LruCache类

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。如果添加的数据大于设置的最大值，就删除最先缓存的数据来调整内存。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。

package com.baiiu.test;

import java.util.LinkedHashMap;
import java.util.Map;

public class HowLruCache<K, V> {
    private final LinkedHashMap<K, V> map;

    /** Size of this cache in units. Not necessarily the number of elements. */
    private int size;// 已经使用的内存大小
    private int maxSize;// 给lrucache分配的总内存大小，用于存储图片缓存

    private int putCount;
    private int createCount;
    private int evictionCount;
    private int hitCount;
    private int missCount;

    /** * @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 HowLruCache(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        this.maxSize = maxSize;

        /** * 该哈希映射的迭代顺序就是最后访问其条目的顺序，<BR> * 从近期访问最少到近期访问最多的顺序（访问顺序）。这种映射很适合构建 LRU 缓存。<BR> * 对于访问顺序，为 true；对于插入顺序，则为 false * * 插入顺序排序，如果需要输出的顺序和输入时的相同，那么就选用LinkedHashMap。 * 访问顺序排序，那么调用get方法后，会将这次访问的元素移至链表尾部，不断访问可以形成按访问顺序排序的链表。 * * 按照访问的次序来排序的含义:当调用LinkedHashMap的get(key)或者put(key, * value)时，碰巧key在map中被包含，那么LinkedHashMap会将该对象放在线性结构的最后。 * */
        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. */
    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++;
                return mapValue;
            }
            missCount++;
        }

        /* * 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. */

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

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

            if (mapValue != null) {
                // There was a conflict so undo that last 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");
        }

        /* * 将该键值对存入内存中，并加总目前已经占用的内存 */
        V previous;
        synchronized (this) {
            putCount++;
            size += safeSizeOf(key, value);
            previous = map.put(key, value);// 重新存入
            if (previous != null) {
                // 该对象已经存在，避免再次加总
                size -= safeSizeOf(key, previous);
            }
        }

        if (previous != null) {
            entryRemoved(false, key, previous, value);// 空方法
        }

        trimToSize(maxSize);
        return previous;
    }

    /** * Remove the eldest entries until the total of remaining entries is at or * below the requested size. * * 获取、存入方法都要调用该方法 * * @param maxSize * the maximum size of the cache before returning. May be -1 to * evict even 0-sized elements. */
    public void trimToSize(int maxSize) {
        while (true) {
            K key;
            V value;
            synchronized (this) {
                // 健壮性判断
                if (size < 0 || (map.isEmpty() && size != 0)) {
                    throw new IllegalStateException(getClass().getName()
                            + ".sizeOf() is reporting inconsistent results!");
                }

                // 不满足判断，有剩余内存就不用删除啦
                if (size <= maxSize || map.isEmpty()) {
                    break;
                }

                // 移除头部元素，即不经常使用元素，经常使用的被重新排列到末尾去了
                Map.Entry<K, V> toEvict = map.entrySet().iterator().next();
                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. * * @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. * * <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}. * @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. * * <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. * * <p> * An entry's size must not change while it is in the cache. 测量每个item的大小 */
    protected int sizeOf(K key, V value) {
        return 1;
    }

    /** * Clear the cache, calling {@link #entryRemoved} on each removed entry. */
    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. */
    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. */
    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. */
    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);
    }
}

转载：http://www.cnblogs.com/baiiu/p/4291277.html