findViewById in a custom View to find child view

I defined a view, it extends RelativeLayout, I want to extract its children whose id are specified in layout xml file.

In HeaderView.java:

  1  package com.example.test; 
  2   
  3  public class HeaderView extends RelativeLayout { 
  4                         
  5      public HeaderView(Context context, AttributeSet attrs) {  
  6          this(context, attrs, 0);        
  7   
  8          TypedArray arr = context.obtainStyledAttributes(attrs, R.styleable.HeaderView, 0, 0);  
  9   
 10          int headerId = 0; 
 11          int containerId = 0;            
 12                         
 13          //  find attributes
 14          if (arr != null) { 
 15              if (arr.hasValue(R.styleable.HeaderView_headerview_header)) { 
 16                  headerId = arr.getResourceId(R.styleable.HeaderView_headerview_header, 0); 
 17              }  
 18              if (arr.hasValue(R.styleable.HeaderView_headerview_container)) { 
 19                  containerId = arr.getResourceId(R.styleable.HeaderView_headerview_container, 0); 
 20              }  
 21              arr.recycle(); 
 22          }  
 23   
 24          Log.d("test", String.format("headerId: %s, containerId %s", headerId, containerId)); 
 25          // headerId: 2131296260, containerId 2131296259 
 26           
 27          // here: mHeaderContainer is null  
 28          mHeaderContainer = (RelativeLayout) findViewById(headerId); 
 29          mContentViewContainer = (RelativeLayout) findViewById(containerId); 
 30      }  
 31  }  

I defined its attribute in headview_attrs.xml:

<?xml version="1.0" encoding="utf-8"?>
<resources>

    <declare-styleable name="HeaderView">
        <attr name="headerview_header" format="reference" />
        <attr name="headerview_container" format="reference" />
    </declare-styleable>

</resources>

I use this view in a layout xml file, in activity_headerview.xml:

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:test="http://schemas.android.com/apk/res/com.example.test"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <com.example.test.HeaderView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        test:headerview_container="@+id/headerview_content"
        test:headerview_header="@+id/headerview_header" >

        <RelativeLayout
            android:id="@+id/headerview_header"
            android:layout_width="match_parent"
            android:layout_height="30dp" >
        </RelativeLayout>

        <RelativeLayout
            android:id="@+id/headerview_content"
            android:layout_width="match_parent"
            android:layout_height="match_parent" >
        </RelativeLayout>
    </com.example.test.HeaderView>

</RelativeLayout>

Here the problem comes: in HeaderView.java, line 28, 29, mHeaderContainer and mContentViewContainer are null.

It looks like that in activity_headerview.xml, the RelativeLayout whose id is headerview_header and the RelativeLayout whose id is headerview_content are not the children of HeaderView.

Did I do something wrong? Any help will be great!