FragmentTransaction#replace和commit不生效

现在问题如图下面的四个导航，每一个应该能对应加载一个Fragment，但是全部加载不出来
切换文字没有问题，也用debugger检查过了，可以确定bottombar的listener是正常的，代码可以执行关键代码activity_main.xml<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:id="@+id/container"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">    <LinearLayout
        android:id="@+id/content"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1"
        android:orientation="horizontal">        //<Textview/> 省略        <FrameLayout
            android:layout_width="match_parent"
            android:layout_height="fill_parent"
            android:id="@+id/main_content">
        </FrameLayout>    </LinearLayout>    <com.roughike.bottombar.BottomBar
        android:id="@+id/bottomBar"
        android:layout_width="match_parent"
        android:layout_height="60dp"
        android:layout_alignParentBottom="true"
        app:bb_tabXmlResource="@xml/bottombar_tabs" />
</LinearLayout>
activity_main.java
loadFragmentHome() 也加载不出来import android.app.FragmentManager;
import android.app.FragmentTransaction;
import android.app.Fragment;public class MainActivity extends AppCompatActivity{    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);        mTextMessage = (TextView) findViewById(R.id.message);        BottomBar bottomBar = (BottomBar) findViewById(R.id.bottomBar);
        bottomBar.setOnTabSelectListener(new OnTabSelectListener() {
            @Override
            public void onTabSelected(@IdRes int tabId) {
                FragmentManager fm=getFragmentManager();
                FragmentTransaction ft=fm.beginTransaction();
                switch(tabId) {//四个都无法加载
                    case R.id.tab_home: {
                        mTextMessage.setText(R.string.title_home);
                        mHome = new HomeFragment();
                        ft.replace(R.id.main_content,mHome);
                        ft.setTransitionStyle(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
                        ft.addToBackStack(null);
                        ft.commitAllowingStateLoss();
                        break;
                    }
                    case R.id.tab_dashboard: {
                        mTextMessage.setText(R.string.title_dashboard);
                        mDashboard = new DashboardFragment();
                        ft.replace(R.id.main_content, mDashboard);
                        ft.setTransitionStyle(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
                        ft.addToBackStack(null);
                        ft.commitAllowingStateLoss();
                        break;
                    }
                    case R.id.tab_bbs: {
                        mTextMessage.setText(R.string.title_bbs);
                        mBBS = new BBSFragment();
                        ft.replace(R.id.main_content, mBBS);
                        ft.setTransitionStyle(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
                        ft.addToBackStack(null);
                        ft.commitAllowingStateLoss();
                        break;
                    }
                    case R.id.tab_profile: {
                        mTextMessage.setText(R.string.title_profile);
                        mProfile = new ProfileFragment();
                        ft.replace(R.id.main_content, mProfile);
                        ft.setTransitionStyle(FragmentTransaction.TRANSIT_FRAGMENT_FADE);
                        ft.addToBackStack(null);
                        ft.commitAllowingStateLoss();
                        break;
                    }
                    default:{
                        Toast.makeText(getApplicationContext(),R.string.error_nulltab,Toast.LENGTH_SHORT).show();
                    }
                }
            }
        });        loadFragmentHome();
    }   //这个也加载不出来
    public void loadFragmentHome(){
        FragmentManager fm=getFragmentManager();
        FragmentTransaction ft=fm.beginTransaction();
        mHome=new HomeFragment();
        ft.replace(R.id.main_content,mHome);
        ft.commitAllowingStateLoss();
    }
    //其它省略
}
fragment_home.xml(随便举个例子，四个都加载不出来)<?xml version="1.0" encoding="utf-8"?>
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">    <TextView
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:text="@string/title_Home" /></FrameLayout>
HomeFragment.javaimport android.app.Fragment;public class HomeFragment extends Fragment{
    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        // Inflate the layout for this fragment
        return inflater.inflate(R.layout.fragment_home, container, false);
    }
    //其它省略
}