关于ViewUtils一些问题

Question

package com.example.testxutils;

import com.lidroid.xutils.ViewUtils;
import com.lidroid.xutils.view.annotation.ViewInject;
import com.lidroid.xutils.view.annotation.event.OnClick;

import android.app.Activity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.TextView;

public class MainActivity extends Activity {

@ViewInject(R.id.txt_test)
private TextView txt_test;

@OnClick(R.id.btn_httptest)
public void btnHttpTestClick(View arg0) {
    txt_test.setText("Hello World!");
}

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

    ViewUtils.inject(this);//为什么我去掉这句话之后编译可以通过，不去掉通不过
    会显示java.lang.NoClassDefFoundError: com.lidroid.xutils.ViewUtils，我已经导入相关的jar包了

}

}