Android 获得imei 和 mac ，平板有时无法获取imei

package com.example.hellotest;import android.net.wifi.WifiInfo;import android.net.wifi.WifiManager;import android.os.Bundle;import android.app.Activity;import android.content.ContentValues;import android.database.Cursor;import android.database.sqlite.SQLiteDatabase;import android.util.Log;import android.view.Menu;import android.view.MenuItem;import android.widget.TextView;public class MainActivity extends Activity {//public static String IMEI;//private TelephonyManager mTelManager;public TextView tv;private SQLiteDatabase  mSqLiteDatabase = null;public static final String TAG = "HELLO";public static final String DB_NAME = "test.db";public static final String TABLE_NAME = "table_test";public static final String id = "_id";public static final String mac = "mac";public static final String num = "num";String sql = "create table " + TABLE_NAME+ "(" + id + " integer primary key, "+ mac + " varchar(50), "+ num + " interger);";String deleteSql = "DELETE FROM " + TABLE_NAME +";";public static String mString = null;public int mCount = 0;@Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        tv = (TextView)findViewById(R.id.tv);    WifiManager wifi = (WifiManager) getSystemService(WIFI_SERVICE);   WifiInfo info = wifi.getConnectionInfo();//mTelManager = (TelephonyManager) this.getSystemService(TELEPHONY_SERVICE);//IMEI = mTelManager.getDeviceId();Log.i("MAC", info.getMacAddress());tv.setText(info.getMacAddress());mString = info.getMacAddress();mSqLiteDatabase = this.openOrCreateDatabase(DB_NAME, MODE_PRIVATE, null);try {mSqLiteDatabase.execSQL(sql);}catch (Exception e) {Log.i(TAG, "Wrong");// TODO: handle exception}Cursor cur = mSqLiteDatabase.rawQuery("SELECT * FROM table_test", null);if (cur != null) {if (cur.moveToFirst()) {int numColumn = cur.getColumnIndex("num");int number = cur.getInt(numColumn);int stringColumn = cur.getColumnIndex("mac");String str = cur.getString(stringColumn);Log.i("num", Integer.toString(number));Log.i("string", str);}}mSqLiteDatabase.execSQL(deleteSql);    }public void AddData() {ContentValues cv = new ContentValues();cv.put(mac, mString);cv.put(num, 7);mSqLiteDatabase.insert(TABLE_NAME, null, cv);Log.i("Success", "AddSuccess...");}public void UpData() {ContentValues cv = new ContentValues();cv.put(mac, mString);cv.put(num, 5);mSqLiteDatabase.update(TABLE_NAME, cv, mac + "=" + mac, null);Log.i("Success", "UpdataSuccess...");}    @Override    public boolean onCreateOptionsMenu(Menu menu) {        getMenuInflater().inflate(R.menu.activity_main, menu);        return true;    }@Overridepublic boolean onOptionsItemSelected(MenuItem item) {// TODO Auto-generated method stubint item_id = item.getItemId();switch (item_id) {case R.id.menu_settings:AddData();MainActivity.this.finish();break;default:break;}return super.onOptionsItemSelected(item);}    }