ScheduledThreadPoolExecutor does not work on Android. Why?

Question

I am quite newbie with android development and I am trying to figure out why this does not work. I run this idea on Eclipse and it works fine. But I cant make it work on any of my devices. The app shows the value at start but it dont refresh the value anymore.

public class MainActivity extends AppCompatActivity {

    private TextView txtCrono;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        this.txtCrono = (TextView) findViewById(R.id.txtTiempo); //activity_main TextView

        ScheduledThreadPoolExecutor exec = new ScheduledThreadPoolExecutor(1);


        exec.scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {
                txtCrono.setText(String.valueOf(System.currentTimeMillis()));
            }
        }, 0, 100, TimeUnit.MILLISECONDS);

    }
}