onCreate的savedInstanceState为何一直是null？

onCreate
onSaveInstanceState
onRestoreInstanceState
各是什么时机运行？package com.androidbook;import android.app.Activity;
import android.os.Bundle;
import android.util.Log;public class HelloActivity extends Activity {
private static final String TAG = "MyNewLog";

    /** Called when the activity is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        /*
         * If an instance of this activity had previously stopped, we can
         * get the original text it started with.
         * */
        if (null != savedInstanceState)
        {
            int IntTest = savedInstanceState.getInt("IntTest");
            String StrTest = savedInstanceState.getString("StrTest");
            Log.e(TAG, "onCreate get the savedInstanceState+IntTest="+IntTest+"+StrTest="+StrTest);        
        }
        setContentView(R.layout.main);
        Log.e(TAG, "onCreate");
    }
    
    @Override
    public void onSaveInstanceState(Bundle savedInstanceState) {
     /*
         * Save away the original text, so we still have it if the activity
         * needs to be killed while paused.
         * */
savedInstanceState.putInt("IntTest", 0);
savedInstanceState.putString("StrTest", "savedInstanceState test");
super.onSaveInstanceState(savedInstanceState);
Log.e(TAG, "onSaveInstanceState");
    }
    
    @Override
    public void onRestoreInstanceState(Bundle savedInstanceState) {
super.onRestoreInstanceState(savedInstanceState);
int IntTest = savedInstanceState.getInt("IntTest");
String StrTest = savedInstanceState.getString("StrTest");
Log.e(TAG, "onRestoreInstanceState+IntTest="+IntTest+"+StrTest="+StrTest);
    }
}