Webapp 项目中在java包下新建一个服务端类
使用JOptionPane框架组件 调用showMessageDialog的方法实现
四个参数null,"这是一个信息对话框","信息",JOptionPane.INFORMATION_MESSAGE

还有确认对话框的代码showConfirmDialog

package servlet;

import javafx.scene.control.Alert;

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 javax.swing.*;
import java.io.IOException;


@WebServlet(name = "DemoServlet",value = "/DemoServlet")
public class DemoServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("项目成功加载完胜");
        //Alert alert = new Alert(Alert.AlertType.ERROR);
        //System.out.println(alert);
        System.out.println("对话框显示完毕");
        JOptionPane.showMessageDialog(null,"这是一个信息对话框","信息",JOptionPane.INFORMATION_MESSAGE);
        int result = JOptionPane.showConfirmDialog(null, "是否继续", "确认", JOptionPane.YES_OPTION);
        if (result==JOptionPane.YES_OPTION) {
            System.out.println("用户输入了是");
        }else{

        }
        //显示一个输入对话框
        String intput = JOptionPane.showInputDialog("请输入您的姓名:");
        System.out.println("请输入您的姓名"+intput);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("完成了!!!!");
    }
}

具体实现效果

总结

穷不失义，达不离道。——孔丘《论语》