Android screensaver saves the instance code for the png image

import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.Rect;
import android.util.Log;
import android.view.View;
public class ScreenShot {
    //  Access to specify Activity Save to png file 
    private static Bitmap takeScreenShot(Activity activity) {
        // View You need a screenshot View
        View view = activity.getWindow().getDecorView();
        view.setDrawingCacheEnabled(true);
        view.buildDrawingCache();
        Bitmap b1 = view.getDrawingCache();
        //  Gets the height of the status bar 
        Rect frame = new Rect();
        activity.getWindow().getDecorView().getWindowVisibleDisplayFrame(frame);
        int statusBarHeight = frame.top;
        Log.i("TAG", "" + statusBarHeight);
        //  Gets screen length and height 
        int width = activity.getWindowManager().getDefaultDisplay().getWidth();
        int height = activity.getWindowManager().getDefaultDisplay()
                .getHeight();
        //  Get rid of the title bar 
        // Bitmap b = Bitmap.createBitmap(b1, 0, 25, 320, 455);
        Bitmap b = Bitmap.createBitmap(b1, 0, statusBarHeight, width, height
                - statusBarHeight);
        view.destroyDrawingCache();
        return b;
    }
    //  Save to sdcard
    private static void savePic(Bitmap b, String strFileName) {
        FileOutputStream fos = null;
        try {
            fos = new FileOutputStream(strFileName);
            if (null != fos) {
                b.compress(Bitmap.CompressFormat.PNG, 90, fos);
                fos.flush();
                fos.close();
            }
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    //  Program entrance 
    public static void shoot(Activity a) {
        ScreenShot.savePic(ScreenShot.takeScreenShot(a), "sdcard/xx.png");
    }
}

It is important to note that the shoot method can only be called after view has been loaded.
Or in the

@Override
    public void onWindowFocusChanged(boolean hasFocus) {
        // TODO Auto-generated method stub
        super.onWindowFocusChanged(hasFocus);
        ScreenShot.shoot(this);
    } In the call