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。
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