一个小白的起点 android 主线程不能下载图片到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请求
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);

}

});
}}