在 Servlet API 中有一个 ServletContextListener 接口，它能够监听 ServletContext 对象的生命周期，实际上就是监听 Web 应用的生命周期。当Servlet 容器启动或终止Web 应用时，会触发ServletContextEvent 事件，该事件由ServletContextListener 来处理。在 ServletContextListener 接口中定义了处理ServletContextEvent 事件的两个方法。实际上ServeltContextListener是生成ServeltContext对象之后调用的.
如此就可以相当于一个服务，形成每隔一段时间去百度车联网上抓取数据，然后存起来，我们自己的客户端在访问我们提供的接口，来一定程度上突破访问天气次数限制。
首先需要先定义一个TimerTask：

public class WeatherTask extends TimerTask{
        //判断是否在抓取信息
    public static boolean isRunning = false;
     String baseUrl="http://api.map.baidu.com/telematics/v3/weather?";
     //抓取的城市信息
     String[] strArray={"郑州","北京","石家庄"};
    public static HashMap <String,String>hm=new HashMap<String,String>();
    public WeatherTask() {
        super();
     }
    private ServletContext context = null;
    public WeatherTask(ServletContext context) {
        this.context = context;
        
     }
    @Override
    public void run() {
        // TODO Auto-generated method stub
        if (!isRunning) {
             isRunning=true;
            for(int i=0;i<strArray.length;i++){
                String rs=result(strArray[i]);
                if(rs!=null){
                    hm.put(strArray[i], rs);
                }
                          if(i==strArray.length-1){
                    isRunning=false;
                }
            }
        }else{
            isRunning=false;
        }
    }
    public String result(String city){
         HashMap<String,String>hmCity=new HashMap<String,String>();
         hmCity.put("location", city);
         hmCity.put("output", "json");
         hmCity.put("ak", "IuRqX0hMiTPnMwGKweGYBb3OnC20YICE");
         HttpRequest req = HttpRequest.get(baseUrl, hmCity, true).readTimeout(10000).connectTimeout(1000000)
                    .uncompress(true).trustAllCerts().trustAllHosts().ignoreCloseExceptions(true);
         System.out.println(req); 
         if (req.code() == 200) {
             return req.body();
            } else{
                return null;
            }
        
    }

}

其次，实现ServletContextListener

public class ContextListener extends HttpServlet implements
        ServletContextListener {
    private static final long serialVersionUID = 1L;
    private java.util.Timer timer = null;
    private WeatherTask myjob;
    private JSONObject js;

    public ContextListener() {
    }

    @Override
    public void contextDestroyed(ServletContextEvent event) {
        // TODO Auto-generated method stub
        timer.cancel();
        System.out.println("定时器销毁");
        event.getServletContext().log("定时器销毁");
    }

    @Override
    public void contextInitialized(ServletContextEvent event) {
        // TODO Auto-generated method stub
        timer = new java.util.Timer(true);
        event.getServletContext().log("定时器已启动");
        myjob = new WeatherTask(event.getServletContext());
        timer.schedule(myjob, 0, 3000000);
        event.getServletContext().log("已经添加任务调度表");
    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        doPost(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp)
            throws ServletException, IOException {
        req.setCharacterEncoding("UTF-8");
        resp.setCharacterEncoding("UTF-8");
        String city = req.getParameter("city");
        city = new String(city.getBytes("iso-8859-1"), "utf-8");
        String outStr;
        if(!myjob.isRunning){
            outStr= myjob.hm.get(city);
        }else{
            outStr="正在抓取";
        
        }
        JSONObject js = new JSONObject();
        js.put("outstr", outStr);
        PrintWriter out = resp.getWriter();
        out.print(outStr);
    }

注意：该项目get请求用了com.github.kevinsawicki.http.HttpRequest以及com.alibaba.fastjson.JSONObject
需要另行导入：
测试接口：

public class Test {
     public static void main(String[] args) {
         String baseUrl="http://localhost:8080/Weather/weather";
         HashMap<String,String>hm=new HashMap();
         hm.put("city", "郑州");
         HttpRequest req = HttpRequest.get(baseUrl, hm, true).readTimeout(1000000).connectTimeout(1000000)
                    .uncompress(true).trustAllCerts().trustAllHosts().ignoreCloseExceptions(true);
         System.out.println(req); 
         if (req.code() == 200) {
                System.out.println("re:"+req.body()); 
            } else {
                System.out.println("ccc"); 
            }
         
    }
}

截图.PNG