android imageview加载网络图片没显示

package study_imageput.com.study_apktointent;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.widget.ImageView;

import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.URL;

public class MainActivity extends Activity {
//定义一个图片显示控件
private ImageView imageView;
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
imageView = (ImageView) this.findViewById(R.id.ivPic);
String url = "http://images.cnitblog.com/blog/430074/201302/01220037-4e6a57c1199748fea9f8391e7e0548d7.jpg";
setPicBitmap(imageView, url);

}

public static void setPicBitmap(final ImageView ivPic,final String pic_url){

new Thread(new Runnable() {
@Override
public void run() {
try{
HttpURLConnection conn = (HttpURLConnection) new URL(pic_url).openConnection();
conn.connect();
InputStream is = conn.getInputStream();
Bitmap bitmap =BitmapFactory.decodeStream(is);
ivPic.setImageBitmap(bitmap);
is.close();
} catch (Exception e){
e.printStackTrace();
}
}
});
}
}
用这样的方式加载出的图片是空白