Java资源缓存 之 LruCache

例如对 网络加载图片进行缓存 ：

  // 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 ：

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

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);
  }
}