android:没有成功的invalidate()

package com.invalidate;
import android.app.Activity;
import android.os.Bundle;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
import android.widget.TextView;
public class InvalidateDemoActivity extends Activity { 
    TextView tv;
    Handler h = new Handler(){
        @Override
        public void handleMessage(Message msg) {
            // TODO Auto-generated method stub
            super.handleMessage(msg);
            switch(msg.what){
            case 1:
                /*Log.i("gang", "gang coming");
                tv.setText("helllllllll0");*/
                tv.invalidate();
                break;
            }
        }
    };
    String str = "sun";
    int i = 0;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        tv = new TextView(this);
        tv.setText(str);
        setContentView(tv);
        new Thread(new Runnable(){

            @Override
            public void run() {
                // TODO Auto-generated method stub
                while(true){
                    try {
                        Thread.sleep(1000);
                        i++;
                        str = "sun"+i;
                        Message msg = new Message();
                        msg.what = 1;
                        h.sendMessage(msg);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                }
            }
        }).start();
    }
}