Servlet principle and ServletContext application

Servlet principle

1. Servlet is invoked by the web server, and after receiving the browser request, the web server will:
   

6.4, Mapping problem

1. A request can specify a mapping path

	<servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>

2. A request can specify multiple mapping paths

	<servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello1</url-pattern>
    </servlet-mapping>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello2</url-pattern>
    </servlet-mapping>

3. A request can specify a generic mapping path

<!--通用映射路径-->
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello/*</url-pattern>
    </servlet-mapping>

4. default request path

<!--默认请求路径-->
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>

5. Specify some suffix or prefix, etc...

<!--可以自定义后缀实现请求映射，注意点*前面不能加映射的路径
hello/sfsfssd.tong
-->
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>*.tong</url-pattern>
    </servlet-mapping>

6. priority issue

​ Specifies that the inherent mapping path has the highest priority, and if it cannot be found, it will find the default processing request

<!--404跳转到自己写的界面-->
    <servlet>
        <servlet-name>error</servlet-name>
        <servlet-class>com.tong.servlet.ErrorServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>error</servlet-name>
        <url-pattern>/*</url-pattern>
    </servlet-mapping>

6.5、ServletContext

When the web container starts, it will create a corresponding ServletContext object for each web program, which represents the current web application;

1. Sharing data

​ The data saved in this Servlet can be obtained in another Servlet;

 //存数据
@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        //Servlet上下文
        ServletContext context = this.getServletContext();

        String name = "tom";//数据
        context.setAttribute("username",name);//讲一个数据保存在了ServletContext中，名字为：username，值为tom

    }

//取数据
@Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        ServletContext context = this.getServletContext();
        String username = (String) context.getAttribute("username");

        resp.setContentType("text/html");
        resp.setCharacterEncoding("utf-8");
        resp.getWriter().print("名字"+username);
    }

//配置xml
<?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">
  <servlet>
    <servlet-name>Hello</servlet-name>
    <servlet-class>com.tong.servlet.HelloServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>Hello</servlet-name>
    <url-pattern>/hello</url-pattern>
  </servlet-mapping>
  
  <servlet>
    <servlet-name>getChar</servlet-name>
    <servlet-class>com.tong.servlet.GetServlet</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>getChar</servlet-name>
    <url-pattern>/get</url-pattern>
  </servlet-mapping>
</web-app>

Test the access result:
access hello and store the name in it,

access get to retrieve the data and display it

2. Obtain initialization parameters

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
            ServletContext context = this.getServletContext();
            String url = context.getInitParameter("url");
            resp.getWriter().print(url);
    }

<!--配置一些web应用初始化参数-->
  <context-param>
    <param-name>url</param-name>
    <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
  </context-param>
  <servlet>
    <servlet-name>gp</servlet-name>
    <servlet-class>com.tong.servlet.ServeltDemo03</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>gp</servlet-name>
    <url-pattern>/gp</url-pattern>
  </servlet-mapping>

3. Request forwarding

   @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    
        ServletContext context = this.getServletContext();
        System.out.println("进入demo04");
//两句话可以合并
//        RequestDispatcher requestDispatcher = context.getRequestDispatcher("/gp");//转发的请求路径
//        requestDispatcher.forward(req,resp);//调用forward实现请求转发
        context.getRequestDispatcher("/gp").forward(req,resp);
    }

  <servlet>
    <servlet-name>sd4</servlet-name>
    <servlet-class>com.tong.servlet.ServletDemo04</servlet-class>
  </servlet>
  <servlet-mapping>
    <servlet-name>sd4</servlet-name>
    <url-pattern>/sd4</url-pattern>
  </servlet-mapping>

Added: Redirection

4. Read resource files

properties

• Create new properties in the java directory
• Create new properties in the resources directory

It was found that they were all packaged in the same path: classes, and the path of Express is classpath;

requires a file stream

username=root
password=123456

    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    
    	
        InputStream in = this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
        Properties prop = new Properties();
        prop.load(in);
        String username = prop.getProperty("username");
        String password = prop.getProperty("password");

        resp.getWriter().print(username+":"+password);
    }

Access test results