前后端传值案例

20211017 前后端传值案例

前端的值如何传到后端

方式1：前端form表单提交




    
    登录







    用户名:

    密码:

LoginServlet.java

package com.servlet;

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;

//地址全称为:http://localhost:8080/demo20211015/login.do
@WebServlet("/login.do")
public class LoginServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //解决响应中文乱码
        resp.setContentType("text/html;charset=utf-8");
        //获取前端传来的值
        //req.getParameter通过请求的name获取到请求的值
        //跟前端input name一致
        String username = req.getParameter("username");
        String password = req.getParameter("password");
        System.out.println("用户名为：" + username);
        System.out.println("密码为：" + password);
        if ("admin".equals(username) && "123".equals(password)) {
            //告诉用户登录成功
            //获取响应的字符输出流
            resp.getWriter().write("" + username + "登录成功");
        }
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    }
}

regist.html




    
    注册



    用户名

    密码

    重复密码

    性别(单选框)男女

    兴趣爱好(多选框)javapythonphp

    省份(下拉框)

    自我介绍(多行文本框)

RegistServlet.java

package com.servlet;

import com.bean.User;

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;

@WebServlet("/regist.do")
public class RegistServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //解决响应中文乱码
        resp.setContentType("text/html;charset=utf-8");

        String username = req.getParameter("username");
        String password = req.getParameter("password");
        String gender = req.getParameter("gender");
        //对于多选框等 使用 getParameterValues
        String[] hobby = req.getParameterValues("hobby");
        String province = req.getParameter("province");
        String introduction = req.getParameter("introduction");

        User user = new User(username, password, gender, hobby, province, introduction);
        System.out.println(user);
        //获取响应的字符输出流
        resp.getWriter().write("注册成功");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    }
}

HttpServletRequest

请求

Tomcat将前端所有请求信息（包括请求头和请求体），封装成HttpServletRequest对象。我们在Java端，是处在服务器的角度去编程。解决怎么从前端拿到值。有一种情况特殊，通过HttpServletRequest 给前端传值（请求转发）。

常用API

package com.servlet;

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;

@WebServlet("/demo03.do")
public class Demo03Servlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ///demo20211015/demo03.do
        //获取请求的URI 资源路径
        System.out.println(req.getRequestURI());
        //http://localhost:8083/demo20211015/demo03.do
        //获取请求的URL 资源全路径
        System.out.println(req.getRequestURL());
        //0:0:0:0:0:0:0:1
        //获取客户端的IP地址
        System.out.println(req.getRemoteHost());
        //zh-CN,zh;q=0.9
        //获取请求头信息 
        System.out.println(req.getHeader("Accept-Language"));
        //GET
        //获取请求方式
        System.out.println(req.getMethod());
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //无论get方式还是post方式 都执行同一段逻辑
        doGet(req, resp);
    }
}

请求转发

用的最多的：在Servlet给域属性赋值，在JSP对域属性取值。

服务器收到请求后，从一个资源跳转到另外资源的操作。本质上是服务器内部跳转。

特点：1.地址栏不会变化

2.只能跳转服务器内部资源

3.可以带信息跳转

4.一次请求

package com.servlet;

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;

@WebServlet("/demo04.do")
public class Demo04Servlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //请求转发：在多个Servlet之间进行转发
        //转发的对象是：同一个request对象和同一个response对象
        //可以实现请求参数的共享和request域属性的共享
        String username = req.getParameter("username");

        //域属性设置
        //域：像HashMap一样 以键值对存储数据的对象
        //域的 key 都是String;value 都是Object
        //对域 进行属性的赋值
//        req.setAttribute();
        //对域 进行属性的取值
//        req.getAttribute()
        req.setAttribute("name1", "value1");
        req.getRequestDispatcher("/demo05.do").forward(req, resp);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    }
}

package com.servlet;

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;

@WebServlet("/demo05.do")
public class Demo05Servlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");

        String name1 = (String) req.getAttribute("name1");
        System.out.println(name1);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    }
}

请求转发在实际案例中的应用

    
    protected void page(HttpServletRequest req, HttpServletResponse resp) throws ServletException,
            IOException {
        //1 获取请求的参数 pageNo 和 pageSize
        int pageNo = WebUtils.parseInt(req.getParameter("pageNo"), 1);
        int pageSize = WebUtils.parseInt(req.getParameter("pageSize"), Page.DEFAULT_PAGE_SIZE);
        //2 调用GoodsService.page(pageNo，pageSize)：Page 对象
        Page page = productService.page(pageNo, pageSize);
        //3 保存Page 对象到Request 域中
        req.setAttribute("page", page);
        //4 请求转发到/admin/product_list.jsp 页面
        req.getRequestDispatcher("/admin/product_list.jsp").forward(req, resp);
    }

<%
    //    List products = (List) request.getAttribute("products");
    Page productsPage = (Page) request.getAttribute("page");
%>

                    <%
                        for (int i = 0; i < productsPage.getItems().size(); i++) {
                            Product product = productsPage.getItems().get(i);
                    %>
                    
                        <%= product.getId()%>
                        
                        <%= product.getName()%>
                        
                        <%= product.getPrice()%>
                        
                        <%= product.getCategory()%>
                        
                        <%= product.getpNum()%>
                        
                        
                             "
                                  alt="..."
                                 >
                        
                        <%= product.getDescription()%>
                        
                        
                            
                            
                        
                    
                    <%
                        }
                    %>

域对象

servlet中，域对象有3种。

1.HttpServletRequest：一次请求内有效，所以要有请求转发，在多个Servlet中共享一个Request对象。

2.HttpSession：一次会话有效

3.ServletContext：整个工程运行的过程中都有效。

域对象：和HashMap结构一致，但是以String作为键，以Object作为值。

关键方法：

        public void setAttribute(String name, Object o);
        public Object getAttribute(String name);

区别：作用域的不同

HttpServletResponse

响应

Tomcat将所有需要返回给前端响应的信息，封装成HttpServletResponse对象。解决怎么给前端返回值。

response返回给前端信息 1.字节流

获取响应的字节输出流

resp.getOutputStream()

可以用作文件下载。

两个方法只能同时使用一个

        //500 getWriter() has already been called for this response
        PrintWriter writer = resp.getWriter();
        ServletOutputStream outputStream = resp.getOutputStream();

response实现文件下载

package com.servlet;


import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;

@WebServlet("/download.do")
public class DownloadFileServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //getWriter() has already been called for this response
//        PrintWriter writer = resp.getWriter();
        ServletOutputStream outputStream = resp.getOutputStream();
        //用File文件类 指向作为下载的文件
        File file = new File("D:\imgUpload\1.jpg");
        //通过设置响应头信息，告诉浏览器 接下来的数据 是一个需要被下载的文件
        resp.setHeader("content-disposition", "attachment;filename=" + "1.jpg");
        //获取文件的输入流 对源文件进行读取
        FileInputStream fis = new FileInputStream(file);
        //定义一个缓冲字节数组 用于字节数据的搬运
        byte[] buffer = new byte[1024];
        int length = 0;
        while ((length = fis.read(buffer)) != -1) {
            outputStream.write(buffer);
            outputStream.flush();
        }
        fis.close();
        outputStream.close();
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}




    
    文件下载


点击这里进行文件下载

2.字符流

resp.getWriter()

        //解决响应中文乱码
        resp.setContentType("text/html;charset=utf-8");

重定向

客户端向服务器发送请求，然后服务器给客户端一串地址，让客户端自行访问。原有的网页、网址已经停止服务。

1.两次请求

2.可以跳转到服务器外部的网址

3.无法带信息跳转

登录成功跳转首页，登录失败跳转回登录页面

        if ("admin".equals(username) && "123".equals(password)) {
            //告诉用户登录成功
            //获取响应的字符输出流
//            resp.getWriter().write("" + username + "登录成功");
            //请求重定向
            //http://localhost:8083/ 这里的斜杠是唯一一个后端特殊的斜杠
            //和前端的/意思相同
            //因为这个字符串 最终是给浏览器进行解析跳转的
            resp.sendRedirect("./index.html");
        } else {
            resp.sendRedirect("./login.html");
        }

ServletConfig

Servlet配置信息类

web.xml

    
    
        Demo01Servlet
        
        com.servlet.Demo01Servlet

        
            datasource
            mysql:jdbc://localhost:3306/bookstote
        
    
    
        
        Demo01Servlet
        
        
        
        /demo01.do

    
    @Override
    public void init(ServletConfig config) throws ServletException {
        //1.获取在web.xml中配置的别名 servlet-name
        System.out.println(config.getServletName());
        //2.获取在web.xml中配置的 init-param
        System.out.println(config.getInitParameter("datasource"));
        //3.获取到域对象之一 ServletContext
        System.out.println(config.getServletContext());
        System.out.println("Demo01Servlet 被初始化");
    }

ServletContext

表示Servlet上下文对象。一个web工程只有一个ServletContext对象。是域对象。当工程启动时，创建。工程停止销毁。

作用：1.作为域对象，进行数据的存取

2.获取当前工程路径

3.获取工程在硬盘上的部署绝对路径

package com.servlet;

import javax.servlet.ServletContext;
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;

@WebServlet("/demo06.do")
public class Demo06Servlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //1.获取到ServletContext对象
        ServletContext servletContext = this.getServletContext();
        //2.获取当前的工程路径
        // /demo20211015
        System.out.println(servletContext.getContextPath());
        //3.获取工程部署在硬盘上的绝对路径
        System.out.println("整个工程的绝对路径" + servletContext.getRealPath("/"));
        System.out.println("hello.html的绝对路径" + servletContext.getRealPath("/hello.html"));
        //4.获取web.xml的context-param
        String bookName = servletContext.getInitParameter("bookName");
        System.out.println(bookName);

        //5.作为域对象使用
        servletContext.setAttribute("book", "钢铁是怎样炼成的");
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    }
}

package com.servlet;

import javax.servlet.ServletContext;
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;

@WebServlet("/demo07.do")
public class Demo07Servlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //域对象取值
        ServletContext servletContext = this.getServletContext();
        String book = (String) servletContext.getAttribute("book");
        System.out.println("书名：" + book);
    }

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    }
}

    
        bookName
        book01

作业

在web/client中创建register.html，后端创建LoginServlet和RegisterServlet，实现登录和注册功能。并且使用重定向跳转到对应页面。

前后端传值案例

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