LruMemoryCache源码分析

1. 目前在看UIL,看到了内存缓存，就纪记录下对LruMemoryCache得分析，看名字就知道他主要做得是实现内存缓存。

2.上注释代码

package com.nostra13.universalimageloader.cache.memory.impl;

import android.graphics.Bitmap;

import com.nostra13.universalimageloader.cache.memory.MemoryCache;

import java.util.Collection;
import java.util.HashSet;
import java.util.LinkedHashMap;
import java.util.Map;

/**
 * A cache that holds strong references to a limited number of Bitmaps. Each time a Bitmap is accessed, it is moved to
 * the head of a queue. When a Bitmap is added to a full cache, the Bitmap at the end of that queue is evicted and may
 * become eligible for garbage collection.

 * 

 * NOTE: This cache uses only strong references for stored Bitmaps.
 *
 * @author Sergey Tarasevich (nostra13[at]gmail[dot]com)
 * @since 1.8.1
 */
public class LruMemoryCache implements MemoryCache {
    //创建一个LinkedHashMap
    private final LinkedHashMap map;
    //缓存得最大内存大小
    private final int maxSize;
    /** Size of this cache in bytes */
    private int size;

    /** @param maxSize Maximum sum of the sizes of the Bitmaps in this cache */
    public LruMemoryCache(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }
        this.maxSize = maxSize;
        //LinkedHashMap他是一个双联表结构，通过在构造函数里面设置，accessOrder 为True,他就会记录访问顺序，将最新访问的元素添加到双向链表的表尾，并从原来的位置删除
        //这样就可以实现将最新访问得元素，存放倒底部。
        this.map = new LinkedHashMap(0, 0.75f, true);
    }

    /**
     * Returns the Bitmap for {@code key} if it exists in the cache. If a Bitmap was returned, it is moved to the head
     * of the queue. This returns null if a Bitmap is not cached.
     * 这个方法很简单就是取出Bitmap根据Uri
     */
    @Override
    public final Bitmap get(String key) {
        if (key == null) {
            throw new NullPointerException("key == null");
        }

        synchronized (this) {
            return map.get(key);
        }
    }

    /** Caches {@code Bitmap} for {@code key}. The Bitmap is moved to the head of the queue. */
    @Override
    public final boolean put(String key, Bitmap value) {
        if (key == null || value == null) {
            throw new NullPointerException("key == null || value == null");
        }

        synchronized (this) {
            size += sizeOf(key, value);//每次存入bitmap得时候根据bitmap获取当前bitmap得宽和高相乘得到所占得内存大小以此累加，标记存入集合bitmap得容量
            Bitmap previous = map.put(key, value);//如果这个bitmap缓存中已经有了就会返回改Bitmap。
            if (previous != null) {
                size -= sizeOf(key, previous);//改Bitmap已经存进去过，那么就减去刚次标记得大小
            }
        }
        //计算删除最少使用得bitmap
        trimToSize(maxSize);
        return true;
    }

    /**
     * Remove the eldest entries until the total of remaining entries is at or below the requested size.
     *
     * @param maxSize the maximum size of the cache before returning. May be -1 to evict even 0-sized elements.
     */
    private void trimToSize(int maxSize) {
        while (true) {
            String key;
            Bitmap value;
            synchronized (this) {
                if (size < 0 || (map.isEmpty() && size != 0)) {
                    throw new IllegalStateException(getClass().getName() + ".sizeOf() is reporting inconsistent results!");
                }
                //如果当前得大小不超过指定得大小直接退出循环
                if (size <= maxSize || map.isEmpty()) {
                    break;
                }
                //获取map得栈顶最不常使用得bitmap
                Map.Entry toEvict = map.entrySet().iterator().next();
                if (toEvict == null) {
                    break;
                }
                key = toEvict.getKey();
                value = toEvict.getValue();
                map.remove(key);//删除该bitmap
                size -= sizeOf(key, value);//大小标记减少
            }
        }
    }

    /** Removes the entry for {@code key} if it exists. */
    @Override
    public final Bitmap remove(String key) {
        if (key == null) {
            throw new NullPointerException("key == null");
        }

        synchronized (this) {
            Bitmap previous = map.remove(key);//删除指定得bitmap
            if (previous != null) {
                size -= sizeOf(key, previous);
            }
            return previous;
        }
    }

    @Override
    public Collection keys() {
        synchronized (this) {
            return new HashSet(map.keySet());
        }
    }

    @Override
    public void clear() {//清空所有得缓存
        trimToSize(-1); // -1 will evict 0-sized elements
    }

    /**
     * Returns the size {@code Bitmap} in bytes.
     * 

     * An entry's size must not change while it is in the cache.
     */
    private int sizeOf(String key, Bitmap value) {
        return value.getRowBytes() * value.getHeight();
    }

    @Override
    public synchronized final String toString() {
        return String.format("LruCache[maxSize=%d]", maxSize);
    }
}

已经就是对源码得注释，此处好处是都实现了MemoryCache 接口，所以上层调用只要你实现MemoryCache 接口，如果有一天你觉得他这个缓存实现得不爽，你自己也可以实现自己得缓存实现，只要实现MemoryCache 接口就可以了。

已经同步在博客： https://l123456789jy.github.io/2017/03/25/LruMemoryCache源码分析/