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. * 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);      }  }
原文链接：http://blog.csdn.net/linghu_java/article/details/8574102
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