findFragmentByTag()取得的数据都是null

private static final String FRAGMENT_LEFT_MENU = "fragment_left_menu";
 private static final String FRAGMENT_CONTENT = "fragment_content";
 /**
 * 出社会fragment，将fragment数据填充给布局文件
 */
 private void initFragment() {
 FragmentManager fm=getSupportFragmentManager();
 FragmentTransaction transaction=fm.beginTransaction();
 transaction.replace(R.id.fl_left_menu,new LeftMenuFragment(),FRAGMENT_LEFT_MENU);
 transaction.replace(R.id.fl_main_menu,new ContentFragment(),FRAGMENT_CONTENT);
 transaction.commit();
 LeftMenuFragment lefFragment=(LeftMenuFragment) fm.findFragmentByTag(FRAGMENT_LEFT_MENU);
 }

lefFragment输出结果为null 怎么办