RecyclerView不显示-Android

我在Android中的RecyclerView遇到一些问题。当我运行项目时，没有显示，只有白屏。请帮忙..

MainActivity代码：

public class MainActivity extends AppCompatActivity {


RecyclerView mRecyclerView;
private RecyclerView.LayoutManager mLayoutManager;
private MyAdapter mViewAdapter;

List<News> news_list = new ArrayList<>();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);


    new ParseNewsJSON().execute();

    mRecyclerView = (RecyclerView) findViewById(R.id.recyclerView);
    mRecyclerView.setHasFixedSize(true);
    //mRecyclerView.setItemAnimator(new DefaultItemAnimator());
    mRecyclerView.setLayoutManager(new LinearLayoutManager(getApplicationContext()));
    mViewAdapter= new MyAdapter(news_list);
    mRecyclerView.setAdapter(mViewAdapter);

}

}

MyAdapter代码：

public class MyAdapter extends RecyclerView.Adapter<MyAdapter.ViewHolder> {

private List<News> newsArticleList;

public class ViewHolder extends RecyclerView.ViewHolder{
    public TextView title;

    //description, genre;

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

        title = (TextView) itemView.findViewById(R.id.textView_articleHeading);
//            description = (TextView) itemView.findViewById(R.id.textView_description);
//            image = (TextView) itemView.findViewById(R.id.rating);
    }
}

public MyAdapter (List<News> newsArticleList){

    this.newsArticleList = newsArticleList;
}
@Override
public MyAdapter.ViewHolder onCreateViewHolder(ViewGroup parent, int viewType) {

    View itemView = LayoutInflater.from(parent.getContext()).inflate(R.layout.content_main, parent, false);
    return new ViewHolder(itemView);
}

@Override
public void onBindViewHolder(MyAdapter.ViewHolder holder, int position) {

    News news = newsArticleList.get(position);


    holder.title.setText(news.getHeading());


}

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

JSON解析代码：

public class ParseNewsJSON extends AsyncTask<String, Void, String> {

private final String JSON_URL = "https://newsapi.org/v2/top-headlines?sources=cnn&apiKey=c80ddd850a524fe5975cad881d6f4aba";

String result ="";

ArrayList<String> article_heading = new ArrayList<>();

@Override
protected String doInBackground(String... strings) {

    try {
        URL url = new URL(JSON_URL);

        HttpURLConnection httpURLConnection = (HttpURLConnection) url.openConnection();

        InputStream inputStream = httpURLConnection.getInputStream();

        InputStreamReader reader = new InputStreamReader(inputStream);

        int data = reader.read();

        while(data !=-1){

            char current = (char) data;

            result += current;

            data = reader.read();
        }
    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }

    return result;
}

@Override
protected void onPostExecute(String s) {

    try {
        JSONObject jsonObject = new JSONObject(result);

        JSONArray jsonArray = jsonObject.getJSONArray("articles");

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

            News news = new News();

            news.setHeading(String.valueOf(article_heading.add(jsonArray.getJSONObject(i).optString("title"))));

           // article_heading.add(jsonArray.getJSONObject(i).optString("title"));


            Log.d("news_JSON", article_heading.toString());

            Log.d("news", news.getHeading().toString());
            //article_heading.add(jsonArray.getJSONObject(i).optString("title"));
        }
    } catch (JSONException e) {
        e.printStackTrace();
    }
    super.onPostExecute(s);
}
}

新闻Java

public class News {

private String heading;
//private String descrpiton;
private String img;


public String getHeading() {
    return heading;
}

public void setHeading(String heading) {
    this.heading = heading;
}

public String getImg() {
    return img;
}

public void setImg(String img) {
    this.img = img;
}
}

我读了一些线程，但是它们并没有解决我的问题。这是运行模拟器时的输出

activity_main.xml：

android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto">

<android.support.v7.widget.RecyclerView
    android:layout_height="match_parent"
    android:layout_width="match_parent"
    android:id="@+id/recyclerView"
    xmlns:android="http://schemas.android.com/apk/res/android">
</android.support.v7.widget.RecyclerView>

card_layout.xml：

xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/card_view"
android:layout_width="match_parent"
android:layout_marginTop="15dp"
android:layout_height="200dp"
app:cardCornerRadius="25dp"


>

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="fill_parent"
    android:orientation="vertical"



    >


    <TextView
        android:id="@+id/textView_articleHeading"
        android:layout_marginLeft="5dp"
        android:textStyle="bold"
        android:textColor="#000000"
        android:textSize="18sp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />

    <TextView
        android:layout_height="0dp"
        android:layout_width="match_parent"
        android:layout_weight="1.0"
        android:id="@+id/rating"
        android:scaleType="center"

       />

    <TextView
        android:id="@+id/textView_description"
        android:textSize="15sp"
        android:textColor="#000000"
        android:layout_marginBottom="5dp"
        android:layout_marginLeft="5dp"
        android:layout_width="match_parent"
        android:layout_height="wrap_content" />


</LinearLayout>