代码演示：

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.Map;

@WebServlet("/req2")
public class RequestDemo2 extends HttpServlet {

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //get请求逻辑
        //System.out.println("get...");

        //1获取所有参数的MAP集合
        Map<String, String[]> map = req.getParameterMap();
        for (String key : map.keySet()){
            //username:zhangsan
            System.out.print(key + ":");

            //获取值
            String[] values = map.get(key);
            for (String value : values){
                System.out.print(value + " ");
            }

            System.out.println();

        }

        System.out.println("--------------");

        //获取对应的参数值，数值
        String[] hobbies = req.getParameterValues("hobby");
        for (String hobby : hobbies){
            System.out.println(hobby);
        }

        //3.根据key 获取单个参数值
        String username = req.getParameter("username");
        String password = req.getParameter("password");

        System.out.println(username);
        System.out.println(password);

    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //post请求逻辑
        this.doGet(req, resp);

//            System.out.println("post...");
//
//            //1获取所有参数的MAP集合
//            Map<String, String[]> map = req.getParameterMap();
//            for (String key : map.keySet()){
//                //username:zhangsan
//                System.out.println(key + ":");
//
//                //获取值
//                String[] values = map.get(key);
//                for (String value : values){
//                    System.out.print(value + " ");
//                }
//
//                System.out.println();
//
//            }
//
//            System.out.println("--------------");
//
//            //获取对应的参数值，数值
//            String[] hobbies = req.getParameterValues("hobby");
//            for (String hobby : hobbies){
//                System.out.println(hobby);
//            }
//
//            //3.根据key 获取单个参数值
//            String username = req.getParameter("username");
//            String password = req.getParameter("password");
//
//            System.out.println(username);
//            System.out.println(password);

        }
}