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