LruCache的用法及分析_app

概述1. package android.util; 2. 3. import java.util.LinkedHashMap; 4. import java.util.Map; 5. 6. 7. /** 8. * A cache that holds strong references to a limited nu
package com.jack.test;  import java.util.linkedHashMap;  import java.util.Map;  import java.util.Map.Entry;/**  * 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,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 = (Entry<K,V>) map.entrySet();                  if (toevict == null) {                      break;                  }                  key = toevict.getKey();                  value = toevict.getValue();                  map.remove(key);                  size -= safeSizeOf(key,value);                  evictionCount++;              }              entryRemoved(true,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,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);      }  }