1 Introduction to Servlet

Servlet is a technology developed by sun company to develop dynamic web
Sun provides an interface in these APIs called: Servlet.
To develop a Servlet program, there are only two steps: write a class, implement the Servlet interface, and deploy the developed java class to the web server.

The java program that implements the Servlet interface is called Servlet.

2 HelloServlet

There are two default classes that implement the Servlet interface: HttpServlet and GenericServlet.

Generally, directly inherit HttpServlet and override the doGet() and doPost() methods.

3 steps

1. Maven dependency

There are mainly two packages:

servlet-api
jsp-api

        <!-- https://mvnrepository.com/artifact/javax.servlet/javax.servlet-api -->
        <dependency>
            <groupId>javax.servlet</groupId>
            <artifactId>javax.servlet-api</artifactId>
            <version>4.0.1</version>
            <scope>provided</scope>
        </dependency>
        <!-- https://mvnrepository.com/artifact/javax.servlet.jsp/jsp-api -->
        <dependency>
            <groupId>javax.servlet.jsp</groupId>
            <artifactId>jsp-api</artifactId>
            <version>2.2</version>
            <scope>provided</scope>
        </dependency>

2. Write a class that inherits HttpServlet

Override the doGet() and doPost() methods.
Because get or post is only different in the way of request, the business logic is the same, and they can call each other.

public class MyServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        //获取前端传送参数
        String method =req.getParameter("method");
        if("add".equals(method)){
            req.getSession().setAttribute("msg", "执行一个add方法~");
        }
        if("delete".equals(method)){
            req.getSession().setAttribute("msg", "执行一个delete方法~~");
        }
        //进行业务处理
        //转发或重定向到视图，此处为转发到指定页面
        req.getRequestDispatcher("/WEB-INF/jsp/home.jsp").forward(req, resp);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

3. Write the page home.jsp to be forwarded

After the servlet is processed, it is forwarded to the /WEB-INF/jsp/home.jsp page.
The body of the page displays ${msg}, which represents the content of the msg field returned by the servlet.

<%@ page contentType="text/html;charset=UTF-8" language="java" %>
<html>
 <head>
    <title>zuka</title>
 </head>
 <body>
  ${msg}
  </body>
</html>

4. Write Servlet mapping

Why do I need to map:

The Servlet implementation class is a JAVA program, which is accessed through a browser, and the browser needs to connect to a web server, so we need to register the Servlet we wrote in web.xml.
You also need to provide a path that the browser can access.

[Note]: In /hi, the slash / must not be less, otherwise IDEA will report an error when starting Tomcat.

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://xmlns.jcp.org/xml/ns/javaee"
         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
         xsi:schemaLocation="http://xmlns.jcp.org/xml/ns/javaee http://xmlns.jcp.org/xml/ns/javaee/web-app_4_0.xsd"
         version="4.0">
    <!--注册servlet-->
    <servlet>
        <servlet-name>hi</servlet-name>
        <servlet-class>servlet.MyServlet</servlet-class>
    </servlet>
    <!--servlet请求路径,斜杠/不能少，否则启动Tomcat会报错-->
    <servlet-mapping>
        <servlet-name>hi</servlet-name>
        <url-pattern>/hi</url-pattern>
    </servlet-mapping>
</web-app>

5. Tomcat configuration

First, make sure that the project to be tested has at least one artifact. If there is none, click + Add.

Added a local Tomcat test instance.

6. Start the test

First enter http://localhost:8080/servlet_war/ to display the default page index.jsp.

Add the servlet access path /hi, and send the parameter?method=add, enter the page http://localhost:8080/servlet_war/hi?method=add. The mapped servlet returns the msg content according to the parameter method=add sent by the front-end "Execute an add method~".

What does the MVC framework do

Map URL to java class or method of java class.
Encapsulate the data submitted by the user.
Process the request-call related business processing-encapsulate the response data.
Render the corresponding data, jsp/html and other presentation layer data.

Original link: https://www.cnblogs.com/letsdaydayup/p/14431529.html

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Dynamic web technology: Servlet in detail