用的IDEA，已经导入了fastjson包 为什么还是显示500

import com.alibaba.fastjson.JSON;

import javax.servlet.ServletException;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

@WebServlet("/newslist")
public class NewsListServlet extends HttpServlet {

    public NewsListServlet() {

    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        List<News> list = new ArrayList<>();
        list.add(new News("TIOBE:2018年5月份全球编程语言排行榜", "2018-5-1", "TIOBE", "1.java 2.python 3. c"));
        list.add(new News("TIOBE:2018年6月份全球编程语言排行榜", "2018-6-1", "TIOBE", "1.java 2.python 3. c"));
        list.add(new News("TIOBE:2018年7月份全球编程语言排行榜", "2018-7-1", "TIOBE", "1.java 2.python 3. c"));
        list.add(new News("TIOBE:2018年8月份全球编程语言排行榜", "2018-8-1", "TIOBE", "1.java 2.python 3. c"));
        String json= JSON.toJSONString(list);
        System.out.println(json);
        response.setContentType("text/html;charset=utf-8");
        response.getWriter().println(json);
    }
}

Type Exception Report

Message Servlet execution threw an exception

Description The server encountered an unexpected condition that prevented it from fulfilling the request.

Exception

javax.servlet.ServletException: Servlet execution threw an exception
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

Root Cause

java.lang.NoClassDefFoundError: com/alibaba/fastjson/JSON
NewsListServlet.doGet(NewsListServlet.java:27)
javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

Root Cause

java.lang.ClassNotFoundException: com.alibaba.fastjson.JSON
org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1364)
org.apache.catalina.loader.WebappClassLoaderBase.loadClass(WebappClassLoaderBase.java:1185)
NewsListServlet.doGet(NewsListServlet.java:27)
javax.servlet.http.HttpServlet.service(HttpServlet.java:634)
javax.servlet.http.HttpServlet.service(HttpServlet.java:741)
org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

Note The full stack trace of the root cause is available in the server logs.