移动架构-图片加载框架设计

图片加载核心就那些东西，这里设计一个图片加载框架，涉及到本地加载和网络加载，内存缓存和硬盘缓存，等等

思路

在getView的时候开始框架的调用

1. 配置一系列环境，包括加载策略，缓存策略，线程数量
2. 调用图片显示，从而添加请求到执行队列
3. 请求及转发请求，调用加载器，根据需要从本地或者网络得到图片
4. 得到的图片再选择缓存策略，硬盘缓存或者内存缓存
5. 最后将图片显示出来

实现的功能和用到的知识

根据用户需求可以灵活配置
支持高并发，图片加载的优先级
支持可以选择不同的加载策略，对加载策略进行扩展
二级缓存：加载图片时内存中已经加载了，则从内存中加载，不存在去外置卡中加载，外置还不存在则从网络下载
并对缓存策略可以扩展
支持从加载过程中显示默认加载图片
支持加载失败时，显示默认错误图片
图片显示自适应，从网络加载下来的图片经最佳比例压缩后显示
不能失真变形
支持请求转发，下载

用到的模式：
生产者 消费者模式
建造者模式
单例模式
模板方法模式
策略模式

用到的知识点
内存缓存 LruCache技术
硬盘缓存技术DiskLruCache技术
图片下载时请求转发

实现代码

首先是配置类，DisplayConfig和ImageLoaderConfig，这两个类主要用于显示及下载的初始化配置

//显示图片配置
public class DisplayConfig {
    //默认显示的图片ID
    public int loadingImage = -1;
    public int failedImage = -1;
}

//图片下载配置
public class ImageLoaderConfig {
    //缓存策略
    private BitmapCache bitmapCache = new MemoryCache();
    //加载策略
    private LoadPolicy loadPolicy = new ReversePolicy();
    //默认线程数
    private int threadCount = Runtime.getRuntime().availableProcessors();
    //加载过程显示图片
    private DisplayConfig displayConfig= new DisplayConfig();
    private ImageLoaderConfig() {}

    //建造者模式，使用链式建造
    public static class Builder {
        private ImageLoaderConfig config;

        public Builder() {
            config = new ImageLoaderConfig();
        }

        //设置缓存策略
        public Builder setCachePolicy(BitmapCache bitmapCache) {
            config.bitmapCache = bitmapCache;
            return this;
        }

        //设置加载策略
        public Builder setLoadPolicy(LoadPolicy loadPolicy) {
            config.loadPolicy = loadPolicy;
            return this;
        }

        //设置线程数量
        public Builder setThreadCount(int count) {
            config.threadCount = count;
            return this;
        }

        //设置加载过程中的图片
        public Builder setLoadingImage(int resID) {
            config.displayConfig.loadingImage = resID;
            return this;
        }

        //设置加载失败的图片
        public Builder setFaildImage(int resID){
            config.displayConfig.failedImage = resID;
            return this;
        }

        //返回配置
        public ImageLoaderConfig build() {
            return config;
        }
    }

    public BitmapCache getBitmapCache() {
        return bitmapCache;
    }

    public LoadPolicy getLoadPolicy() {
        return loadPolicy;
    }

    public int getThreadCount() {
        return threadCount;
    }

    public DisplayConfig getDisplayConfig() {
        return displayConfig;
    }
}

然后是图片的请求类，包括BitmapRequest，RequestDispatcher和RequestQueue，用于完成Bitmap请求的封装，请求的转发及请求队列的管理

//bitmap请求
public class BitmapRequest implements Comparable<BitmapRequest> {
    //加载策略
    private LoadPolicy loadPolicy = SimpeImageLoader.getInstance().getConfig().getLoadPolicy();
    //编号
    private int serialNo;
    //持有ImageView的软引用
    private SoftReference<ImageView> imageViewSoft;
    //图片路径
    private String imageUrl;
    //MD5图片路径
    private String imageUrlMD5;
    //下载完成监听
    public SimpeImageLoader.ImageListener imageListener;
    //设置显示配置
    private DisplayConfig displayConfig;

    public BitmapRequest(ImageView imageView, String imageUrl, DisplayConfig displayConfig,
                         SimpeImageLoader.ImageListener imageListener) {
        this.imageViewSoft = new SoftReference<>(imageView);
        //设置可见Image的Tag，防止图片错位
        imageView.setTag(imageUrl);
        this.imageUrl = imageUrl;
        this.imageUrlMD5 = MD5Utils.toMD5(imageUrl);
        if (displayConfig != null) {
            this.displayConfig = displayConfig;
        }
        this.imageListener = imageListener;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        BitmapRequest that = (BitmapRequest) o;
        return serialNo == that.serialNo &&
                Objects.equals(loadPolicy, that.loadPolicy);
    }

    @Override
    public int hashCode() {

        return Objects.hash(loadPolicy, serialNo);
    }

    public int getSerialNo() {
        return serialNo;
    }

    public void setSerialNo(int serialNo) {
        this.serialNo = serialNo;
    }

    public ImageView getImageView() {
        return imageViewSoft.get();
    }

    public String getImageUrl() {
        return imageUrl;
    }

    public String getImageUrlMD5() {
        return imageUrlMD5;
    }

    public DisplayConfig getDisplayConfig() {
        return displayConfig;
    }

    //间接比较，确定优先级
    @Override
    public int compareTo(@NonNull BitmapRequest o) {
        return loadPolicy.compareto(o, this);
    }
}

//转发器，请求转发线程，从请求队列中获取请求
public class RequestDispatcher extends Thread{
    private static final String TAG = "RequestDispatcher";
    //请求队列
    private BlockingQueue<BitmapRequest> requests;

    public RequestDispatcher(BlockingQueue<BitmapRequest> requests) {
        this.requests = requests;
    }

    @Override
    public void run() {
        while(!isInterrupted()){
            try {
                BitmapRequest request = requests.take();
                //处理请求对象
                //解析请求头
                String schema = pareSchema(request.getImageUrl());
                //获取加载器
                Loader loader = LoaderManager.getInstance().getLoader(schema);
                //加载图片
                loader.loadImage(request);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }

    //判断图片路劲来源
    private String pareSchema(String imageUrl) {
        if(imageUrl.contains("://")){
            return imageUrl.split("://")[0];
        }else {
            Log.d(TAG, "不支持此文件类型");
        }
        return null;
    }
}

//请求队列
public class RequestQueue {
    private static final String TAG = "RequestQueue";
    //阻塞式队列，多线程共享
    private BlockingQueue<BitmapRequest> requests = new PriorityBlockingQueue<>();
    //转发器数量
    private int threadCount;
    //一组转发器
    private RequestDispatcher[] dispatchers;
    //请求编号
    private AtomicInteger count = new AtomicInteger(0);

    public RequestQueue(int threadCount) {
        this.threadCount = threadCount;
    }

    //添加请求对象
    public void addRequest(BitmapRequest request) {
        if (!requests.contains(request)) {
            //给请求进行编号
            request.setSerialNo(count.incrementAndGet());
            requests.add(request);
        } else {
            Log.d(TAG, "请求已存在：" + request.getSerialNo());
        }
    }

    //开始请求
    public void start() {
        stop(); //开始前要先停止
        starDispatchers();
    }

    private void starDispatchers() {
        dispatchers = new RequestDispatcher[threadCount];
        for (int i = 0; i < threadCount; i++) {
            RequestDispatcher dispatcher = new RequestDispatcher(requests);
            dispatchers[i] = dispatcher;
            dispatchers[i].start();
        }
    }

    //停止请求
    public void stop() {

    }
}

缓存策略类包括缓存接口BitmapCache，硬盘缓存DiskCache，内存缓存MemoryCache和双缓存DoubleCache

//缓存策略接口
public interface BitmapCache {
    //缓存Bitmap
    void put(BitmapRequest request, Bitmap bitmap);
    //获取Bitmap
    Bitmap get(BitmapRequest request);
    //移除缓存
    void remove(BitmapRequest request);
}

//硬盘缓存策略
public class DiskCache implements BitmapCache {
    //缓存路径
    private String cacheDir = "Image";
    //MB
    private static final int MB = 1024 * 1024;
    private DiskLruCache diskLruCache;
    //单利
    private static DiskCache instance;

    private DiskCache(Context context) {
        initDiskCache(context);
    }

    private void initDiskCache(Context context) {
        //缓存目录
        File dir = getDiskCache(cacheDir, context);
        if (!dir.exists()) {
            dir.mkdir();
        }
        try {
            //设置缓存容量
            diskLruCache = DiskLruCache.open(dir, 1, 1, 50 * MB);
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private File getDiskCache(String cacheDir, Context context) {
        //默认存储路径
        return new File(Environment.getExternalStorageDirectory(), cacheDir);
    }

    public static DiskCache getInstance(Context context) {
        if (instance == null) {
            synchronized (DiskCache.class) {
                if (instance == null) {
                    instance = new DiskCache(context);
                }
            }
        }
        return instance;
    }

    @Override
    public void put(BitmapRequest request, Bitmap bitmap) {
        DiskLruCache.Editor editor = null;
        OutputStream outputStream = null;
        try {
            editor = diskLruCache.edit(request.getImageUrlMD5());
            //一个key对应一个文件
            outputStream = editor.newOutputStream(0);
            if (persistBitmap2Disk(bitmap, outputStream)) {
                editor.commit();
            } else {
                editor.abort();
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    private boolean persistBitmap2Disk(Bitmap bitmap, OutputStream outputStream) {
        BufferedOutputStream bos = new BufferedOutputStream(outputStream);
        bitmap.compress(Bitmap.CompressFormat.JPEG, 100, bos);
        try {
            bos.flush();
        } catch (IOException e) {
            e.printStackTrace();
        } finally {
            IOUtil.closeQuietly(bos);
        }
        return true;
    }

    @Override
    public Bitmap get(BitmapRequest request) {
        try {
            DiskLruCache.Snapshot snapshot = diskLruCache.get(request.getImageUrlMD5());
            if(snapshot != null){
                InputStream inputStream = snapshot.getInputStream(0);
                return BitmapFactory.decodeStream(inputStream);
            }
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    @Override
    public void remove(BitmapRequest request) {
        try {
            diskLruCache.remove(request.getImageUrlMD5());
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

//内存缓存策略
public class MemoryCache implements BitmapCache {

    private LruCache<String, Bitmap> lruCache;

    public MemoryCache() {
        //设置最大缓存值
        int maxSize = (int) (Runtime.getRuntime().freeMemory() / 1024 / 8);
        lruCache = new LruCache<String, Bitmap>(maxSize) {
            //告诉如何计算
            @Override
            protected int sizeOf(String key, Bitmap value) {
                return value.getRowBytes() * value.getHeight();
            }
        };
    }

    @Override
    public void put(BitmapRequest request, Bitmap bitmap) {
        lruCache.put(request.getImageUrlMD5(), bitmap);
    }

    @Override
    public Bitmap get(BitmapRequest request) {
        return lruCache.get(request.getImageUrlMD5());
    }

    @Override
    public void remove(BitmapRequest request) {
        lruCache.remove(request.getImageUrlMD5());
    }
}

public class DoubleCache implements BitmapCache{

    //内存缓存
    private MemoryCache memoryCache = new MemoryCache();
    //硬盘缓存
    private DiskCache diskCache;

    public DoubleCache(Context context){
        diskCache = DiskCache.getInstance(context);
    }

    @Override
    public void put(BitmapRequest request, Bitmap bitmap) {
        memoryCache.put(request,bitmap);
        diskCache.put(request,bitmap);
    }

    @Override
    public Bitmap get(BitmapRequest request) {
        Bitmap bitmap = memoryCache.get(request);
        if(bitmap == null){
            bitmap = diskCache.get(request);
            if(bitmap != null){
                //放在内存，方便读取
                memoryCache.put(request,bitmap);
            }
        }
        return bitmap;
    }

    @Override
    public void remove(BitmapRequest request) {
        memoryCache.remove(request);
        diskCache.remove(request);
    }
}

硬盘缓存调用了一个开源库DiskLruCache，用到了其中的DiskLruCache.java,IOUtils.java和StrictLineReader.java
接下来时加载策略，包括加载策略接口LoadPolicy，ReversePolicy和SerialPolicy

//加载策略接口
public interface LoadPolicy {
    //优先级比较
    int compareto(BitmapRequest request1,BitmapRequest request2);
}

//逆序加载策略
public class ReversePolicy implements LoadPolicy{
    @Override
    public int compareto(BitmapRequest request1, BitmapRequest request2) {
        return request2.getSerialNo() - request1.getSerialNo();
    }
}

//顺序加载策略
public class SerialPolicy implements LoadPolicy{
    @Override
    public int compareto(BitmapRequest request1, BitmapRequest request2) {
        return request1.getSerialNo() - request2.getSerialNo();
    }
}

然后是工具类，包括图片解码类BitmapDecoder，图片宽高计算类ImageViewHelper和MD5工具类MD5Utils

//解码图片
public abstract class BitmapDecoder {
    public Bitmap decodeBitmap(int reqWidth, int reqHeight) {
        //初始化Options
        BitmapFactory.Options options = new BitmapFactory.Options();
        //读取部分信息，获得图片宽高
        options.inJustDecodeBounds = true;
        //根据bitmap加载图片
        decodeBitmapWithOption(options);
        //计算图片缩放比例
        caculateSizeWithOption(options, reqWidth, reqHeight);
        //返回缩放后的Bitmap
        return decodeBitmapWithOption(options);
    }

    private void caculateSizeWithOption(BitmapFactory.Options options, int reqWidth, int reqHeight) {
        int width = options.outWidth;
        int height = options.outHeight;
        int inSampleSize = 1;
        if (width > reqWidth || height > reqHeight) {
            int widthRatio = Math.round((float) width / (float) reqWidth);
            int heightRatio = Math.round((float) height / (float) reqHeight);
            inSampleSize = Math.max(widthRatio, heightRatio);
        }
        options.inSampleSize = inSampleSize;
        options.inPreferredConfig = Bitmap.Config.RGB_565;
        options.inJustDecodeBounds = false;
        //内存不足时回收Bitmap
        options.inPurgeable = true;
        options.inInputShareable = true;
    }

    public abstract Bitmap decodeBitmapWithOption(BitmapFactory.Options options);
}

public class ImageViewHelper {

	//默认的图片宽高
	private static int DEFAULT_WIDTH = 200;
	private static int DEFAULT_HEIGHT = 200;
	
	//获取ImageView控件的宽度
	public static int getImageViewWidth(ImageView imageView){
		if(imageView != null){
			LayoutParams params = imageView.getLayoutParams();
			int width = 0;
			if(params != null && params.width != LayoutParams.WRAP_CONTENT){
				width = imageView.getWidth();
			}
			if(width <= 0 && params != null){
				width = params.width;
			}
			if(width <= 0){
				width = getImageViewFieldValue(imageView,"mMaxWidth");
			}
			return width;
		}
		return DEFAULT_WIDTH;
	}
	
	//获取图片的高度
	public static int getImageViewHeight(ImageView imageView){
		if(imageView != null){
			LayoutParams params = imageView.getLayoutParams();
			int height = 0;
			if(params != null && params.height != LayoutParams.WRAP_CONTENT){
				height = imageView.getWidth();
			}
			if(height <= 0 && params != null){
				height = params.height;
			}
			if(height <= 0){
				height = getImageViewFieldValue(imageView,"mMaxHeight");
			}
			return height;
		}
		return DEFAULT_HEIGHT;
	}
	
	private static int getImageViewFieldValue(ImageView imageView,String fieldName) {
		try {
			Field field = ImageView.class.getDeclaredField(fieldName);
			field.setAccessible(true);
			int fieldValue = (Integer)field.get(imageView);
			if(fieldValue > 0 && fieldValue < Integer.MAX_VALUE){
				return fieldValue;
			}
		} catch (Exception e) {
			e.printStackTrace();
		}
		return 0;
	}
}

public class MD5Utils {
    private static final String TAG = "MD5Utils";
    private static MessageDigest digest;

    static {
        try {
            digest = MessageDigest.getInstance("MD5");
        } catch (NoSuchAlgorithmException e) {
            e.printStackTrace();
            Log.d(TAG, "MD5算法不支持");
        }
    }

    //MD5加密
    public static String toMD5(String key) {
        if (digest == null) {
            return String.valueOf(key.hashCode());
        }
        //更新字节
        digest.update(key.getBytes());
        //获取最终的摘要
        return convert2HexString(digest.digest());
    }

    //转为16进制字符串
    private static String convert2HexString(byte[] bytes) {
        StringBuffer sb = new StringBuffer();
        for (byte b : bytes) {
            String hex = Integer.toHexString(0xFF & b);
            if (hex.length() == 1) {
                sb.append('0');
            }
            sb.append(hex);
        }
        return sb.toString();
    }
}

最后是加载类，包括加载接口Loader，加载抽象类AbstractLoader，硬盘加载器LocalLoader，网络加载器UrlLoader，空加载器NullLoader，图片加载器SimpeImageLoader，加载器管理LoaderManager

//加载器接口
public interface Loader {
    //加载图片
    void loadImage(BitmapRequest request);
}

//抽象加载器
public abstract class AbstractLoader implements Loader {
    //持有缓存策略，得到自定义缓存策略
    private BitmapCache bitmapCache = SimpeImageLoader.getInstance().getConfig().getBitmapCache();
    //拿到显示配置
    private DisplayConfig displayConfig = SimpeImageLoader.getInstance().getConfig().getDisplayConfig();

    @Override
    public void loadImage(BitmapRequest request) {
        //从缓存中读取bitmap
        Bitmap bitmap = bitmapCache.get(request);
        if (bitmap == null) {
            //显示默认加载图片
            showLoadingImage(request);
            //加载图片
            bitmap = onLoad(request);
            //缓存图片
            cacheBitmap(request, bitmap);
        }
        deliveryToUIThread(request, bitmap);
    }

    //交给主线程显示
    protected void deliveryToUIThread(final BitmapRequest request, final Bitmap bitmap) {
        ImageView imageView = request.getImageView();
        if (imageView != null) {
            imageView.post(new Runnable() {
                @Override
                public void run() {
                    updateImageView(request, bitmap);
                }
            });
        }
    }

    //更新ImageView
    private void updateImageView(BitmapRequest request, Bitmap bitmap) {
        ImageView imageView = request.getImageView();
        //加载正常
        if (bitmap != null && imageView.getTag().equals(request.getImageUrl())) {
            imageView.setImageBitmap(bitmap);
        }
        //加载失败
        if (bitmap == null && request.getDisplayConfig() != null &&
                request.getDisplayConfig().failedImage != -1) {
            imageView.setImageResource(displayConfig.failedImage);
        }
        //监听 回调
        if (request.imageListener != null) {
            request.imageListener.onComplete(imageView, bitmap, request.getImageUrl());
        }
    }

    //缓存图片
    private void cacheBitmap(BitmapRequest request, Bitmap bitmap) {
        if (request != null && bitmap != null) {
            synchronized (AbstractLoader.class) {
                bitmapCache.put(request, bitmap);
            }
        }
    }

    //抽象的加载方法，由子类去实现
    protected abstract Bitmap onLoad(BitmapRequest request);

    //加载前显示的图片
    protected void showLoadingImage(BitmapRequest request) {
        if (hasLoadingPlaceHolder()) {
            final ImageView imageView = request.getImageView();
            if (imageView != null) {
                imageView.post(new Runnable() {
                    @Override
                    public void run() {
                        imageView.setImageResource(displayConfig.loadingImage);
                    }
                });
            }
        }
    }

    protected boolean hasLoadingPlaceHolder() {
        return (displayConfig != null && displayConfig.loadingImage > 0);
    }

    protected boolean hasFailedPlaceHolder() {
        return (displayConfig != null && displayConfig.failedImage > 0);
    }
}

//硬盘加载器
public class LocalLoader extends AbstractLoader{
    @Override
    protected Bitmap onLoad(BitmapRequest request) {
        //得到本地图片路径
        final String path = Uri.parse(request.getImageUrl()).getPath();
        File file = new File(path);
        if(!file.exists()){
            return null;
        }
        BitmapDecoder decoder = new BitmapDecoder() {
            @Override
            public Bitmap decodeBitmapWithOption(BitmapFactory.Options options) {
                return BitmapFactory.decodeFile(path,options);
            }
        };
        return decoder.decodeBitmap(ImageViewHelper.getImageViewWidth(request.getImageView()),
                ImageViewHelper.getImageViewHeight(request.getImageView()));
    }
}

//网络加载器
public class UrlLoader extends AbstractLoader {
    @Override
    protected Bitmap onLoad(final BitmapRequest request) {
        //下载之后读取
        downloadImgByUrl(request.getImageUrl(), getCache(request.getImageUrlMD5()));
        BitmapDecoder decoder = new BitmapDecoder() {
            @Override
            public Bitmap decodeBitmapWithOption(BitmapFactory.Options options) {
                return BitmapFactory.decodeFile(getCache(request.getImageUrlMD5()).getAbsolutePath(), options);
            }
        };
        return decoder.decodeBitmap(ImageViewHelper.getImageViewWidth(request.getImageView())
                , ImageViewHelper.getImageViewHeight(request.getImageView()));
    }

    public static boolean downloadImgByUrl(String urlStr, File file) {
        FileOutputStream fos = null;
        InputStream is = null;
        try {
            URL url = new URL(urlStr);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            is = conn.getInputStream();
            fos = new FileOutputStream(file);
            byte[] buf = new byte[512];
            int len = 0;
            while ((len = is.read(buf)) != -1) {
                fos.write(buf, 0, len);
            }
            fos.flush();
            return true;
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                if (is != null)
                    is.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
            try {
                if (fos != null)
                    fos.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return false;
    }

    private File getCache(String unipue) {
        File file = new File(Environment.getExternalStorageDirectory(), "ImageLoader");
        if (!file.exists()) {
            file.mkdir();
        }
        return new File(file, unipue);
    }
}

public class NullLoader extends AbstractLoader {
    @Override
    protected Bitmap onLoad(BitmapRequest request) {
        return null;
    }
}

//图片加载器，单利对象
public class SimpeImageLoader {
    //配置文件
    private ImageLoaderConfig config;
    //请求队列
    private RequestQueue queue;
    //单利
    private static volatile SimpeImageLoader instance;

    private SimpeImageLoader() {
    }

    private SimpeImageLoader(ImageLoaderConfig config) {
        this.config = config;
        queue = new RequestQueue(config.getThreadCount());
        //开启请求队列
        queue.start();
    }

    public static SimpeImageLoader getInstance(ImageLoaderConfig config) {
        if (instance == null) {
            synchronized (SimpeImageLoader.class) {
                if (instance == null) {
                    instance = new SimpeImageLoader(config);
                }
            }
        }
        return instance;
    }

    //第二次获取单利
    public static SimpeImageLoader getInstance() {
        if (instance == null) {
            throw new UnsupportedOperationException("未初始化参数");
        }
        return instance;
    }

    //获取全局配置
    public ImageLoaderConfig getConfig(){
        return config;
    }

    //获取图片
    public void displayImage(ImageView imageView, String url) {
        displayImage(imageView, url, null, null);
    }

    //扩展，重载
    public void displayImage(ImageView imageView, String url,
                             DisplayConfig displayConfig, ImageListener imageListener) {
        //实例化请求
        BitmapRequest bitmapRequest = new BitmapRequest(imageView,url,displayConfig,imageListener);
        //添加请求到队列
        queue.addRequest(bitmapRequest);

    }

    //扩展接口
    public interface ImageListener {
        void onComplete(ImageView imageView, Bitmap bitmap, String url);
    }
}

//加载器管理
public class LoaderManager {
    //缓存支持的Loader类型
    private Map<String, Loader> loaderMap = new HashMap<>();
    //单例模式
    private static LoaderManager instance = new LoaderManager();

    private LoaderManager() {
        register("http", new UrlLoader());
        register("https", new UrlLoader());
        register("file", new LocalLoader());
    }

    public static LoaderManager getInstance() {
        return instance;
    }

    private void register(String schema, Loader Loader) {
        loaderMap.put(schema, Loader);
    }

    public Loader getLoader(String schema){
        if(loaderMap.containsKey(schema)){
            return loaderMap.get(schema);
        }
        return new NullLoader();
    }
}

最后是测试类，这里我是用tomcat搭建服务器，使用975张图片做测试

public class MainActivity extends AppCompatActivity {

    private SimpeImageLoader imageLoader;
    private static final int COUNT = 975;
    private static final String path = "http://192.168.1.2:8080/test/";

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.list);
        GridView listview = (GridView) findViewById(R.id.listview);
        listview.setAdapter(new MyAdapter(this));

        //配置
        ImageLoaderConfig.Builder build = new ImageLoaderConfig.Builder();
        build.setThreadCount(3) //线程数量
                .setLoadPolicy(new ReversePolicy()) //加载策略
                .setCachePolicy(new DoubleCache(this)) //缓存策略
                .setLoadingImage(R.drawable.loading)
                .setFaildImage(R.drawable.not_found);

        ImageLoaderConfig config = build.build();
        //初始化
        imageLoader = SimpeImageLoader.getInstance(config);
    }

    class MyAdapter extends BaseAdapter {

        private LayoutInflater inflater;

        public MyAdapter(Context context) {
            inflater = LayoutInflater.from(context);
        }

        @Override
        public int getCount() {
            return COUNT;
        }

        @Override
        public Object getItem(int position) {
            return getUrl(position);
        }

        @Override
        public long getItemId(int position) {
            return position;
        }

        @Override
        public View getView(int position, View convertView, ViewGroup parent) {
            View item = inflater.inflate(R.layout.item, null);
            ImageView imageView = (ImageView) item.findViewById(R.id.iv);
            //请求图片
            imageLoader.displayImage(imageView, getUrl(position));
            return item;
        }
    }

    public String getUrl(int position) {
        if (position < 10)
            return path + "00" + position + ".jpg";
        else if (position < 100)
            return path + "0" + position + ".jpg";
        else if (position < COUNT)
            return path + position + ".jpg";
        else
            return null;
    }
}

其实，这就是缩水版的Glide