Android提供的LruCache类简介

        LruCache 这个类在一个项目中看大就百度了一下，看到博主的博客讲解很详细就转载了：http://blog.csdn.net/linghu_java/article/details/8574102

        以下是源码分析：

      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);          }      }