Requested resource isn't available (Servlet)

I created a dynamic web project in Eclipse 4.5, called HelloServlet and modified web.xml. I also added Apache 8 in Eclipse. I think I did everything right, but I still can't run the servlet.

In Project explorer: /src/HelloServlet/HelloWorldServlet.java.

HelloWorldServlet.java:

package HelloServlet;

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

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

    /**
     * @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);
    }

}

Web.xml:

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns:xsi="http://ift.tt/ra1lAU" xmlns="http://ift.tt/19L2NlC" xsi:schemaLocation="http://ift.tt/19L2NlC http://ift.tt/1drxgYl" id="WebApp_ID" version="3.1">
  <display-name>HelloServlet</display-name>
  <welcome-file-list>
    <welcome-file>index.html</welcome-file>
    <welcome-file>index.htm</welcome-file>
    <welcome-file>index.jsp</welcome-file>
    <welcome-file>default.html</welcome-file>
    <welcome-file>default.htm</welcome-file>
    <welcome-file>default.jsp</welcome-file>
  </welcome-file-list>

  <servlet>
    <servlet-name>HelloWorldServlet</servlet-name>
    <servlet-class>HelloServlet.HelloWorldServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>HelloWorldServlet</servlet-name>
    <url-pattern>/HelloWorldServlet</url-pattern>
  </servlet-mapping>
</web-app>

Sometimes, I get class not found without /HelloWorldServlet. I searched and knew how to set servlet-class correctly, but it still doesn't run.