Android编程学习之异步加载图片的方法

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。