android.support.v4包中的LruCache源码简读

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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