关于sessionId的问题求前辈们解个惑

Question

package day3.cookie;

import java.io.IOException;
import java.io.PrintWriter;

import javax.servlet.ServletException;
import javax.servlet.http.Cookie;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

public class C extends HttpServlet {

@Override
protected void service(HttpServletRequest req, HttpServletResponse resp)
        throws ServletException, IOException {
    String path = req.getServletPath();
    String contextPath = req.getContextPath();
    if(path.contains("add")){
        add(req,resp);
    }
    else if(path.contains("select")){
        select(req,resp);
    }
    else if(path.contains("update")){
        update(req,resp);
    }
}

private void update(HttpServletRequest req, HttpServletResponse resp) {
    Cookie c = new Cookie("a1","hello");
    resp.addCookie(c);

}

private void select(HttpServletRequest req, HttpServletResponse resp) {
    Cookie[] arr = req.getCookies();
    System.out.println(arr.length);
    for(Cookie c : arr){
        System.out.println(c.getName()+","+c.getValue());
    }

}

private void add(HttpServletRequest req, HttpServletResponse resp) {
    Cookie c = new Cookie("a1","123");
    c.setMaxAge(7*24*60*60);
    //c.setPath("/servletStu");
    resp.addCookie(c);
    //System.out.println(c.getPath());
}

}

在先执行添加后执行查询，只打印了我添加的cookie，没有打印出sessionId，然后连续访问查询都不会有sessionId，想问下各位前辈是为什么。sessionId是第一次访问创建出来所以不会打印出来我懂，那我在浏览器上连续刷新访问几次都不打印是为什么？究竟是怎样才算第二次访问呢？
希望得到各位大佬的解答，感谢