在前后端的交互中，前端通过以下三种方式来与后端进行交互🌟

✅query string

✅form表单

✅JSON字符串

下面我们将书写这三种方式的后端代码并进行讲解

1、Query String

QueryString即在url中写入键值对，一般用doGet方法进行交互

代码如下 

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

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

        resp.getWriter().write("ok");
    }
}

用postman验证

 

2、form表单

 form表单一般用doPost方法进行交互

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("/postParameter")
public class postParameterServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String username = req.getParameter("username");
        String password = req.getParameter("password");

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

        resp.getWriter().write("ok");
    }
}

3、Jason字符串

 （1）关于Jason

Jason是什么

JSON 是存储和交换文本信息的语法，类似 XML。JSON采用完全独立于任何程序语言的文本格式

要在Servlet中实现Jason相关操作，我们使用Jackson解析器

（2）jackson的配置

怎样配置Jackson呢？🧐

在中央仓库搜索Jackson，选中Jackson Databind👆

 点击进去之后，选择2.15.0版本👆

（Jackson2.15.0对应Tomcat8、对应Servlet3.1.0）

进去后，点击复制下面这段代码📃

<!-- https://mvnrepository.com/artifact/com.fasterxml.jackson.core/jackson-databind -->
<dependency>
    <groupId>com.fasterxml.jackson.core</groupId>
    <artifactId>jackson-databind</artifactId>
    <version>2.15.0</version>
</dependency>

粘贴到pom.xml文件中

之后会出现Maven的按钮，点击进行下载 

下载完成后，原本出现的红字报错消失

（3）使用json字符串进行前后端交互

 在配置好json的解析器后，我们书写代码进行前后端交互

import com.fasterxml.jackson.databind.ObjectMapper;

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;

//定一个Request类
class Request{
    public String username;
    public String password;
}

class Response{
    public Boolean ok;
}

@WebServlet("/json")
public class JsonParameterSerlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ObjectMapper mapper = new ObjectMapper();
        Request request = mapper.readValue(req.getInputStream(),Request.class);
        //把json字符串映射成Request类的java对象
        System.out.println("username" + request.username + "password" + request.password);
        Response response = new Response();
        response.ok = true;
        //将Response类的java对象映射成json字符串
        String respjson = mapper.writeValueAsString(response);
        resp.setContentType("applicaton/json;charset=utf8");
        resp.getWriter().write(respjson);
    }
}

使用postman进行验证