实例化

当用户通过浏览器输入一个路径，这个路径对应的servlet被调用的时候，该Servlet就会被实例化

image.png
无论访问了多少次LoginServlet，构造方法只会执行一次，所以Servlet是单例的

初始化

LoginServlet继承了HttpServlet，同时也继承了init(ServletConfig)方法
init初始化也只会执行一次
代码如下

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

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

public class LoginServlet extends HttpServlet {
    
    public LoginServlet() {
        System.out.println( "LoginServlet 构造方法 被调用");
    }
    
    @Override
    public void init(ServletConfig config) throws ServletException {
        System.out.println("init(ServletConfig)");
    }
    
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        
        String name = req.getParameter("name");
        String password = req.getParameter("password");
        
        String html = null;
        
        if ("admin".equals(name) && "123".equals(password)) {
            html = "<div style='color:green'>登录成功</div>";
        } else {
            html = "<div style='color:red'>登录失败</div>";
        }
        
        resp.setContentType("text/html; charset=UTF-8");
        resp.getWriter().println(html);
    }
}