import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
 
/*
   后端用来处理登录请求的servlet
   1.继承HttpServlet
   2.重写方法
   Servlet接口,是javaWeb最顶级接口,定义servlet中的方法
   abstract GenericServlet implements Servlet
       destroy();
       init();
   abstract class HttpServlet extends GenericServlet
       service();
   LoginServlet extends HttpServlet  也就遵循servlet规范,请求到服务器时.服务器也要遵照servlet规范调用对应的方法
 */
public class LoginServlet_back extends HttpServlet {
 
    /*
       创建阶段
       serlvet对象不由我们创建,是由服务器创建的,创建有两种情况:
          1.默认的,在第一访问对应的servlet时创建.
          2.1 >=0 就会在服务器启动时将servlet创建并初始化.
          服务器为每个servlet只创建一个对象(单例的),
     */
  public  LoginServlet(){
        System.out.println("LoginServlet无参构造方法");
    }
 
    /*
      初始化阶段:
         在servlet创建成功后,执行完构造方法后,会立即调用init(),来实现一些初始化操作(只执行一次)
     */
    @Override
      public void init(ServletConfig config) throws ServletException {
        System.out.println(config.getServletName());
        System.out.println("init"+config);
     }
 
    /*
       服务阶段:
           对象创建,初始化完成后,一旦有客户端请求,就会执行service(),提供服务.
           HttpServletRequest req,(表示请求)
           HttpServletResponse resp(表示响应)
     */
    @Override
    protected void service(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("service");
        System.out.println("service"+req);
        System.out.println("service"+resp);
    }
 
    /*
     销毁阶段: 当服务器关闭,servlet在真正的销毁之前,服务器会调用destroy(),掉用的是已经生成的servlet对象的destory()方法执行一些最终的操作
                例如打印日志,例如将内存数据保存到硬盘
     */
        @Override
        public void destroy() {
            System.out.println("destroy");
        }
 
 
}

HttpServlet类中具有servic方法和doGet和post方法，系统会自动调用service方法，在service方法中会自动识别前端发来的请求，可以通过请求的类型去判断请求的方式，掉用doGet或者Post方法，我们需要在继承HttpServlet类的子类中去重写我们需要的doGet方法或者Post方法

@WebServlet("/lg")   //通过该标签可以快速配置该类的访问地址，不用再去web.xml中进行配置
public class LoginServlet_2 extends HttpServlet {
 
 
    /*
      处理doGet方式请求
     */
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("doGet");
        System.out.println(req.getParameter("name"));
        System.out.println(req.getParameter("age"));
    }
 
    /*
      处理post方式请求
     */
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        System.out.println("doPost");
        req.setCharacterEncoding("UTF-8");
        String account = req.getParameter("account");
        String password = req.getParameter("password");
        System.out.println(account);
        System.out.println(password);
        System.out.println(req.getParameter("xb"));
        System.out.println(Arrays.toString( req.getParameterValues("ck")));//在接收多选框的时候使用getParameterValues
 
       
        //响应
       resp.setContentType("text/html;charset=utf-8");
       PrintWriter printWriter = resp.getWriter();
       printWriter.print("
登录成功
");
       //printWriter.print("账号或密码错误");
    }
}

 在doPost方法中文容易出现乱码：

1.防止在使用请求时出现乱码，在使用请求前先进行处理

 req.setCharacterEncoding("UTF-8");

resp.setContentType("text/html;charset=utf-8");

session

 
import javax.servlet.*;
import javax.servlet.annotation.WebFilter;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
 
@WebFilter(urlPatterns = "/*")
public class CorsFilter implements Filter {
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain)
            throws IOException, ServletException {
        HttpServletResponse httpResponse = (HttpServletResponse) servletResponse;
        HttpServletRequest httpRequest = (HttpServletRequest) servletRequest;
        System.out.println("跨域过滤器");
        //允许携带Cookie时不能设置为* 否则前端报错
        httpResponse.setHeader("Access-Control-Allow-Origin", httpRequest.getHeader("origin"));//允许所有请求跨域
                                                              // 在使用时将origin替换成信任的IP
        httpResponse.setHeader("Access-Control-Allow-Methods", "*");//允许跨域的请求方法GET, POST, HEAD 等
        httpResponse.setHeader("Access-Control-Allow-Headers", "*");//允许跨域的请求头
        httpResponse.setHeader("Access-Control-Allow-Credentials", "true");//是否携带cookie
 
        filterChain.doFilter(servletRequest, servletResponse);
    }
}

 
import javax.servlet.*;
import java.io.IOException;
 
public class EncodFilter implements Filter {
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        System.out.println("编码过滤器");
        servletRequest.setCharacterEncoding("utf-8");
        servletResponse.setCharacterEncoding("utf-8");
        filterChain.doFilter(servletRequest,servletResponse);//向下执行
    }
}

 
import com.ffyc.webback.model.Admin;
import javax.servlet.*;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpSession;
import java.io.IOException;
import java.io.PrintWriter;
 
public class IsLoginFilter implements Filter {
 
    @Override
    public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
        HttpServletRequest request=(HttpServletRequest)servletRequest;
        HttpSession session=request.getSession();
        System.out.println("登录权限");
        Admin admin=(Admin) session.getAttribute("admin");
        if(admin==null){   //后端验证失败
            PrintWriter printWriter=servletResponse.getWriter();
            printWriter.print(201);   //告诉后端验证失败
        }else{
            filterChain.doFilter(servletRequest,servletResponse);//向下执行
        }
    }
}

在web.xml中配置上述三个过滤器