使用下面的类来下载和加载listview中的图像。它会在下载后缓存每个图像。还加载图像和延迟加载。

package com.fudiyoxpress.images;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.Collections;
import java.util.Map;
import java.util.WeakHashMap;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Handler;
import android.widget.ImageView;

import com.fudiyoxpress.R;
import com.fudiyoxpress.config.Config;
import com.fudiyoxpress.twitter.ScaleBitmap;

public class ImageLoader {

    // Initialize MemoryCache
    MemoryCache memoryCache = new MemoryCache();

    FileCache fileCache;

    Context C;

    // Create Map (collection) to store image and image url in key value pair
    private Map<ImageView, String> imageViews = Collections
            .synchronizedMap(new WeakHashMap<ImageView, String>());
    ExecutorService executorService;

    // handler to display images in UI thread
    Handler handler = new Handler();

    public ImageLoader(Context context) {

        C = context;
        fileCache = new FileCache(context);

        // Creates a thread pool that reuses a fixed number of
        // threads operating off a shared unbounded queue.
        executorService = Executors.newFixedThreadPool(5);

    }

    // default image show in list (Before online image download)
    final int stub_id = R.drawable.restlogoplaceholder;

    public void DisplayImage(String url, ImageView imageView, Context context,
            boolean header_flag) {

        Bitmap largeIcon = BitmapFactory.decodeResource(context.getResources(),
                R.drawable.restlogoplaceholder);
        header_flag = false;
        // Store image and url in Map
        imageViews.put(imageView, url);

        // Check image is stored in MemoryCache Map or not (see
        // MemoryCache.java)
        Bitmap bitmap = memoryCache.get(url);

        if (bitmap != null) {
            // if image is stored in MemoryCache Map then
            // Show image in listview row
            Bitmap b = ScaleBitmap
                    .getScaledBitmap(context, bitmap, header_flag);
            imageView.setImageBitmap(b);

        } else {
            // queue Photo to download from url
            queuePhoto(url, imageView, header_flag);

            // Before downloading image show default image
            imageView.setImageBitmap(ScaleBitmap.getScaledBitmap(context,
                    largeIcon, header_flag));

        }
    }



    private void queuePhoto(String url, ImageView imageView, boolean header_flag) {
        // Store image and url in PhotoToLoad object
        PhotoToLoad p = new PhotoToLoad(url, imageView, header_flag);

        // pass PhotoToLoad object to PhotosLoader runnable class
        // and submit PhotosLoader runnable to executers to run runnable
        // Submits a PhotosLoader runnable task for execution

        executorService.submit(new PhotosLoader(p));
    }

    // Task for the queue
    private class PhotoToLoad {
        public String url;
        public ImageView imageView;
        public boolean b;

        public PhotoToLoad(String u, ImageView i, boolean header_flag) {
            url = u;
            imageView = i;
            b = header_flag;
        }
    }

    class PhotosLoader implements Runnable {
        PhotoToLoad photoToLoad;

        PhotosLoader(PhotoToLoad photoToLoad) {
            this.photoToLoad = photoToLoad;
        }

        @Override
        public void run() {
            try {
                // Check if image already downloaded
                if (imageViewReused(photoToLoad))
                    return;
                // download image from web url
                Bitmap bmp = getBitmap(photoToLoad.url);

                // set image data in Memory Cache
                memoryCache.put(photoToLoad.url, bmp);

                if (imageViewReused(photoToLoad))
                    return;

                // Get bitmap to display
                BitmapDisplayer bd = new BitmapDisplayer(bmp, photoToLoad);

                // Causes the Runnable bd (BitmapDisplayer) to be added to the
                // message queue.
                // The runnable will be run on the thread to which this handler
                // is attached.
                // BitmapDisplayer run method will call
                handler.post(bd);

            } catch (Throwable th) {
                // th.printStackTrace();
            }
        }
    }

    private Bitmap getBitmap(String url) {
        File f = fileCache.getFile(url);

        // from SD cache
        // CHECK : if trying to decode file which not exist in cache return null
        Bitmap b = decodeFile(f);
        if (b != null)
            return b;

        // Download image file from web
        try {

            // // download the image
            Bitmap bitmap = null;

            URL imageURL = null;
            try {

                imageURL = new URL(Config.WEB_URL + "/ServeBlob?id=" + url);

                HttpURLConnection connection = (HttpURLConnection) imageURL
                        .openConnection();
                connection.setDoInput(true);
                connection.connect();
                // if(!(new File(imageURL.toString())).exists())
                // {
                // imageURL=new URL("");
                // }
                InputStream inputStream = connection.getInputStream();

                // Constructs a new FileOutputStream that writes to
                // file
                // if file not exist then it will create file
                OutputStream os = new FileOutputStream(f);

                // See Utils class CopyStream method
                // It will each pixel from input stream and
                // write pixels to output stream (file)
                Utils.CopyStream(inputStream, os);

                os.close();

                BitmapFactory.Options options = new BitmapFactory.Options();
                options.inSampleSize = 8;

                bitmap = BitmapFactory.decodeStream(inputStream, null, options);

            } catch (IOException e) {

                // e.printStackTrace();
            }

            // Now file created and going to resize file with defined height
            // Decodes image and scales it to reduce memory consumption
            bitmap = decodeFile(f);

            return bitmap;

        } catch (Throwable ex) {
            ex.printStackTrace();
            if (ex instanceof OutOfMemoryError)
                memoryCache.clear();
            return null;
        }
    }

    // Decodes image and scales it to reduce memory consumption
    private Bitmap decodeFile(File f) {

        try {

            // Decode image size
            BitmapFactory.Options o = new BitmapFactory.Options();
            o.inJustDecodeBounds = true;
            FileInputStream stream1 = new FileInputStream(f);
            BitmapFactory.decodeStream(stream1, null, o);
            stream1.close();

            // Find the correct scale value. It should be the power of 2.

            // Set width/height of recreated image
            final int REQUIRED_SIZE = 85;

            int width_tmp = o.outWidth, height_tmp = o.outHeight;
            int scale = 1;
            while (true) {
                if (width_tmp / 2 < REQUIRED_SIZE
                        || height_tmp / 2 < REQUIRED_SIZE)
                    break;
                width_tmp /= 2;
                height_tmp /= 2;
                scale *= 2;
            }

            // decode with current scale values
            BitmapFactory.Options o2 = new BitmapFactory.Options();
            o2.inSampleSize = scale;
            FileInputStream stream2 = new FileInputStream(f);
            Bitmap bitmap = BitmapFactory.decodeStream(stream2, null, o2);
            stream2.close();
            return bitmap;

        } catch (FileNotFoundException e) {
        } catch (IOException e) {
            e.printStackTrace();
        }
        return null;
    }

    boolean imageViewReused(PhotoToLoad photoToLoad) {

        String tag = imageViews.get(photoToLoad.imageView);
        // Check url is already exist in imageViews MAP
        if (tag == null || !tag.equals(photoToLoad.url))
            return true;
        return false;
    }

    // Used to display bitmap in the UI thread
    class BitmapDisplayer implements Runnable {
        Bitmap bitmap;
        PhotoToLoad photoToLoad;

        public BitmapDisplayer(Bitmap b, PhotoToLoad p) {
            bitmap = b;
            photoToLoad = p;
        }

        public void run() {
            if (imageViewReused(photoToLoad))
                return;

            // Show bitmap on UI
            if (bitmap != null) {
                photoToLoad.imageView.setImageBitmap(ScaleBitmap
                        .getScaledBitmap(C, bitmap, photoToLoad.b));
            } else {

            }
            // photoToLoad.imageView.setImageResource(stub_id);

        }
    }

    public void clearCache() {
        // Clear cache directory downloaded images and stored data in maps
        memoryCache.clear();
        fileCache.clear();
    }

}




package com.fudiyoxpress.images;

import java.util.Collections;
import java.util.Iterator;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Map.Entry;
import android.graphics.Bitmap;
import android.util.Log;

public class MemoryCache {

    private static final String TAG = "MemoryCache";

    //Last argument true for LRU ordering
    private Map<String, Bitmap> cache = Collections.synchronizedMap(
            new LinkedHashMap<String, Bitmap>(10,1.5f,true));

   //current allocated size
    private long size=0; 

    //max memory cache folder used to download images in bytes
    private long limit = 1000000; 

    public MemoryCache(){

        //use 25% of available heap size
        setLimit(Runtime.getRuntime().maxMemory()/4);
    }

    public void setLimit(long new_limit){

        limit=new_limit;
        Log.i(TAG, "MemoryCache will use up to "+limit/1024./1024.+"MB");
    }

    public Bitmap get(String id){
        try{
            if(!cache.containsKey(id))
                return null;
            //NullPointerException sometimes happen here http://code.google.com/p/osmdroid/issues/detail?id=78 
            return cache.get(id);
        }catch(NullPointerException ex){
            ex.printStackTrace();
            return null;
        }
    }

    public void put(String id, Bitmap bitmap){
        try{
            if(cache.containsKey(id))
                size-=getSizeInBytes(cache.get(id));
            cache.put(id, bitmap);
            size+=getSizeInBytes(bitmap);
            checkSize();
        }catch(Throwable th){
            th.printStackTrace();
        }
    }

    private void checkSize() {
        Log.i(TAG, "cache size="+size+" length="+cache.size());
        if(size>limit){
            Iterator<Entry<String, Bitmap>> iter=cache.entrySet().iterator();//least recently accessed item will be the first one iterated  
            while(iter.hasNext()){
                Entry<String, Bitmap> entry=iter.next();
                size-=getSizeInBytes(entry.getValue());
                iter.remove();
                if(size<=limit)
                    break;
            }
            Log.i(TAG, "Clean cache. New size "+cache.size());
        }
    }

    public void clear() {
        try{
            //NullPointerException sometimes happen here http://code.google.com/p/osmdroid/issues/detail?id=78 
            cache.clear();
            size=0;
        }catch(NullPointerException ex){
            ex.printStackTrace();
        }
    }

    long getSizeInBytes(Bitmap bitmap) {
        if(bitmap==null)
            return 0;
        return bitmap.getRowBytes() * bitmap.getHeight();
    }
}




package com.fudiyoxpress.images;

import java.io.InputStream;
import java.io.OutputStream;

public class Utils {
    public static void CopyStream(InputStream is, OutputStream os)
    {
        final int buffer_size=1024;
        try
        {

            byte[] bytes=new byte[buffer_size];
            for(;;)
            {
              //Read byte from input stream

              int count=is.read(bytes, 0, buffer_size);
              if(count==-1)
                  break;

              //Write byte from output stream
              os.write(bytes, 0, count);
            }
        }
        catch(Exception ex){}
    }
}

我写了一篇教程，解释如何在列表视图中延迟加载图像。我详细讨论了回收和并发的问题。我还使用固定线程池来防止产生大量线程。

在Listview教程中延迟加载图像

除了异步加载数据缓存外，您可能需要UI缓存，如setViewCacheSize

除了加载可见项数据外，您可能需要加载近似的可见项数据

AndroidX分页库是另一个选项，例如，您可以从SQLite数据库加载、缓存和显示10000000个项目到RecyclerView。参考PagedList

例子：假设列表视图可见项为[6,7,8,9,10]，您可能需要加载[6,7,8,10]并预加载项[1，2，3，4，5]和[11，12，13，14，15]，因为用户可能会滚动到前页或后页

你必须试试这个通用加载器。我在做了很多关于延迟加载的RnD之后使用了这个。

通用图像加载器

特征

多线程图像加载（异步或同步）ImageLoader配置的广泛定制（线程执行器、下载器、解码器、内存和磁盘缓存、显示图像选项等）每个显示图像调用都有许多自定义选项（存根图像、缓存开关、解码选项、位图处理和显示等）内存和/或磁盘上的图像缓存（设备的文件系统或SD卡）收听加载过程（包括下载进度）

Android 2.0+支持

高性能加载器-在检查了这里建议的方法之后，我用了Ben的解决方案-

我意识到使用drawable比使用位图更快，所以我使用了drawable使用SoftReference很好，但它会使缓存的图像太频繁地被删除，所以我添加了一个保存图像引用的链接列表，防止图像被删除，直到达到预定义的大小为了打开InputStream，我使用java.net.URLConnection，它允许我使用web缓存（您需要先设置响应缓存，但这是另一回事）

我的代码：

import java.util.Map; 
import java.util.HashMap; 
import java.util.LinkedList; 
import java.util.Collections; 
import java.util.WeakHashMap; 
import java.lang.ref.SoftReference; 
import java.util.concurrent.Executors; 
import java.util.concurrent.ExecutorService; 
import android.graphics.drawable.Drawable;
import android.widget.ImageView;
import android.os.Handler;
import android.os.Message;
import java.io.InputStream;
import java.net.MalformedURLException; 
import java.io.IOException; 
import java.net.URL;
import java.net.URLConnection;

public class DrawableBackgroundDownloader {    

private final Map<String, SoftReference<Drawable>> mCache = new HashMap<String, SoftReference<Drawable>>();   
private final LinkedList <Drawable> mChacheController = new LinkedList <Drawable> ();
private ExecutorService mThreadPool;  
private final Map<ImageView, String> mImageViews = Collections.synchronizedMap(new WeakHashMap<ImageView, String>());  

public static int MAX_CACHE_SIZE = 80; 
public int THREAD_POOL_SIZE = 3;

/**
 * Constructor
 */
public DrawableBackgroundDownloader() {  
    mThreadPool = Executors.newFixedThreadPool(THREAD_POOL_SIZE);  
}  


/**
 * Clears all instance data and stops running threads
 */
public void Reset() {
    ExecutorService oldThreadPool = mThreadPool;
    mThreadPool = Executors.newFixedThreadPool(THREAD_POOL_SIZE);
    oldThreadPool.shutdownNow();

    mChacheController.clear();
    mCache.clear();
    mImageViews.clear();
}  

public void loadDrawable(final String url, final ImageView imageView,Drawable placeholder) {  
    mImageViews.put(imageView, url);  
    Drawable drawable = getDrawableFromCache(url);  

    // check in UI thread, so no concurrency issues  
    if (drawable != null) {  
        //Log.d(null, "Item loaded from mCache: " + url);  
        imageView.setImageDrawable(drawable);  
    } else {  
        imageView.setImageDrawable(placeholder);  
        queueJob(url, imageView, placeholder);  
    }  
} 


private Drawable getDrawableFromCache(String url) {  
    if (mCache.containsKey(url)) {  
        return mCache.get(url).get();  
    }  

    return null;  
}

private synchronized void putDrawableInCache(String url,Drawable drawable) {  
    int chacheControllerSize = mChacheController.size();
    if (chacheControllerSize > MAX_CACHE_SIZE) 
        mChacheController.subList(0, MAX_CACHE_SIZE/2).clear();

    mChacheController.addLast(drawable);
    mCache.put(url, new SoftReference<Drawable>(drawable));

}  

private void queueJob(final String url, final ImageView imageView,final Drawable placeholder) {  
    /* Create handler in UI thread. */  
    final Handler handler = new Handler() {  
        @Override  
        public void handleMessage(Message msg) {  
            String tag = mImageViews.get(imageView);  
            if (tag != null && tag.equals(url)) {
                if (imageView.isShown())
                    if (msg.obj != null) {
                        imageView.setImageDrawable((Drawable) msg.obj);  
                    } else {  
                        imageView.setImageDrawable(placeholder);  
                        //Log.d(null, "fail " + url);  
                    } 
            }  
        }  
    };  

    mThreadPool.submit(new Runnable() {  
        @Override  
        public void run() {  
            final Drawable bmp = downloadDrawable(url);
            // if the view is not visible anymore, the image will be ready for next time in cache
            if (imageView.isShown())
            {
                Message message = Message.obtain();  
                message.obj = bmp;
                //Log.d(null, "Item downloaded: " + url);  

                handler.sendMessage(message);
            }
        }  
    });  
}  



private Drawable downloadDrawable(String url) {  
    try {  
        InputStream is = getInputStream(url);

        Drawable drawable = Drawable.createFromStream(is, url);
        putDrawableInCache(url,drawable);  
        return drawable;  

    } catch (MalformedURLException e) {  
        e.printStackTrace();  
    } catch (IOException e) {  
        e.printStackTrace();  
    }  

    return null;  
}  


private InputStream getInputStream(String urlString) throws MalformedURLException, IOException {
    URL url = new URL(urlString);
    URLConnection connection;
    connection = url.openConnection();
    connection.setUseCaches(true); 
    connection.connect();
    InputStream response = connection.getInputStream();

    return response;
}
}

public class ImageDownloader {

Map<String, Bitmap> imageCache;

public ImageDownloader() {
    imageCache = new HashMap<String, Bitmap>();

}

// download function
public void download(String url, ImageView imageView) {
    if (cancelPotentialDownload(url, imageView)) {

        // Caching code right here
        String filename = String.valueOf(url.hashCode());
        File f = new File(getCacheDirectory(imageView.getContext()),
                filename);

        // Is the bitmap in our memory cache?
        Bitmap bitmap = null;

        bitmap = (Bitmap) imageCache.get(f.getPath());

        if (bitmap == null) {

            bitmap = BitmapFactory.decodeFile(f.getPath());

            if (bitmap != null) {
                imageCache.put(f.getPath(), bitmap);
            }

        }
        // No? download it
        if (bitmap == null) {
            try {
                BitmapDownloaderTask task = new BitmapDownloaderTask(
                        imageView);
                DownloadedDrawable downloadedDrawable = new DownloadedDrawable(
                        task);
                imageView.setImageDrawable(downloadedDrawable);
                task.execute(url);
            } catch (Exception e) {
                Log.e("Error==>", e.toString());
            }

        } else {
            // Yes? set the image
            imageView.setImageBitmap(bitmap);
        }
    }
}

// cancel a download (internal only)
private static boolean cancelPotentialDownload(String url,
        ImageView imageView) {
    BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);

    if (bitmapDownloaderTask != null) {
        String bitmapUrl = bitmapDownloaderTask.url;
        if ((bitmapUrl == null) || (!bitmapUrl.equals(url))) {
            bitmapDownloaderTask.cancel(true);
        } else {
            // The same URL is already being downloaded.
            return false;
        }
    }
    return true;
}

// gets an existing download if one exists for the imageview
private static BitmapDownloaderTask getBitmapDownloaderTask(
        ImageView imageView) {
    if (imageView != null) {
        Drawable drawable = imageView.getDrawable();
        if (drawable instanceof DownloadedDrawable) {
            DownloadedDrawable downloadedDrawable = (DownloadedDrawable) drawable;
            return downloadedDrawable.getBitmapDownloaderTask();
        }
    }
    return null;
}

// our caching functions
// Find the dir to save cached images
private static File getCacheDirectory(Context context) {
    String sdState = android.os.Environment.getExternalStorageState();
    File cacheDir;

    if (sdState.equals(android.os.Environment.MEDIA_MOUNTED)) {
        File sdDir = android.os.Environment.getExternalStorageDirectory();

        // TODO : Change your diretcory here
        cacheDir = new File(sdDir, "data/ToDo/images");
    } else
        cacheDir = context.getCacheDir();

    if (!cacheDir.exists())
        cacheDir.mkdirs();
    return cacheDir;
}

private void writeFile(Bitmap bmp, File f) {
    FileOutputStream out = null;

    try {
        out = new FileOutputStream(f);
        bmp.compress(Bitmap.CompressFormat.PNG, 80, out);
    } catch (Exception e) {
        e.printStackTrace();
    } finally {
        try {
            if (out != null)
                out.close();
        } catch (Exception ex) {
        }
    }
}

// download asynctask
public class BitmapDownloaderTask extends AsyncTask<String, Void, Bitmap> {
    private String url;
    private final WeakReference<ImageView> imageViewReference;

    public BitmapDownloaderTask(ImageView imageView) {
        imageViewReference = new WeakReference<ImageView>(imageView);
    }

    @Override
    // Actual download method, run in the task thread
    protected Bitmap doInBackground(String... params) {
        // params comes from the execute() call: params[0] is the url.
        url = (String) params[0];
        return downloadBitmap(params[0]);
    }

    @Override
    // Once the image is downloaded, associates it to the imageView
    protected void onPostExecute(Bitmap bitmap) {
        if (isCancelled()) {
            bitmap = null;
        }

        if (imageViewReference != null) {
            ImageView imageView = imageViewReference.get();
            BitmapDownloaderTask bitmapDownloaderTask = getBitmapDownloaderTask(imageView);
            // Change bitmap only if this process is still associated with
            // it
            if (this == bitmapDownloaderTask) {
                imageView.setImageBitmap(bitmap);

                // cache the image

                String filename = String.valueOf(url.hashCode());
                File f = new File(
                        getCacheDirectory(imageView.getContext()), filename);

                imageCache.put(f.getPath(), bitmap);

                writeFile(bitmap, f);
            }
        }
    }

}

static class DownloadedDrawable extends ColorDrawable {
    private final WeakReference<BitmapDownloaderTask> bitmapDownloaderTaskReference;

    public DownloadedDrawable(BitmapDownloaderTask bitmapDownloaderTask) {
        super(Color.WHITE);
        bitmapDownloaderTaskReference = new WeakReference<BitmapDownloaderTask>(
                bitmapDownloaderTask);
    }

    public BitmapDownloaderTask getBitmapDownloaderTask() {
        return bitmapDownloaderTaskReference.get();
    }
}

// the actual download code
static Bitmap downloadBitmap(String url) {
    HttpParams params = new BasicHttpParams();
    params.setParameter(CoreProtocolPNames.PROTOCOL_VERSION,
            HttpVersion.HTTP_1_1);
    HttpClient client = new DefaultHttpClient(params);
    final HttpGet getRequest = new HttpGet(url);

    try {
        HttpResponse response = client.execute(getRequest);
        final int statusCode = response.getStatusLine().getStatusCode();
        if (statusCode != HttpStatus.SC_OK) {
            Log.w("ImageDownloader", "Error " + statusCode
                    + " while retrieving bitmap from " + url);
            return null;
        }

        final HttpEntity entity = response.getEntity();
        if (entity != null) {
            InputStream inputStream = null;
            try {
                inputStream = entity.getContent();
                final Bitmap bitmap = BitmapFactory
                        .decodeStream(inputStream);
                return bitmap;
            } finally {
                if (inputStream != null) {
                    inputStream.close();
                }
                entity.consumeContent();
            }
        }
    } catch (Exception e) {
        // Could provide a more explicit error message for IOException or
        // IllegalStateException
        getRequest.abort();
        Log.w("ImageDownloader", "Error while retrieving bitmap from "
                + url + e.toString());
    } finally {
        if (client != null) {
            // client.close();
        }
    }
    return null;
 }
}