Android 非同步擷取網狀圖片並處理導致記憶體溢出問題解決方案

測試環境為Adnroid 2.1以上。
1.AndroidManifest.xml 許可權配置：
添加互連網存取權限： 複製代碼 代碼如下:<uses-permission android:name="android.permission.INTERNET" />

2.非同步圖片類 ImageDownloadTask 複製代碼 代碼如下:import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.net.MalformedURLException;
import java.net.URL;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.AsyncTask;
import android.widget.ImageView;
public class ImageDownloadTask extends AsyncTask<Object, Object, Bitmap> {
private ImageView imageView = null;
/***
* 這裡擷取到手機的解析度大小
* */
public void setDisplayWidth(int width) {
_displaywidth = width;
}
public int getDisplayWidth() {
return _displaywidth;
}
public void setDisplayHeight(int height) {
_displayheight = height;
}
public int getDisplayHeight() {
return _displayheight;
}
public int getDisplayPixels() {
return _displaypixels;
}
private int _displaywidth = 480;
private int _displayheight = 800;
private int _displaypixels = _displaywidth * _displayheight;
@Override
protected Bitmap doInBackground(Object... params) {
// TODO Auto-generated method stub
Bitmap bmp = null;
imageView = (ImageView) params[1];
try {
String url = (String) params[0];
bmp = getBitmap(url, _displaypixels,true);
} catch (Exception e) {
return null;
}
return bmp;
}
protected void onPostExecute(Bitmap result) {
if (imageView != null&&result!=null) {
imageView.setImageBitmap(result);
if (null != result && result.isRecycled() == false)
System.gc();
}
}
/**
* 通過URL獲得網片。如:http://www.xxxxxx.com/xx.jpg
* */
public Bitmap getBitmap(String url, int displaypixels, Boolean isBig) throws MalformedURLException, IOException {
Bitmap bmp = null;
BitmapFactory.Options opts = new BitmapFactory.Options();
InputStream stream = new URL(url).openStream();
byte[] bytes = getBytes(stream);
//這3句是處理圖片溢出的begin( 如果不需要處理溢出直接 opts.inSampleSize=1;)
opts.inJustDecodeBounds = true;
BitmapFactory.decodeByteArray(bytes, 0, bytes.length, opts);
opts.inSampleSize = computeSampleSize(opts, -1, displaypixels);
//end
opts.inJustDecodeBounds = false;
bmp = BitmapFactory.decodeByteArray(bytes, 0, bytes.length, opts);
return bmp;
}
/**
* 資料流轉成btyle[]數組
* */
private byte[] getBytes(InputStream is) {
ByteArrayOutputStream baos = new ByteArrayOutputStream();
byte[] b = new byte[2048];
int len = 0;
try {
while ((len = is.read(b, 0, 2048)) != -1) {
baos.write(b, 0, len);
baos.flush();
}
} catch (IOException e) {
e.printStackTrace();
}
byte[] bytes = baos.toByteArray();
return bytes;
}
/****
* 處理圖片bitmap size exceeds VM budget （Out Of Memory 記憶體溢出）
*/
private int computeSampleSize(BitmapFactory.Options options,
int minSideLength, int maxNumOfPixels) {
int initialSize = computeInitialSampleSize(options, minSideLength,
maxNumOfPixels);
int roundedSize;
if (initialSize <= 8) {
roundedSize = 1;
while (roundedSize < initialSize) {
roundedSize <<= 1;
}
} else {
roundedSize = (initialSize + 7) / 8 * 8;
}
return roundedSize;
}
private int computeInitialSampleSize(BitmapFactory.Options options,
int minSideLength, int maxNumOfPixels) {
double w = options.outWidth;
double h = options.outHeight;
int lowerBound = (maxNumOfPixels == -1) ? 1 : (int) Math.ceil(Math
.sqrt(w * h / maxNumOfPixels));
int upperBound = (minSideLength == -1) ? 128 : (int) Math.min(
Math.floor(w / minSideLength), Math.floor(h / minSideLength));
if (upperBound < lowerBound) {
return lowerBound;
}
if ((maxNumOfPixels == -1) && (minSideLength == -1)) {
return 1;
} else if (minSideLength == -1) {
return lowerBound;
} else {
return upperBound;
}
}
}

3.測試調用代碼： 複製代碼 代碼如下:@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.main);
ImageDownloadTask imgtask =new ImageDownloadTask();
/**這裡是擷取手機螢幕的解析度用來處理 圖片 溢出問題的。begin*/
DisplayMetrics dm = new DisplayMetrics();
getWindowManager().getDefaultDisplay().getMetrics(dm);
imgtask.setDisplayWidth(dm.widthPixels);
imgtask.setDisplayHeight(dm.heightPixels);
//end
ImageView imageView_test= (ImageView)findViewById(R.id.imageView_test);
imgtask.execute("http://pic.qukantu.com/big/7515/201201031116491.jpg",imageView_test);
}

4.小結：
主要是通過 extends AsyncTask<Object, Object, Bitmap> 來實現非同步。
圖片Out Of Memory 記憶體溢出 這一塊操作，在實際應用中應該考慮淡定抽取出來。這裡為了方便放進來了。 溢出處理實際上就是獲得裝置解析度把圖片進行壓縮。