Android 中LruCache 原理与编程

Android用LruCache来取代原来强引用和软引用实现内存缓存，因为据说自2.3以后Android将更频繁的调用GC，导致软引用缓存的数据极易被释放。

LruCache使用一个LinkedHashMap简单的实现内存的缓存，没有软引用，都是强引用。根据LinkedHashMap的结构原理，最新的应该在尾端，旧的应该在头部。如果添加的数据大于设置的最大值，就删除最先缓存（头部）的数据来调整内存。他的主要原理在trimToSize方法中。需要了解两个主要的变量size和maxSize

maxSize是通过构造方法初始化的值，他表示这个缓存能缓存的最大值是多少。

size在添加和移除缓存都被更新值，他通过safeSizeOf这个方法更新值。safeSizeOf默认返回1，但一般我们会根据maxSize重写这个方法，比如认为maxSize代表是KB的话，那么就以KB为单位返回该项所占的内存大小。

除异常外首先会判断size是否超过maxSize,，如果超过了就取出最先插入的缓存，如果不为空就删掉（一般来说只要map不为空都不会返回null，因为他是个双休链表），并把size减去该项所占的大小。这个操作将一直循环下去，直到size比maxSize小或者缓存为空。

Android提供的LruCache类简介

 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.      *  保证当前缓存的大小不大于maxSize!     */      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);      }  }  

第177行，Map.Entry< K, V> toEvict = map.eldest(); 这句中的eldest()方法是哪里来的？代码不是support-v4包里的LruCache而是android framework里面的LruCache，eldest是隐藏的API。

    /**      * Returns the eldest entry in the map, or {@code null} if the map is empty.      * @hide      */      public Entry<K, V> eldest() {          LinkedEntry<K, V> eldest = header.nxt;          return eldest != header ? eldest : null;      }  

编程引用：

郭霖大师博文
- Android高效加载大图、多图解决方案，有效避免程序OOM
- Android照片墙应用实现，再多的图片也不怕崩溃

其他
- Android 异步加载图片，使用LruCache和SD卡或手机缓存，效果非常的流畅
- 图片缓存之内存缓存技术LruCache,软引用