LruCache原理分析

使用LruCache作为图片的内存缓存，其内部使用LinkedHashMap作为实现基础，并且全部使用强引用。弱引用、软应用在android API9之后会被更容易回收，使得有潜在浪费资源的情况。
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参考： http://blog.csdn.net/yudajun/article/details/41620647

难点：
    LinkedHashMap插入顺序理解。
            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。
原理：
    在获取get()、  存入put()方法调用后，都要调用trimToSize()方法，开启循环进行判断。
    发现有溢出现象，则溢出头部元素，即最不常用元素。

源代码：(看注释)

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);
    }
}