请问 为什么不能弹出 popupwindow

可以弹出来啊Activity
package app.example.t;import android.os.Bundle;
import android.app.Activity;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.ImageView;
import android.widget.PopupWindow;public class MainActivity extends Activity { private PopupWindow mPopupWindow;
private ImageView imageview_popwindow; protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main); // imageview_popwindow 按钮 imageview_popwindow = (ImageView) findViewById(R.id.imageview_popwindow); imageview_popwindow.setOnClickListener(new OnClickListener() {
public void onClick(View v) {
getPopupWindowInstance();
mPopupWindow.showAsDropDown(imageview_popwindow); // 想在imageview_popwindow
// 下方弹出
// popupwindow
}
}); } private void getPopupWindowInstance() {
if (mPopupWindow != null) {
mPopupWindow.dismiss();
} else {
initPopupWindow();
}
} private void initPopupWindow() {
LayoutInflater layoutInflater = LayoutInflater.from(this);
View popupWindow = layoutInflater.inflate(R.layout.popup_window, null); // 布局文件
// popup_window.xml
// 里边有个textview
mPopupWindow = new PopupWindow(popupWindow, 100, 130);
}
}
activity_main.xml
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context=".MainActivity" >    <ImageView
        android:id="@+id/imageview_popwindow"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="#ff33cc" /></RelativeLayout>
popup_window.xml<?xml version="1.0" encoding="utf-8"?>
<TextView xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:text="一本正经" ></TextView>