why occurred this error “Can't create handler inside thread that has not called Looper.prepare()”

package com.example.internetimageview;

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

import javax.net.ssl.HttpsURLConnection;

import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.BitmapFactory;
import android.os.Bundle;
import android.os.Handler;
import android.os.Looper;
import android.os.Message;
import android.text.TextUtils;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ImageView;
import android.widget.Toast;

public class MainActivity extends Activity implements OnClickListener {

	protected static final int CHANG_UI = 1;

	protected static final int SHOW_MSG = 2;

	private Handler handler  = new Handler(){

		@Override
		public void handleMessage(Message msg) {
			super.handleMessage(msg);
			
			if (msg.what == CHANG_UI){
				Bitmap bm = (Bitmap) msg.obj;
				iv_image.setImageBitmap(bm);
			}else if(msg.what == SHOW_MSG){
				Toast.makeText(MainActivity.this, msg.obj.toString(), 0).show();
			}
		}
		
	};
	
	private Button bt_show = null;
	private EditText et_url = null;
	private ImageView iv_image = null;
	
	@Override
	protected void onCreate(Bundle savedInstanceState)  {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		
		bt_show = (Button) findViewById(R.id.bt_show);
		et_url = (EditText) findViewById(R.id.et_url);
		iv_image = (ImageView) findViewById(R.id.iv_image);
		
		bt_show.setOnClickListener(this);
		et_url.setText("http://www.baidu.com/img/baidu_jgylogo3.gif");
	}

	@Override
	public void onClick(View v) {
		
		new Thread(){

			@Override
			public void run() {
				//Looper.prepare();
				String path = et_url.getText().toString();
				if (TextUtils.isEmpty(path)){
					showMsg("图片路径不能为空");
					return;
				}
				
				try {
					URL url = new URL(path);
					HttpURLConnection conn= (HttpURLConnection) url.openConnection();
					
					conn.setRequestMethod("GET"); //必须大写
					conn.setConnectTimeout(5000);
					conn.setReadTimeout(5000);
					
					int responseCode=  conn.getResponseCode();
					
					//显示响应状态码
					showMsg(responseCode + "");
					
					if (responseCode == 200){
						InputStream is = conn.getInputStream();
						Bitmap bitmap = BitmapFactory.decodeStream(is);
						
						Message message = new Message();
						message.what = CHANG_UI;
						message.obj = bitmap;
						handler.handleMessage(message);
						
//						第二种方法
//						Looper.prepare();
//						iv_image.setImageBitmap(bitmap);
//						Looper.loop();
						
						
					}else{
						showMsg("图片响应失败");
					}
					
				}catch(MalformedURLException e){
					showMsg("图片路径可能错误");
				}catch (IOException e) {
					e.printStackTrace();
					showMsg("图片IO失败");
				} catch (Exception e) {
					e.printStackTrace();
					showMsg("图片获取失败");
				}
			}

			/**显示消息
			 * @param msg 要显示的消息文本
			 */
			private void showMsg(CharSequence msg){
				Message message = new Message();
				message.what = SHOW_MSG;
				message.obj = msg;
				handler.handleMessage(message);
			}
			
		}.start();
		
	}
	
	
	
}