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); 
  } 
}