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。