1.修改菜单文件bottom_bar_menu.xml中item的showAsAction属性如下：

<item android:id="@id/menu_item"
android:title="text"
android:icon="@drawable/drawable_resource_name"
android:showAsAction="always|withText" />

2.创建BottomNavigationViewHelper.java类，代码如下：

import android.annotation.SuppressLint;
import android.support.design.internal.BottomNavigationItemView;
import android.support.design.internal.BottomNavigationMenuView;
import android.support.design.widget.BottomNavigationView;
import java.lang.reflect.Field;

    public class BottomNavigationViewHelper {
        @SuppressLint("RestrictedApi")
        public static void removeShiftMode(BottomNavigationView view) {
            BottomNavigationMenuView menuView = (BottomNavigationMenuView) view.getChildAt(0);
            try {
                Field shiftingMode = menuView.getClass().getDeclaredField("mShiftingMode");
                shiftingMode.setAccessible(true);
                shiftingMode.setBoolean(menuView, false);
                shiftingMode.setAccessible(false);
                for (int i = 0; i < menuView.getChildCount(); i++) {
                    BottomNavigationItemView item = (BottomNavigationItemView) menuView.getChildAt(i);
                    //noinspection RestrictedApi
                    item.setShiftingMode(false);
                    // set once again checked value, so view will be updated
                    //noinspection RestrictedApi
                    item.setChecked(item.getItemData().isChecked());
                }
            } catch (NoSuchFieldException e) {
                Log.e("BottomNav", "Unable to get shift mode field", e);
            } catch (IllegalAccessException e) {
                Log.e("BottomNav", "Unable to change value of shift mode", e);
            }
        }
    }

3.当底部当行的items超过3个时使用removeShiftMode()方法

BottomNavigationView bottomNavigationView = (BottomNavigationView)findViewById(R.id.bottom_navigation);
BottomNavigationViewHelper.removeShiftMode(bottomNavigationView);

参考资料：https://stackoverflow.com/questions/41718633/bottomnavigationview-with-more-than-3-items-tab-title-is-hiding