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Java资源缓存 之 LruCache

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例如对 网络加载图片进行缓存 :

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// 得到 应用程序 被分配的最大的内存
   int maxMemory=( int ) Runtime.getRuntime().maxMemory();
   // 取处内存的 1/5 用来当 缓存 大小
   int cachSize=maxMemory/ 5 ;
   // 实例化 LruCache
   lruCache= new lruCache<String, Bitmap>(cachSize){
     //内部方法sizeOf设置每一张图片的缓存大小
     protected int sizeOf(String key, Bitmap value) {
       //在每次存入缓存时调用,告诉系统这张缓存图片有多大
       // 相当于 为每次 要缓存的 资源 分配 大小空间
       return value.getByteCount();
     }
   };
  

上面的 代码 一般 放在初始化的 方法 里面 。

其实 可以将 LurCache 类 理解 为 Map 类 map 有 put和 get 方法 。

接下去就调用put 和 get 方法 进行需要缓存资源的存取 。

LurCache 的 add :

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public void putBitmapToCache(String url,Bitmap bitmap){
     if (getBitmapfromCache(url)== null ) { //判断当前缓存是否存在
       lruCache.put(url, bitmap);
     }
   }
LurCache 的 get:
 
public Bitmap getBitmapfromCache(String url){
     return lruCache.get(url); //可将lruCache看成map
   }

调用上面的 add 和 get 方法 就可以对资源进行缓存的 ,还是挺简单的, 。

但要注意一点 LruCache lruCache=new LruCache<String, Bitmap>(cachSize) 只能被new 一次 ,不然不同对象就不同的缓存了 。

附上Android的Lrucache类 。

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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);
   }
}

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