ImageLoader图片加载器

思路：展示一张图片， 先从内存（Lurcache容器）中查找，如果有 从内存中取出展示；没有 再到SD卡中查找，SD卡中有就从SD卡中取出展示，如果都没有 就从网络上下载，保存到内存和SD卡。

//图片加载器
public class ImageLoader {

    //图片缓存(默认缓存)
    private ImageCache mImageCache = new DoubleCache();
    //线程池，数量为CPU的数量
    private ExecutorService mExecutorService = Executors.newFixedThreadPool(Runtime.getRuntime().availableProcessors());

    //注入缓存实现
    public void setImageCache(ImageCache cache) {
        mImageCache = cache;
    }

    public void displayImage(final String imageUrl, final ImageView imageView) {
        Bitmap bitmap = mImageCache.get(imageUrl);
        if (bitmap != null) {
            imageView.setImageBitmap(bitmap);
            return;
        }
        //图片没缓存，提交到线程池中下载图片
        submitLoadRequest(imageUrl, imageView);

    }

    private void submitLoadRequest(final String imageUrl, final ImageView imageView) {
        imageView.setTag(imageUrl);
        mExecutorService.submit(new Runnable() {
            @Override
            public void run() {
                Bitmap bitmap = downloadImage(imageUrl);
                if (bitmap == null) {
                    return;
                }
                if (imageView.getTag().equals(imageUrl)) {
                    imageView.setImageBitmap(bitmap);
                }
                mImageCache.put(imageUrl, bitmap);
            }
        });
    }


    private Bitmap downloadImage(String imageUrl) {
        Bitmap bitmap = null;
        try {
            URL url = new URL(imageUrl);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            bitmap = BitmapFactory.decodeStream(conn.getInputStream());
            conn.disconnect();
        } catch (MalformedURLException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
        return bitmap;
    }
}

//图片缓存接口
public interface ImageCache {

    //从缓存中取出图片
    Bitmap get(String url);

    //将图片put入缓存中
    void put(String url, Bitmap bitmap);

}

//内存加载
public class MemoryCache implements ImageCache{

    private LruCache<String, Bitmap> mMemoryCache;

    //初始化LRU缓存
    public MemoryCache(){
        initImageCache();
    }


    private void initImageCache(){
        //计算可用的最大内存
        int maxMemory = (int)(Runtime.getRuntime().maxMemory() / 1024);
        //取四分之一可用内存作缓存
        int cacheSize = maxMemory / 4;
        mMemoryCache = new LruCache<String, Bitmap>(cacheSize){
            @Override
            protected int sizeOf(String key, Bitmap bitmap) {
                return bitmap.getRowBytes() * bitmap.getHeight() / 1024;
            }
        };
    }


    @Override
    public Bitmap get(String url) {
        return mMemoryCache.get(url);
    }

    @Override
    public void put(String url, Bitmap bitmap) {
        mMemoryCache.put(url, bitmap);
    }
}

//内存缓存
public class DiskCache implements ImageCache {

    private static String cacheDir = "sdcard/cache/";

    //从本地获取该图片
    @Override
    public Bitmap get(String url) {
        return BitmapFactory.decodeFile(cacheDir + url);
    }

    //将bitmap写入文件中
    @Override
    public void put(String url, Bitmap bitmap) {
        FileOutputStream fileOutputStream = null;
        try {
            fileOutputStream = new FileOutputStream(cacheDir + url);
            bitmap.compress(Bitmap.CompressFormat.PNG, 100, fileOutputStream);
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } finally {
            if (fileOutputStream != null) {
                try {
                    fileOutputStream.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

//双缓存DoubleCache类
public class DoubleCache implements ImageCache {

    private ImageCache mMemoryCache = new MemoryCache();
    private ImageCache mDiskCache = new DiskCache();

    //先从内存中获取图片，如果没有，再从SD卡中获取
    @Override
    public Bitmap get(String url) {
        Bitmap bitmap = mMemoryCache.get(url);
        if (bitmap == null){
            bitmap = mDiskCache.get(url);
        }
        return bitmap;
    }

    //将图片缓存到内存和SD卡中
    @Override
    public void put(String url, Bitmap bitmap) {
        mMemoryCache.put(url, bitmap);
        mDiskCache.put(url, bitmap);
    }
}

摘录自：Android源码设计模式解析与实战。