {
66 throw new IllegalArgumentException("maxSize <= 0");
67 }
68 this.maxSize = maxSize;
69 this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
70 }
71
72 /**
73 * Returns the value for {@code key} if it exists in the cache or can be
74 * created by {@code #create}. If a value was returned, it is moved to the
75 * head of the queue. This returns null if a value is not cached and cannot
76 * be created. 通过key返回相应的item，或者创建返回相应的item。相应的item会移动到队列的头部，
77 * 如果item的value没有被cache或者不能被创建，则返回null。
78 */
79 public final V get(K key) {
80 if (key == null) {
81 throw new NullPointerException("key == null");
82 }
83
84 V mapValue;
85 synchronized (this) {
86 mapValue = map.get(key);
87 if (mapValue != null) {
88 hitCount++; //命中
89 return mapValue;
90 }
91 missCount++; //丢失
92 }
93
94 /*
95 * Attempt to create a value. This may take a long time, and the map
96 * may be different when create() returns. If a conflicting value was
97 * added to the map while create() was working, we leave that value in
98 * the map and release the created value.
99 * 如果丢失了就试图创建一个item
100 */
101
102 V createdValue = create(key);
103 if (createdValue == null) {
104 return null;
105 }
106
107 synchronized (this) {
108 createCount++;//创建++
109 mapValue = map.put(key, createdValue);
110
111 if (mapValue != null) {
112 // There was a conflict so undo that last put
113 //如果前面存在oldValue，那么撤销put（）
114 map.put(key, mapValue);
115 } else {
116 size += safeSizeOf(key, createdValue);
117 }
118 }
119
120 if (mapValue != null) {
121 entryRemoved(false, key, createdValue, mapValue);
122 return mapValue;
123 } else {
124 trimToSize(maxSize);
125 return createdValue;
126 }
127 }
128
129 /**
130 * Caches {@code value} for {@code key}. The value is moved to the head of
131 * the queue.
132 *
133 * @return the previous value mapped by {@code key}.
134 */
135 public final V put(K key, V value) {
136 if (key == null || value == null) {
137 throw new NullPointerException("key == null || value == null");
138 }
139
140 V previous;
141 synchronized (this) {
142 putCount++;
143 size += safeSizeOf(key, value);
144 previous = map.put(key, value);
145 if (previous != null) { //返回的先前的value值
146 size -= safeSizeOf(key, previous);
147 }
148 }
149
150 if (previous != null) {
151 entryRemoved(false, key, previous, value);
152 }
153
154 trimToSize(maxSize);
155 return previous;
156 }
157
158 /**
159 * @param maxSize the maximum size of the cache before returning. May be -1
160 * to evict even 0-sized elements.
161 * 清空cache空间
162 */
163 private void trimToSize(int maxSize) {
164 while (true) {
165 K key;
166 V value;
167 synchronized (this) {
168 if (size < 0 || (map.isEmpty() && size != 0)) {
169 throw new IllegalStateException(getClass().getName()
170 + ".sizeOf() is reporting inconsistent results!");
171 }
172
173 if (size <= maxSize) {
174 break;
175 }
176
177 Map.Entry<K, V> toEvict = map.eldest();
178 if (toEvict == null) {
179 break;
180 }
181
182 key = toEvict.getKey();
183 value = toEvict.getValue();
184 map.remove(key);
185 size -= safeSizeOf(key, value);
186 evictionCount++;
187 }
188
189 entryRemoved(true, key, value, null);
190 }
191 }
192
193 /**
194 * Removes the entry for {@code key} if it exists.
195 * 删除key相应的cache项，返回相应的value
196 * @return the previous value mapped by {@code key}.
197 */
198 public final V remove(K key) {
199 if (key == null) {
200 throw new NullPointerException("key == null");
201 }
202
203 V previous;
204 synchronized (this) {
205 previous = map.remove(key);
206 if (previous != null) {
207 size -= safeSizeOf(key, previous);
208 }
209 }
210
211 if (previous != null) {
212 entryRemoved(false, key, previous, null);
213 }
214
215 return previous;
216 }
217
218 /**
219 * Called for entries that have been evicted or removed. This method is
220 * invoked when a value is evicted to make space, removed by a call to
221 * {@link #remove}, or replaced by a call to {@link #put}. The default
222 * implementation does nothing.
223 * 当item被回收或者删掉时调用。改方法当value被回收释放存储空间时被remove调用，
224 * 或者替换item值时put调用，默认实现什么都没做。
225 * <p>The method is called without synchronization: other threads may
226 * access the cache while this method is executing.
227 *
228 * @param evicted true if the entry is being removed to make space, false
229 * if the removal was caused by a {@link #put} or {@link #remove}.
230 * true---为释放空间被删除；false---put或remove导致
231 * @param newValue the new value for {@code key}, if it exists. If non-null,
232 * this removal was caused by a {@link #put}. Otherwise it was caused by
233 * an eviction or a {@link #remove}.
234 */
235 protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}
236
237 /**
238 * Called after a cache miss to compute a value for the corresponding key.
239 * Returns the computed value or null if no value can be computed. The
240 * default implementation returns null.
241 * 当某Item丢失时会调用到，返回计算的相应的value或者null
242 * <p>The method is called without synchronization: other threads may
243 * access the cache while this method is executing.
244 *
245 * <p>If a value for {@code key} exists in the cache when this method
246 * returns, the created value will be released with {@link #entryRemoved}
247 * and discarded. This can occur when multiple threads request the same key
248 * at the same time (causing multiple values to be created), or when one
249 * thread calls {@link #put} while another is creating a value for the same
250 * key.
251 */
252 protected V create(K key) {
253 return null;
254 }
255
256 private int safeSizeOf(K key, V value) {
257 int result = sizeOf(key, value);
258 if (result < 0) {
259 throw new IllegalStateException("Negative size: " + key + "=" + value);
260 }
261 return result;
262 }
263
264 /**
265 * Returns the size of the entry for {@code key} and {@code value} in
266 * user-defined units. The default implementation returns 1 so that size
267 * is the number of entries and max size is the maximum number of entries.
268 * 返回用户定义的item的大小，默认返回1代表item的数量，最大size就是最大item值
269 * <p>An entry's size must not change while it is in the cache.
270 */
271 protected int sizeOf(K key, V value) {
272 return 1;
273 }
274
275 /**
276 * Clear the cache, calling {@link #entryRemoved} on each removed entry.
277 * 清空cacke
278 */
279 public final void evictAll() {
280 trimToSize(-1); // -1 will evict 0-sized elements
281 }
282
283 /**
284 * For caches that do not override {@link #sizeOf}, this returns the number
285 * of entries in the cache. For all other caches, this returns the sum of
286 * the sizes of the entries in this cache.
287 */
288 public synchronized final int size() {
289 return size;
290 }
291
292 /**
293 * For caches that do not override {@link #sizeOf}, this returns the maximum
294 * number of entries in the cache. For all other caches, this returns the
295 * maximum sum of the sizes of the entries in this cache.
296 */
297 public synchronized final int maxSize() {
298 return maxSize;
299 }
300
301 /**
302 * Returns the number of times {@link #get} returned a value that was
303 * already present in the cache.
304 */
305 public synchronized final int hitCount() {
306 return hitCount;
307 }
308
309 /**
310 * Returns the number of times {@link #get} returned null or required a new
311 * value to be created.
312 */
313 public synchronized final int missCount() {
314 return missCount;
315 }
316
317 /**
318 * Returns the number of times {@link #create(Object)} returned a value.
319 */
320 public synchronized final int createCount() {
321 return createCount;
322 }
323
324 /**
325 * Returns the number of times {@link #put} was called.
326 */
327 public synchronized final int putCount() {
328 return putCount;
329 }
330
331 /**
332 * Returns the number of values that have been evicted.
333 * 返回被回收的数量
334 */
335 public synchronized final int evictionCount() {
336 return evictionCount;
337 }
338
339 /**
340 * Returns a copy of the current contents of the cache, ordered from least
341 * recently accessed to most recently accessed. 返回当前cache的副本，从最近最少访问到最多访问
342 */
343 public synchronized final Map<K, V> snapshot() {
344 return new LinkedHashMap<K, V>(map);
345 }
346
347 @Override public synchronized final String toString() {
348 int accesses = hitCount + missCount;
349 int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;
350 return String.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",
351 maxSize, hitCount, missCount, hitPercent);
352 }
353 }
