本文实例讲述了Android编程学习之异步加载图片的方法。分享给大家供大家参考，具体如下：

最近在android开发中碰到比较棘手的问题，就是加载图片内存溢出。我开发的是一个新闻应用，应用中用到大量的图片，一个界面中可能会有上百张图片。开发android应用的朋友可能或多或少碰到加载图片内存溢出问题，一般情况下，加载一张大图就会导致内存溢出，同样，加载多张图片内存溢出的概率也很高。

列一下网络上查到的一般做法：

1.使用BitmapFactory.Options对图片进行压缩
2.优化加载图片的adapter中的getView方法，使之尽可能少占用内存
3.使用异步加载图片的方式，使图片在页面加载后慢慢载入进来。

1、2步骤是必须做足的工作，但是对于大量图片的列表仍然无法解决内存溢出的问题，采用异步加载图片的方式才能有效解决图片加载内存溢出问题。

测试的效果图如下：

在这里我把主要的代码贴出来，给大家分享一下。

1、首先是MainActivity和activity_main.xml布局文件的代码。

（1）、MainActivity的代码如下：

package net.loonggg.test; import java.util.List; import net.loonggg.adapter.MyAdapter; import net.loonggg.bean.Menu; import net.loonggg.util.HttpUtil; import net.loonggg.util.Utils; import android.app.Activity; import android.app.ProgressDialog; import android.os.AsyncTask; import android.os.Bundle; import android.view.Window; import android.widget.ListView; public class MainActivity extends Activity {  private ListView lv;  private MyAdapter adapter;  private ProgressDialog pd;  @Override  protected void onCreate(Bundle savedInstanceState) {   requestWindowFeature(Window.FEATURE_NO_TITLE);   super.onCreate(savedInstanceState);   setContentView(R.layout.activity_main);   lv = (ListView) findViewById(R.id.lv);   pd = new ProgressDialog(this);   pd.setTitle("加载菜单");   pd.setMessage("正在加载");   adapter = new MyAdapter(this);   new MyTask().execute("1");  }  public class MyTask extends AsyncTask<String, Void, List<Menu>> {   @Override   protected void onPreExecute() {    super.onPreExecute();    pd.show();   }   @Override   protected void onPostExecute(List<Menu> result) {    super.onPostExecute(result);    adapter.setData(result);    lv.setAdapter(adapter);    pd.dismiss();   }   @Override   protected List<Menu> doInBackground(String... params) {    String menuListStr = getListDishesInfo(params[0]);    return Utils.getInstance().parseMenusJSON(menuListStr);   }  }  private String getListDishesInfo(String sortId) {   // url   String url = HttpUtil.BASE_URL + "servlet/MenuInfoServlet?sortId="     + sortId + "&flag=1";   // 查询返回结果   return HttpUtil.queryStringForPost(url);  } } 

(2)、activity_main.xml的布局文件如下：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"  xmlns:tools="http://schemas.android.com/tools"  android:layout_width="match_parent"  android:layout_height="match_parent"  android:background="#ffffff"  android:orientation="vertical" >  <ListView   android:id="@+id/lv"   android:layout_width="fill_parent"   android:layout_height="wrap_content" >  </ListView> </LinearLayout> 

2、这是自定义的ListView的adapter的代码：

package net.loonggg.adapter; import java.util.List; import net.loonggg.bean.Menu; import net.loonggg.test.R; import net.loonggg.util.ImageLoader; import android.app.Activity; import android.content.Context; import android.view.LayoutInflater; import android.view.View; import android.view.ViewGroup; import android.widget.BaseAdapter; import android.widget.ImageView; import android.widget.TextView; public class MyAdapter extends BaseAdapter {  private List<Menu> list;  private Context context;  private Activity activity;  private ImageLoader imageLoader;  private ViewHolder viewHolder;  public MyAdapter(Context context) {   this.context = context;   this.activity = (Activity) context;   imageLoader = new ImageLoader(context);  }  public void setData(List<Menu> list) {   this.list = list;  }  @Override  public int getCount() {   return list.size();  }  @Override  public Object getItem(int position) {   return list.get(position);  }  @Override  public long getItemId(int position) {   return position;  }  @Override  public View getView(int position, View convertView, ViewGroup parent) {   if (convertView == null) {    convertView = LayoutInflater.from(context).inflate(      R.layout.listview_item, null);    viewHolder = new ViewHolder();    viewHolder.tv = (TextView) convertView.findViewById(R.id.item_tv);    viewHolder.iv = (ImageView) convertView.findViewById(R.id.item_iv);    convertView.setTag(viewHolder);   } else {    viewHolder = (ViewHolder) convertView.getTag();   }   viewHolder.tv.setText(list.get(position).getDishes());   imageLoader.DisplayImage(list.get(position).getPicPath(), activity,     viewHolder.iv);   return convertView;  }  private class ViewHolder {   private ImageView iv;   private TextView tv;  } } 

3、这是最重要的一部分代码，这就是异步加载图片的一个类，这里我就不解释了，代码中附有注释。代码如下：

package net.loonggg.util; import java.io.File; import java.io.FileInputStream; import java.io.FileNotFoundException; import java.io.FileOutputStream; 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.Stack; import java.util.WeakHashMap; import net.loonggg.test.R; import android.app.Activity; import android.content.Context; import android.graphics.Bitmap; import android.graphics.BitmapFactory; import android.widget.ImageView; /**  * 异步加载图片类  *  * @author loonggg  *  */ public class ImageLoader {  // 手机中的缓存  private MemoryCache memoryCache = new MemoryCache();  // sd卡缓存  private FileCache fileCache;  private PicturesLoader pictureLoaderThread = new PicturesLoader();  private PicturesQueue picturesQueue = new PicturesQueue();  private Map<ImageView, String> imageViews = Collections    .synchronizedMap(new WeakHashMap<ImageView, String>());  public ImageLoader(Context context) {   // 设置线程的优先级   pictureLoaderThread.setPriority(Thread.NORM_PRIORITY - 1);   fileCache = new FileCache(context);  }  // 在找不到图片时，默认的图片  final int stub_id = R.drawable.stub;  public void DisplayImage(String url, Activity activity, ImageView imageView) {   imageViews.put(imageView, url);   Bitmap bitmap = memoryCache.get(url);   if (bitmap != null)    imageView.setImageBitmap(bitmap);   else {// 如果手机内存缓存中没有图片，则调用任务队列，并先设置默认图片    queuePhoto(url, activity, imageView);    imageView.setImageResource(stub_id);   }  }  private void queuePhoto(String url, Activity activity, ImageView imageView) {   // 这ImageView可能之前被用于其它图像。所以可能会有一些旧的任务队列。我们需要清理掉它们。   picturesQueue.Clean(imageView);   PictureToLoad p = new PictureToLoad(url, imageView);   synchronized (picturesQueue.picturesToLoad) {    picturesQueue.picturesToLoad.push(p);    picturesQueue.picturesToLoad.notifyAll();   }   // 如果这个线程还没有启动，则启动线程   if (pictureLoaderThread.getState() == Thread.State.NEW)    pictureLoaderThread.start();  }  /**   * 根据url获取相应的图片的Bitmap   *   * @param url   * @return   */  private Bitmap getBitmap(String url) {   File f = fileCache.getFile(url);   // 从SD卡缓存中获取   Bitmap b = decodeFile(f);   if (b != null)    return b;   // 否则从网络中获取   try {    Bitmap bitmap = null;    URL imageUrl = new URL(url);    HttpURLConnection conn = (HttpURLConnection) imageUrl      .openConnection();    conn.setConnectTimeout(30000);    conn.setReadTimeout(30000);    InputStream is = conn.getInputStream();    OutputStream os = new FileOutputStream(f);    // 将图片写到sd卡目录中去    ImageUtil.CopyStream(is, os);    os.close();    bitmap = decodeFile(f);    return bitmap;   } catch (Exception ex) {    ex.printStackTrace();    return null;   }  }  // 解码图像和缩放以减少内存的消耗  private Bitmap decodeFile(File f) {   try {    // 解码图像尺寸    BitmapFactory.Options o = new BitmapFactory.Options();    o.inJustDecodeBounds = true;    BitmapFactory.decodeStream(new FileInputStream(f), null, o);    // 找到正确的缩放值。这应该是2的幂。    final int REQUIRED_SIZE = 70;    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;    }    // 设置恰当的inSampleSize可以使BitmapFactory分配更少的空间    // 用正确恰当的inSampleSize进行decode    BitmapFactory.Options o2 = new BitmapFactory.Options();    o2.inSampleSize = scale;    return BitmapFactory.decodeStream(new FileInputStream(f), null, o2);   } catch (FileNotFoundException e) {   }   return null;  }  /**   * PictureToLoad类(包括图片的地址和ImageView对象)   *   * @author loonggg   *   */  private class PictureToLoad {   public String url;   public ImageView imageView;   public PictureToLoad(String u, ImageView i) {    url = u;    imageView = i;   }  }  public void stopThread() {   pictureLoaderThread.interrupt();  }  // 存储下载的照片列表  class PicturesQueue {   private Stack<PictureToLoad> picturesToLoad = new Stack<PictureToLoad>();   // 删除这个ImageView的所有实例   public void Clean(ImageView image) {    for (int j = 0; j < picturesToLoad.size();) {     if (picturesToLoad.get(j).imageView == image)      picturesToLoad.remove(j);     else      ++j;    }   }  }  // 图片加载线程  class PicturesLoader extends Thread {   public void run() {    try {     while (true) {      // 线程等待直到有图片加载在队列中      if (picturesQueue.picturesToLoad.size() == 0)       synchronized (picturesQueue.picturesToLoad) {        picturesQueue.picturesToLoad.wait();       }      if (picturesQueue.picturesToLoad.size() != 0) {       PictureToLoad photoToLoad;       synchronized (picturesQueue.picturesToLoad) {        photoToLoad = picturesQueue.picturesToLoad.pop();       }       Bitmap bmp = getBitmap(photoToLoad.url);       // 写到手机内存中       memoryCache.put(photoToLoad.url, bmp);       String tag = imageViews.get(photoToLoad.imageView);       if (tag != null && tag.equals(photoToLoad.url)) {        BitmapDisplayer bd = new BitmapDisplayer(bmp,          photoToLoad.imageView);        Activity activity = (Activity) photoToLoad.imageView          .getContext();        activity.runOnUiThread(bd);       }      }      if (Thread.interrupted())       break;     }    } catch (InterruptedException e) {     // 在这里允许线程退出    }   }  }  // 在UI线程中显示Bitmap图像  class BitmapDisplayer implements Runnable {   Bitmap bitmap;   ImageView imageView;   public BitmapDisplayer(Bitmap bitmap, ImageView imageView) {    this.bitmap = bitmap;    this.imageView = imageView;   }   public void run() {    if (bitmap != null)     imageView.setImageBitmap(bitmap);    else     imageView.setImageResource(stub_id);   }  }  public void clearCache() {   memoryCache.clear();   fileCache.clear();  } } 

4、紧接着是几个实体类，一个是缓存到SD卡中的实体类，还有一个是缓存到手机内存中的实体类。代码如下：

（1）、缓存到sd卡的实体类：

package net.loonggg.util; import java.io.File; import android.content.Context; public class FileCache {  private File cacheDir;  public FileCache(Context context) {   // 找到保存缓存的图片目录   if (android.os.Environment.getExternalStorageState().equals(     android.os.Environment.MEDIA_MOUNTED))    cacheDir = new File(      android.os.Environment.getExternalStorageDirectory(),      "newnews");   else    cacheDir = context.getCacheDir();   if (!cacheDir.exists())    cacheDir.mkdirs();  }  public File getFile(String url) {   String filename = String.valueOf(url.hashCode());   File f = new File(cacheDir, filename);   return f;  }  public void clear() {   File[] files = cacheDir.listFiles();   for (File f : files)    f.delete();  } } 

（2）、缓存到手机内存的实体类：

package net.loonggg.util; import java.lang.ref.SoftReference; import java.util.HashMap; import android.graphics.Bitmap; public class MemoryCache {  private HashMap<String, SoftReference<Bitmap>> cache=new HashMap<String, SoftReference<Bitmap>>();  public Bitmap get(String id){   if(!cache.containsKey(id))    return null;   SoftReference<Bitmap> ref=cache.get(id);   return ref.get();  }  public void put(String id, Bitmap bitmap){   cache.put(id, new SoftReference<Bitmap>(bitmap));  }  public void clear() {   cache.clear();  } } 

5、这个是输入输出流转换的类，及方法：

package net.loonggg.util; import java.io.InputStream; import java.io.OutputStream; public class ImageUtil {  public static void CopyStream(InputStream is, OutputStream os) {   final int buffer_size = 1024;   try {    byte[] bytes = new byte[buffer_size];    for (;;) {     int count = is.read(bytes, 0, buffer_size);     if (count == -1)      break;     os.write(bytes, 0, count);    }   } catch (Exception ex) {   }  } } 

到这里基本就完成了。

希望本文所述对大家Android程序设计有所帮助。