Android - 提供的LruCache類簡介

 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保存一個強引用來限制內容數量，每當Item被訪問的時候，此Item就會移動到隊列的頭部。
 * 當cache已滿的時候加入新的item時，在隊列尾部的item會被回收。
 * <p>If your cached values hold resources that need to be explicitly released, 
 * override {@link #entryRemoved}. 
 * 如果你cache的某個值需要明確釋放，重寫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相對應的item丟掉啦，重寫create().這簡化了調用代碼，即使丟失了也總會返回。
 * <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: 默認cache大小是測量的item的數量，重寫sizeof計算不同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.
 * 不允許key或者value爲null
 *  當get（），put（），remove（）返回值爲null時，key相應的項不在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; //已經存儲的大小
    private int maxSize; //規定的最大存儲空間

    private int putCount;  //put的次數
    private int createCount;  //create的次數
    private int evictionCount;  //回收的次數
    private int hitCount;  //命中的次數
    private int missCount;  //丟失的次數

    /** 
     * @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. 通過key返回相應的item，或者創建返回相應的item。相應的item會移動到隊列的頭部，
     * 如果item的value沒有被cache或者不能被創建，則返回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++;  //丟失
        }  

        /* 
         * 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. 
         * 如果丟失了就試圖創建一個item
         */  

        V createdValue = create(key);  
        if (createdValue == null) {  
            return null;  
        }  

        synchronized (this) {  
            createCount++;//創建++  
            mapValue = map.put(key, createdValue);  

            if (mapValue != null) {  
                // There was a conflict so undo that last put  
                //如果前面存在oldValue，那麼撤銷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空間
     */  
    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. 
     * 刪除key相應的cache項，返回相應的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. 
     * 當item被回收或者刪掉時調用。改方法當value被回收釋放存儲空間時被remove調用，
     * 或者替換item值時put調用，默認實現什麼都沒做。
     * <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---爲釋放空間被刪除；false---put或remove導致
     * @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. 
     * 當某Item丟失時會調用到，返回計算的相應的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. 
     * 返回用戶定義的item的大小，默認返回1代表item的數量，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. 
     * 返回被回收的數量
     */  
    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. 返回當前cache的副本，從最近最少訪問到最多訪問
     */  
    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);  
    }  
}