一個小白的起點 android 主線程不能下載圖片到imageview

package com.example.app000;
import java.io.InputStream;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;


import org.apache.http.HttpResponse;
import org.apache.http.NameValuePair;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.entity.UrlEncodedFormEntity;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;
import org.apache.http.protocol.HTTP;
import org.apache.http.util.EntityUtils;


import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;


public class MainActivity extends Activity {
private Button button1;
private ImageView iv;


@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
button1=(Button)findViewById(R.id.button1);
iv=(ImageView)findViewById(R.id.imageView1);
button1.setOnClickListener(new OnClickListener(){
@Override
public void onClick(View v) {
//get請求
try {
getImage();
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();
}

}


private void getImage() throws Exception {
String imageUrl="http://dimg03.c-ctrip.com/images/fd/tg/g2/M05/3A/8A/"
+ "CghzgVWtqdKAdn9LAAGye73fzZs667_R_1024_10000_Q90.jpg";
URL url=new URL(imageUrl);
HttpURLConnection conn=(HttpURLConnection) url.openConnection();
conn.connect();
InputStream is=conn.getInputStream();
Bitmap bm=BitmapFactory.decodeStream(is);
is.close();
iv.setScaleType(ImageView.ScaleType.FIT_CENTER);
   iv.setImageBitmap(bm);

}


});
}}