安卓缓存-LruCache介绍

在Android中，有一个叫做LruCache类专门用来做图片缓存处理的。
它有一个特点，当缓存的图片达到了预先设定的值的时候，那么近期使用次数最少的图片就会被回收掉。
我们来看看它的源码（注意是在android.support.v4.util下）:

package android.support.v4.util;

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

public class LruCache<K, V> {
    //内部维护了一个LinkedHashMap，为什么要用这个，后面介绍
    private final LinkedHashMap<K, V> map;

    private int size; //已缓存的大小
    private int maxSize; //缓存最大值

    private int putCount; //往集合里put的次数
    private int createCount; //创建的个数
    private int evictionCount;//移除的个数
    private int hitCount;//命中次数
    private int missCount;//丢失的次数

    /** * 构造方法 * @param maxSize 缓存的最大值 */
    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);
    }

    /** * 重新设置最大缓存大小 * * @param maxSize */
    public void resize(int maxSize) {
        if (maxSize <= 0) {
            throw new IllegalArgumentException("maxSize <= 0");
        }

        synchronized (this) {
            this.maxSize = maxSize;
        }
        trimToSize(maxSize);
    }

    /** * 通过key获取相应的value，或者创建返回相应的value。相应的value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++;
                return mapValue;
            }
            missCount++;
        }

        //如果丢失了就试图创建一个value
        V createdValue = create(key);
        if (createdValue == null) {
            return null;
        }
        //同步，保证多线程下访问安全
        synchronized (this) {
            createCount++;
            mapValue = map.put(key, createdValue);

            if (mapValue != null) {
                // 如果前面存在一个旧的value，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;
        }
    }

    /** * 在缓存集合（LinkedHashMap）队首添加value * * @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
                size -= safeSizeOf(key, previous);
            }
        }

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

        trimToSize(maxSize);
        return previous;
    }

    /** * 调整cache空间 * 当缓存的大小超过原来设定的maxSize时，遍历map，将多余的项（代码中对应toEvict）剔除掉，直到当 * 前cache的大小等于或小于限定的大小 * @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) {
                if (size < 0 || (map.isEmpty() && size != 0)) {
                    throw new IllegalStateException(getClass().getName()
                            + ".sizeOf() is reporting inconsistent results!");
                }

                if (size <= maxSize || map.isEmpty()) {
                    break;
                }

                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);
        }
    }

    /** * 从缓存中移除掉key对应的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;
    }

    /** * 当item被回收或者删掉时调用。该方法当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。 * * @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) {}

    /** * 当某Item丢失时会调用到，返回计算的相应的value或者null * <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;
    }
    /** * 计算某个item的大小,如果是Bitmap，这应该是bitmap.getByteCount(); */
    private int safeSizeOf(K key, V value) {
        int result = sizeOf(key, value);
        if (result < 0) {
            throw new IllegalStateException("Negative size: " + key + "=" + value);
        }
        return result;
    }


    protected int sizeOf(K key, V value) {
        return 1;
    }

    /** * 清空缓存 */
    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;
    }

    /** * 返回get()为null的次数或者创建的次数 */
    public synchronized final int missCount() {
        return missCount;
    }

    /** * 返回创建create()的次数 */
    public synchronized final int createCount() {
        return createCount;
    }

    /** * 返回put的次数 */
    public synchronized final int putCount() {
        return putCount;
    }

    /** * 返回被回收的数量 */
    public synchronized final int evictionCount() {
        return evictionCount;
    }

    /** * 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);
    }
}

我们来分析下LinkedHashMap：
LinkedHashMap中的get()方法不仅返回所匹配的值，并且在返回前还会将所匹配的key对应的entry调整在列表中的顺序（LinkedHashMap使用双链表来保存数据），让它处于链表的最后。当然，这种情况必须是在LinkedHashMap中accessOrder==true的情况下才生效的，反之就是get()方法不会改变被匹配的key对应的entry在列表中的位置，这样将大大方便了缓存顺序的调整和缓存的移除。这就是为什么要使用LinkedHashMap的原因。

        @Override 
        public V get(Object key) {
        /* * This method is overridden to eliminate the need for a polymorphic * invocation in superclass at the expense of code duplication. */
        if (key == null) {
            HashMapEntry<K, V> e = entryForNullKey;
            if (e == null)
                return null;
            if (accessOrder)
                makeTail((LinkedEntry<K, V>) e);
            return e.value;
        }

        int hash = Collections.secondaryHash(key);
        HashMapEntry<K, V>[] tab = table;
        for (HashMapEntry<K, V> e = tab[hash & (tab.length - 1)];
                e != null; e = e.next) {
            K eKey = e.key;
            if (eKey == key || (e.hash == hash && key.equals(eKey))) {
                if (accessOrder) 
                    makeTail((LinkedEntry<K, V>) e);//调整顺序
                return e.value;
            }
        }
        return null;
    }

好了，LruCache就介绍到这里，下面有个使用例子，大家可以看看：

package com.king.imagechooser.util;

import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.util.DisplayMetrics;
import android.util.Log;
import android.util.LruCache;
import android.view.ViewGroup.LayoutParams;
import android.widget.ImageView;

import java.lang.reflect.Field;
import java.util.LinkedList;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

/** * 本地图片加载框架（读取本地图片） * Created by King on 2016/3/21. */
public class ImageLoader {

    /** * 图片缓存 */
    private LruCache<String, Bitmap> mCaches;

    /** * 线程池 */
    private ExecutorService mThreadPool;
    /** * 默认线程数量 */
    private final static int DEFAULT_THREAD_COUNTS = 1;

    /** * 信号量 */
    private Semaphore mSemaphore = new Semaphore(0);
    private Semaphore mSemaphoreThreadPool;

    private Type mType = Type.LIFO;

    public enum Type {
        FIFO, LIFO
    }

    private LinkedList<Runnable> mTaskQueue;

    private Thread mPoolThread;
    private Handler mPoolThreadHandler;

    private Handler mUIHandler;

    /** * 单例 */
    private static ImageLoader mInstance;

    private ImageLoader(int treadCount, Type type) {
        init(treadCount, type);
    }

    public static ImageLoader getInstance(int treadCount, Type type) {
        if (mInstance == null) {
            synchronized (ImageLoader.class) {
                if (mInstance == null) {
                    mInstance = new ImageLoader(treadCount, type);
                }
            }
        }
        return mInstance;
    }


    /** * 初始化 * * @param threadCount * @param type */
    public void init(int threadCount, Type type) {

        mPoolThread = new Thread() {
            @Override
            public void run() {

                Looper.prepare();
                mPoolThreadHandler = new Handler() {
                    @Override
                    public void handleMessage(Message msg) {
                        //从线程池取一个任务去执行
                        mThreadPool.execute(getTaskThread());
                        try {
                            //最多执行threadCount个线程，利用信号量来控制
                            mSemaphoreThreadPool.acquire();
                        } catch (Exception e) {
                            Log.e("King", e.getMessage());
                        }
                    }
                };
                //释放信号量
                mSemaphore.release();
                Looper.loop();
            }
        };
        mPoolThread.start();
        //获取最大可用内存
        int maxMemory = (int) Runtime.getRuntime().maxMemory();
        int cacheMemory = maxMemory / 4;
        //初始化LruCache
        mCaches = new LruCache<String, Bitmap>(cacheMemory) {
            @Override
            protected int sizeOf(String key, Bitmap value) {
                return value.getRowBytes() * value.getHeight();
            }
        };
        //创建线程池
        mThreadPool = Executors.newFixedThreadPool(threadCount);
        mTaskQueue = new LinkedList<Runnable>();
        mType = type;

        mSemaphoreThreadPool = new Semaphore(threadCount);

    }

    /** * 显示图片到ImageView */
    public void loadImage(final String path, final ImageView imageView) {
        imageView.setTag(path);
        if (mUIHandler == null) {
            mUIHandler = new Handler() {
                @Override
                public void handleMessage(Message msg) {
                    //获取图片，为ImageView回调设置图片
                    ImageHolder holder = (ImageHolder) msg.obj;
                    Bitmap bm = holder.bitmap;
                    ImageView iv = holder.imageView;
                    String path = holder.path;
                    if (iv.getTag().toString().equals(path)) {
                        iv.setImageBitmap(bm);
                    }
                }
            };
        }

        Bitmap bitmap = getBitmapFromCache(path);

        if (bitmap != null) {
            refreshBitmap(path, imageView, bitmap);
        } else {
            addTaskThread(new Runnable() {
                @Override
                public void run() {
                    //获取imageView大小
                    ImageSize imageSize = getImageViewSize(imageView);
                    //压缩图片
                    Bitmap bitmap = decodeBitmapFromPath(path, imageSize.width, imageSize.height);
                    //将图片加入缓存
                    addBitmapToCache(path, bitmap);
                    //发送消息，更新视图
                    refreshBitmap(path, imageView, bitmap);
                    //释放信号量，保证剩余线程继续执行
                    mSemaphoreThreadPool.release();
                }
            });
        }
    }

    private void refreshBitmap(String path, ImageView imageView, Bitmap bitmap) {
        ImageHolder holder = new ImageHolder();
        holder.bitmap = bitmap;
        holder.imageView = imageView;
        holder.path = path;
        Message msg = Message.obtain();
        msg.obj = holder;
        mUIHandler.sendMessage(msg);
    }
    /** * 获取ImageView的实际大小 */
    protected ImageSize getImageViewSize(ImageView imageView) {
        ImageSize size = new ImageSize();

        DisplayMetrics displayMetrics = imageView.getContext().getResources().getDisplayMetrics();

        LayoutParams lp = imageView.getLayoutParams();
        int width = imageView.getWidth();

        if (width <= 0) {
            width = lp.width; //获取ImageView在layout中声明的宽度
        }
        if (width <= 0) {
            width = getImageViewFieldValue(imageView, "mMaxWidth");//获取ImageView在layout中的最大宽度
        }
        if (width <= 0) {
            width = displayMetrics.widthPixels;
        }
        int height = imageView.getHeight();
        if (height <= 0) {
            height = lp.height; //获取ImageView在layout中声明的高度
        }
        if (height <= 0) {
            height = getImageViewFieldValue(imageView, "mMaxHeight"); //获取ImageView在layout中的最大高度
        }
        if (height <= 0) {
            height = displayMetrics.heightPixels;
        }

        size.width = width;
        size.height = height;
        return size;
    }

    //通过反射获取ImageView的属性（为了兼容低版本Api）
    private static int getImageViewFieldValue(Object object, String fieldName) {
        int value = 0;
        try {
            Field field = ImageView.class.getDeclaredField(fieldName);
            field.setAccessible(true);

            int fieldValue = field.getInt(object);
            if (fieldValue > 0 && fieldValue < Integer.MAX_VALUE) {
                value = fieldValue;
            }
        } catch (NoSuchFieldException e) {
            Log.e("King", e.getMessage());
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        return value;
    }

    /** * 压缩图片 * * @param path * @param width * @param height * @return */
    protected Bitmap decodeBitmapFromPath(String path, int width, int height) {
        try {
            BitmapFactory.Options options = new BitmapFactory.Options();
            options.inJustDecodeBounds = true;
            BitmapFactory.decodeFile(path, options);
            options.inSampleSize = calculateInSampleSize(options, width, height);
            options.inJustDecodeBounds = false;
            return BitmapFactory.decodeFile(path, options);
        } catch (Throwable tr) {
            Log.e("King", tr.getMessage());
        }
        return null;
    }

    // 计算图片的缩放值
    public static int calculateInSampleSize(BitmapFactory.Options options,
                                            int reqWidth, int reqHeight) {
        try {
            final int height = options.outHeight;
            final int width = options.outWidth;
            int inSampleSize = 1;

            if (height > reqHeight || width > reqWidth) {
                final int heightRatio = Math.round((float) height
                        / (float) reqHeight);
                final int widthRatio = Math.round((float) width
                        / (float) reqWidth);
                inSampleSize = Math.max(heightRatio, widthRatio);
            }
            return inSampleSize;
        } catch (Throwable e) {
            Log.e("King", e.getMessage());
        }
        return 1;
    }
    /** * 将图片添加到缓存 * * @param path * @param bm */
    private void addBitmapToCache(String path, Bitmap bm) {
        if (getBitmapFromCache(path) == null) {
            if (bm != null) {
                mCaches.put(path, bm);
            }
        }

    }
    //添加线程到集合中，方便管理
    private synchronized void addTaskThread(Runnable runnable) {
        try {
            mTaskQueue.add(runnable);
            if (mPoolThreadHandler == null) {
                //请求信号量，保证Handler为空时再请求，从而避免重复请求
                mSemaphore.acquire();
            }
            mPoolThreadHandler.sendEmptyMessage(0x110);
        } catch (Throwable tr) {
            Log.e("King", tr.getMessage(), tr);
        }
    }
    /** * 获取线程 * * @return */
    private Runnable getTaskThread() {
        if (mType == Type.FIFO) {

            return mTaskQueue.removeFirst();
        } else if (mType == Type.LIFO) {
            return mTaskQueue.removeLast();
        }
        return null;

    }
    /** * 从缓存中获取图片 */
    public Bitmap getBitmapFromCache(String key) {
        return mCaches.get(key);
    }

    private class ImageHolder {
        Bitmap bitmap;
        ImageView imageView;
        String path;
    }
    private class ImageSize {
        int width;
        int height;
    }

}