我正在尝试从mysql数据库填充recylerview。我能够使用volley库（使用toast测试）正确地从db获取数据，但它没有显示在recyclerview中。我的密码到现在为止。
碎片通知.xml

<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"
xmlns:app="http://schemas.android.com/apk/res-auto"
tools:context="in.techbash.androidhiverecyclerview.NotificationFragment">

<!-- TODO: Update blank fragment layout -->
<RelativeLayout
    android:id="@+id/relativeLayout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    app:layout_behavior="@string/appbar_scrolling_view_behavior">

    <android.support.v7.widget.RecyclerView
        android:id="@+id/recycler_view"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:scrollbars="vertical" />

</RelativeLayout>

</FrameLayout>

我的notificationfragment类
notificationfragment.java文件

public class NotificationFragment extends Fragment {

private List<Notification> notificationList = new ArrayList<>();
private RecyclerView recyclerView;
NotificationsAdapter notificationsAdapter;
private View rootView;
private RecyclerView.LayoutManager mLayoutManager;

public NotificationFragment() {
    // Required empty public constructor
}

@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
                         Bundle savedInstanceState) {
    // Inflate the layout for this fragment
    rootView = inflater.inflate(R.layout.fragment_notification, container, false);

    recyclerView = rootView.findViewById(R.id.recycler_view);

    getNotifications();

    return rootView;
}

private void getNotifications() {

    final String get_notifications = "http://192.168.1.102/notification/get_notifications.php";

    final String user_id = "327875";

    Map<String, String> parameters = new HashMap<String, String>();
    parameters.put("user_id", user_id);

    JSONObject jsonObj = new JSONObject(parameters);

    JsonObjectRequest jsonObjectRequest = new JsonObjectRequest(Request.Method.POST,
            get_notifications, jsonObj, new Response.Listener<JSONObject>() {
        @Override
        public void onResponse(JSONObject response) {
            try {

                JSONArray jsonArray = response.getJSONArray("result");

                for (int i = 0; i < jsonArray.length(); i++) {
                    JSONObject jsonObject = jsonArray.getJSONObject(i);

                    //adding the product to product list
                    notificationList.add(new Notification(
                            jsonObject.getString("msg_title"),
                            jsonObject.getString("message"),
                            jsonObject.getString("sender_name")
                    ));
                }
                //creating adapter object and setting it to recyclerview
                NotificationsAdapter adapter = new NotificationsAdapter(getActivity(), notificationList);
                recyclerView.setAdapter(adapter);

            } catch (JSONException e) {
                e.printStackTrace();
            }
        }
    }, new Response.ErrorListener() {
        @Override
        public void onErrorResponse(VolleyError error) {

        }
    }) {
        @Override
        public String getBodyContentType() {
            return "application/json; charset=utf-8";
        }
    };

    MySingleton.getInstance(getActivity()).addToRequestQueue(jsonObjectRequest);

}

}

我的通知\列表\行.xml文件

<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:background="?android:attr/selectableItemBackground"
android:clickable="true"
android:focusable="true"
android:orientation="vertical"
android:paddingBottom="@dimen/row_padding_vertical"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/row_padding_vertical">

<TextView
    android:id="@+id/title"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_alignParentTop="true"
    android:textColor="@color/title"
    android:textSize="16dp"
    android:textStyle="bold" />

<TextView
    android:id="@+id/message"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:layout_below="@id/title" />

<TextView
    android:id="@+id/sender"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_alignParentRight="true"
    android:textColor="@color/year" />

</RelativeLayout>

通知适配器.java

public class NotificationsAdapter extends 
RecyclerView.Adapter<NotificationsAdapter.MyViewHolder> {

private List<Notification> notificationsList;
private Context mCtx;

public NotificationsAdapter(Context mCtx, List<Notification> notificationsList){
    this.mCtx = mCtx;
    this.notificationsList = notificationsList;
}

@Override
public MyViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {
    //inflating and returning our view holder
    View view = LayoutInflater.from(parent.getContext()).inflate(R.layout.notification_list_row, parent, false);
    return new MyViewHolder(view);
}

@Override
public void onBindViewHolder(MyViewHolder holder, int position) {

    Notification notification = notificationsList.get(position);
    holder.title.setText(notification.getTitle());
    holder.message.setText(notification.getMessage());
    holder.sender.setText(notification.getSender());

}

@Override
public int getItemCount() {
    return notificationsList.size();
}

public class MyViewHolder extends RecyclerView.ViewHolder {

    public TextView title, message, sender;

    public MyViewHolder(View itemView) {
        super(itemView);

        title = (TextView) itemView.findViewById(R.id.title);
        message = (TextView) itemView.findViewById(R.id.message);
        sender = (TextView) itemView.findViewById(R.id.sender);
    }
}
}

我的通知获取者/设置者

public class Notification {

String sender, title, message;

public Notification(String title, String message, String sender){
    this.title = title;
    this.message = message;
    this.sender = sender;
}

public String getSender() {
    return sender;
}

public void setSender(String sender) {
    this.sender = sender;
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}

public String getMessage() {
    return message;
}

public void setMessage(String message) {
    this.message = message;
}
}

我是新来的安卓。请帮我解决这个问题。