XML : How to set a layout gravity center

I am just designing a layout for login. However, I want to set a layout gravity center (full layout content set into the center of the screen). I am using gravity center but it doesn't give me the proper output, So may be I am having problem with another issue. Bellow is my layout xml code. Hence, Can someone suggest me where I am having problem !!!

  <?xml version="1.0" encoding="utf-8"?>  <LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"      android:orientation="vertical"      android:layout_width="match_parent"      android:layout_height="match_parent">        <LinearLayout          android:layout_width="match_parent"          android:layout_height="wrap_content"          android:layout_marginRight="30dp"          android:paddingLeft="20dp"          android:layout_marginTop="10dp"          android:background="#458221"          >          <TextView              android:layout_width="match_parent"              android:layout_height="wrap_content"              android:text="Hello"              />      </LinearLayout>      <LinearLayout          android:layout_width="match_parent"          android:layout_height="wrap_content"          android:orientation="vertical"          android:layout_gravity="center"      <!-- I want to set this layout into the center of the screen ,This layout cann't set the gravity center -->              >          <TextView              android:layout_width="match_parent"              android:layout_height="wrap_content"              android:text="User Name: "              />          <EditText              android:layout_width="match_parent"              android:layout_height="wrap_content"              android:hint="user"              />          <TextView              android:layout_width="match_parent"              android:layout_height="wrap_content"              android:text="password"              />          <EditText              android:layout_width="match_parent"              android:layout_height="wrap_content"              android:hint="pass"              />      </LinearLayout>    </LinearLayout>