摘自：http://blog.csdn.net/linghu_java/article/details/8574102
- package android.util;
- import java.util.LinkedHashMap;
- import java.util.Map;
- /**
- * A cache that holds strong references to a limited number of values. Each time
- * a value is accessed, it is moved to the head of a queue. When a value is
- * added to a full cache, the value at the end of that queue is evicted and may
- * become eligible for garbage collection.
- * 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。
- * 当cache已满的时候加入新的item时，在队列尾部的item会被回收。
- * <p>If your cached values hold resources that need to be explicitly released,
- * override {@link #entryRemoved}.
- * 如果你cache的某个值需要明确释放，重写entryRemoved()
- * <p>If a cache miss should be computed on demand for the corresponding keys,
- * override {@link #create}. This simplifies the calling code, allowing it to
- * assume a value will always be returned, even when there's a cache miss.
- * 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。
- * <p>By default, the cache size is measured in the number of entries. Override
- * {@link #sizeOf} to size the cache in different units. For example, this cache
- * 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
- * 大小。
- * <pre> {@code
- * int cacheSize = 4 * 1024 * 1024; // 4MiB
- * LruCache<String, Bitmap> bitmapCache = new LruCache<String, Bitmap>(cacheSize) {
- * protected int sizeOf(String key, Bitmap value) {
- * return value.getByteCount();
- * }
- * }}</pre>
- *
- * <p>This class is thread-safe. Perform multiple cache operations atomically by
- * synchronizing on the cache: <pre> {@code
- * synchronized (cache) {
- * if (cache.get(key) == null) {
- * cache.put(key, value);
- * }
- * }}</pre>
- *
- * <p>This class does not allow null to be used as a key or value. A return
- * value of null from {@link #get}, {@link #put} or {@link #remove} is
- * unambiguous: the key was not in the cache.
- * 不允许key或者value为null
- * 当get（），put（），remove（）返回值为null时，key相应的项不在cache中
- */
- public class LruCache<K, V> {
- private final LinkedHashMap<K, V> map;
- /** Size of this cache in units. Not necessarily the number of elements. */
- private int size; //已经存储的大小
- private int maxSize; //规定的最大存储空间
- private int putCount; //put的次数
- private int createCount; //create的次数
- private int evictionCount; //回收的次数
- private int hitCount; //命中的次数
- private int missCount; //丢失的次数
- /**
- * @param maxSize for caches that do not override {@link #sizeOf}, this is
- * the maximum number of entries in the cache. For all other caches,
- * this is the maximum sum of the sizes of the entries in this cache.
- */
- public LruCache(int maxSize) {
- if (maxSize <= 0) {
- throw new IllegalArgumentException("maxSize <= 0");
- }
- this.maxSize = maxSize;
- this.map = new LinkedHashMap<K, V>(0, 0.75f, true);
- }
- /**
- * Returns the value for {@code key} if it exists in the cache or can be
- * created by {@code #create}. If a value was returned, it is moved to the
- * head of the queue. This returns null if a value is not cached and cannot
- * 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，
- * 如果item的value没有被cache或者不能被创建，则返回null。
- */
- public final V get(K key) {
- if (key == null) {
- throw new NullPointerException("key == null");
- }
- V mapValue;
- synchronized (this) {
- mapValue = map.get(key);
- if (mapValue != null) {
- hitCount++; //命中
- return mapValue;
- }
- missCount++; //丢失
- }
- /*
- * Attempt to create a value. This may take a long time, and the map
- * may be different when create() returns. If a conflicting value was
- * added to the map while create() was working, we leave that value in
- * the map and release the created value.
- * 如果丢失了就试图创建一个item
- */
- V createdValue = create(key);
- if (createdValue == null) {
- return null;
- }
- synchronized (this) {
- createCount++;//创建++
- mapValue = map.put(key, createdValue);
- if (mapValue != null) {
- // There was a conflict so undo that last put
- //如果前面存在oldValue，那么撤销put（）
- map.put(key, mapValue);
- } else {
- size += safeSizeOf(key, createdValue);
- }
- }
- if (mapValue != null) {
- entryRemoved(false, key, createdValue, mapValue);
- return mapValue;
- } else {
- trimToSize(maxSize);
- return createdValue;
- }
- }
- /**
- * Caches {@code value} for {@code key}. The value is moved to the head of
- * the queue.
- *
- * @return the previous value mapped by {@code key}.
- */
- public final V put(K key, V value) {
- if (key == null || value == null) {
- throw new NullPointerException("key == null || value == null");
- }
- V previous;
- synchronized (this) {
- putCount++;
- size += safeSizeOf(key, value);
- previous = map.put(key, value);
- if (previous != null) { //返回的先前的value值
- size -= safeSizeOf(key, previous);
- }
- }
- if (previous != null) {
- entryRemoved(false, key, previous, value);
- }
- trimToSize(maxSize);
- return previous;
- }
- /**
- * @param maxSize the maximum size of the cache before returning. May be -1
- * to evict even 0-sized elements.
- * 清空cache空间
- */
- private void trimToSize(int maxSize) {
- while (true) {
- K key;
- V value;
- synchronized (this) {
- if (size < 0 || (map.isEmpty() && size != 0)) {
- throw new IllegalStateException(getClass().getName()
- + ".sizeOf() is reporting inconsistent results!");
- }
- if (size <= maxSize) {
- break;
- }
- Map.Entry<K, V> toEvict = map.eldest();
- if (toEvict == null) {
- break;
- }
- key = toEvict.getKey();
- value = toEvict.getValue();
- map.remove(key);
- size -= safeSizeOf(key, value);
- evictionCount++;
- }
- entryRemoved(true, key, value, null);
- }
- }
- /**
- * Removes the entry for {@code key} if it exists.
- * 删除key相应的cache项，返回相应的value
- * @return the previous value mapped by {@code key}.
- */
- public final V remove(K key) {
- if (key == null) {
- throw new NullPointerException("key == null");
- }
- V previous;
- synchronized (this) {
- previous = map.remove(key);
- if (previous != null) {
- size -= safeSizeOf(key, previous);
- }
- }
- if (previous != null) {
- entryRemoved(false, key, previous, null);
- }
- return previous;
- }
- /**
- * Called for entries that have been evicted or removed. This method is
- * invoked when a value is evicted to make space, removed by a call to
- * {@link #remove}, or replaced by a call to {@link #put}. The default
- * implementation does nothing.
- * 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，
- * 或者替换item值时put调用，默认实现什么都没做。
- * <p>The method is called without synchronization: other threads may
- * access the cache while this method is executing.
- *
- * @param evicted true if the entry is being removed to make space, false
- * if the removal was caused by a {@link #put} or {@link #remove}.
- * true---为释放空间被删除；false---put或remove导致
- * @param newValue the new value for {@code key}, if it exists. If non-null,
- * this removal was caused by a {@link #put}. Otherwise it was caused by
- * an eviction or a {@link #remove}.
- */
- protected void entryRemoved(boolean evicted, K key, V oldValue, V newValue) {}
- /**
- * Called after a cache miss to compute a value for the corresponding key.
- * Returns the computed value or null if no value can be computed. The
- * default implementation returns null.
- * 当某Item丢失时会调用到，返回计算的相应的value或者null
- * <p>The method is called without synchronization: other threads may
- * access the cache while this method is executing.
- *
- * <p>If a value for {@code key} exists in the cache when this method
- * returns, the created value will be released with {@link #entryRemoved}
- * and discarded. This can occur when multiple threads request the same key
- * at the same time (causing multiple values to be created), or when one
- * thread calls {@link #put} while another is creating a value for the same
- * key.
- */
- protected V create(K key) {
- return null;
- }
- private int safeSizeOf(K key, V value) {
- int result = sizeOf(key, value);
- if (result < 0) {
- throw new IllegalStateException("Negative size: " + key + "=" + value);
- }
- return result;
- }
- /**
- * Returns the size of the entry for {@code key} and {@code value} in
- * user-defined units. The default implementation returns 1 so that size
- * is the number of entries and max size is the maximum number of entries.
- * 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
- * <p>An entry's size must not change while it is in the cache.
- */
- protected int sizeOf(K key, V value) {
- return 1;
- }
- /**
- * Clear the cache, calling {@link #entryRemoved} on each removed entry.
- * 清空cacke
- */
- public final void evictAll() {
- trimToSize(-1); // -1 will evict 0-sized elements
- }
- /**
- * For caches that do not override {@link #sizeOf}, this returns the number
- * of entries in the cache. For all other caches, this returns the sum of
- * the sizes of the entries in this cache.
- */
- public synchronized final int size() {
- return size;
- }
- /**
- * For caches that do not override {@link #sizeOf}, this returns the maximum
- * number of entries in the cache. For all other caches, this returns the
- * maximum sum of the sizes of the entries in this cache.
- */
- public synchronized final int maxSize() {
- return maxSize;
- }
- /**
- * Returns the number of times {@link #get} returned a value that was
- * already present in the cache.
- */
- public synchronized final int hitCount() {
- return hitCount;
- }
- /**
- * Returns the number of times {@link #get} returned null or required a new
- * value to be created.
- */
- public synchronized final int missCount() {
- return missCount;
- }
- /**
- * Returns the number of times {@link #create(Object)} returned a value.
- */
- public synchronized final int createCount() {
- return createCount;
- }
- /**
- * Returns the number of times {@link #put} was called.
- */
- public synchronized final int putCount() {
- return putCount;
- }
- /**
- * Returns the number of values that have been evicted.
- * 返回被回收的数量
- */
- public synchronized final int evictionCount() {
- return evictionCount;
- }
- /**
- * Returns a copy of the current contents of the cache, ordered from least
- * 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
- */
- public synchronized final Map<K, V> snapshot() {
- return new LinkedHashMap<K, V>(map);
- }
- @Override public synchronized final String toString() {
- int accesses = hitCount + missCount;
- int hitPercent = accesses != 0 ? (100 * hitCount / accesses) : 0;
- return String.format("LruCache[maxSize=%d,hits=%d,misses=%d,hitRate=%d%%]",
- maxSize, hitCount, missCount, hitPercent);
- }
- }