java - servlet3.1注解不生效

我新建了个servlet项目:

package hello;

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

/**
 * Servlet implementation class HelloServlet
 */
@WebServlet("/Hello")
public class HelloServlet extends HttpServlet {
    private static final long serialVersionUID = 1L;

    /**
     * Default constructor. 
     */
    public HelloServlet() {
        // TODO Auto-generated constructor stub
    }

    /**
     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        response.getWriter().append("Served at: ").append(request.getContextPath());
    }

    /**
     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)
     */
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        // TODO Auto-generated method stub
        doGet(request, response);
    }

}

然后访问http://localhost:8080/hello/H...显示404

但访问别的jsp页面正常
然后我加了个web.xml文件:

http://xmlns.jcp.org/xml/ns/javaee/web-app_3_1.xsd" 
  version="3.1">

就能访问了

但是我再新建个servlet文件,新建的却还是无法访问

@WebServlet("/World")
public class WorldServlet extends HttpServlet {

为什么会这样啊?难道不能用注解了?
我用的tomcat 8.5 , Java 1.8 , servlet 3.1 , eclipse 4.6.0 (Neon)
求解注解为什么不能用啊

项目结构: