Android 中LruCache 原理与编程

Android用LruCache来取代原来强引用和软引用实现内存缓存，因为据说自2.3以后Android将更频繁的调用GC，导致软引用缓存的数据极易被释放。

LruCache使用一个LinkedHashMap简单的实现内存的缓存，没有软引用，都是强引用。根据LinkedHashMap的结构原理，最新的应该在尾端，旧的应该在头部。如果添加的数据大于设置的最大值，就删除最先缓存（头部）的数据来调整内存。他的主要原理在trimToSize方法中。需要了解两个主要的变量size和maxSize

maxSize是通过构造方法初始化的值，他表示这个缓存能缓存的最大值是多少。

size在添加和移除缓存都被更新值，他通过safeSizeOf这个方法更新值。safeSizeOf默认返回1，但一般我们会根据maxSize重写这个方法，比如认为maxSize代表是KB的话，那么就以KB为单位返回该项所占的内存大小。

除异常外首先会判断size是否超过maxSize,，如果超过了就取出最先插入的缓存，如果不为空就删掉（一般来说只要map不为空都不会返回null，因为他是个双休链表），并把size减去该项所占的大小。这个操作将一直循环下去，直到size比maxSize小或者缓存为空。

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. 
     *  保证当前缓存的大小不大于maxSize!
     */  
    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);  
    }  
}  

第177行，Map.Entry< K, V> toEvict = map.eldest(); 这句中的eldest()方法是哪里来的？代码不是support-v4包里的LruCache而是android framework里面的LruCache，eldest是隐藏的API。

    /** 
     * Returns the eldest entry in the map, or {@code null} if the map is empty. 
     * @hide 
     */  
    public Entry<K, V> eldest() {  
        LinkedEntry<K, V> eldest = header.nxt;  
        return eldest != header ? eldest : null;  
    }  

编程引用：

郭霖大师博文
- Android高效加载大图、多图解决方案，有效避免程序OOM
- Android照片墙应用实现，再多的图片也不怕崩溃

其他
- Android 异步加载图片，使用LruCache和SD卡或手机缓存，效果非常的流畅
- 图片缓存之内存缓存技术LruCache,软引用