Android学习笔记（二）之异步加载图片

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。不懂可以给我留言。