url.openStream()报错？?报错

如下代码种的url.openStream()一执行就报错，访问网络的权限也已经给了，鼓捣半天办实在是不知道为什么，求大伙儿帮忙看下

protected void onCreate(Bundle savedInstanceState) {
	super.onCreate(savedInstanceState);
	setContentView(R.layout.activity_main);
	button = (Button) this.findViewById(R.id.btnGet);
	imageView = (ImageView)this.findViewById(R.id.imgView);
	button.setOnClickListener(new View.OnClickListener() {

	    @Override
	    public void onClick(View v) {
		String urlString = "http://x.limgs.cn/f2/c1/up201301/342a0432638526a1c79471d86f3d7192.jpg";
		try {
		    URL url = new URL(urlString);
		    InputStream inputStream = url.openStream();
		    Bitmap bitmap = BitmapFactory.decodeStream(inputStream);
		    imageView.setImageBitmap(bitmap);
		    inputStream.close();

		} catch (MalformedURLException e) {
		    e.printStackTrace();
		} catch (IOException e) {
		    e.printStackTrace();
		}
	    }
	});
    }

异常是这个：

android.os.NetworkOnMainThreadException