android 线程安全Thread+handler解决联网获取数据，ui出错问题

package djx.android;

import djx.downLoad.DownFiles;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.TextView;

public class downLoadPractice extends Activity {
private Button button_submit=null;
private TextView textView=null;
private String content=null;
private Handler handler=null;
    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        //创建属于主线程的handler
        handler=new Handler();
       
        button_submit=(Button)findViewById(R.id.button_submit);
        textView=(TextView)findViewById(R.id.textView);
        button_submit.setOnClickListener(new submitOnClieckListener());
    }
    //为按钮添加监听器
    class submitOnClieckListener implements OnClickListener{
@Override
public void onClick(View v) {
//本地机器部署为服务器，从本地下载a.txt文件内容在textView上显示
final DownFiles df=new DownFiles("http://192.168.75.1:8080/downLoadServer/a.txt");
textView.setText("正在加载......");
new Thread(){
public void run(){
content=df.downLoadFiles();
handler.post(runnableUi);
}
}.start();
}
   
    }

   // 构建Runnable对象，在runnable中更新界面
    Runnable   runnableUi=new  Runnable(){
@Override
public void run() {
//更新界面
textView.setText("the Content is:"+content);
}
   
    };
   
   
}