ServletContext learning diary 1

Catalog

One 、ServletContext object

1.1 ServletContext summary

1.2 obtain ServletContext object

1.3 ServletContext effect

1.3.1 Get the real path of the project

1.3.2 Get the project context path

1.3.3 Global container

1.4 ServletContext characteristic

1.5 ServletContext Application scenarios

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One 、ServletContext object

1.1 ServletContext summary

Global object , Also has scope , Corresponding to one Tomcat Medium Web application

When Web When the server starts , For every individual Web Application creation one Block shared storage area (ServletContext) .

ServletContext stay Web Create... When the server starts , Destroy on server shutdown .

1.2 obtain ServletContext object

GenericServlet Provides getServletContext() Method .( recommend ) this.getServletContext();

HttpServletRequest Provides getServletContext() Method .( recommend )

HttpSession Provides getServletContext() Method .

package com.ha.controller;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import javax.servlet.http.HttpSession;

import java.io.IOException;

public class ServletContextController extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1. adopt this.getServletContext();
        ServletContext servletContext = this.getServletContext();

        //2. adopt request Object acquisition 
        ServletContext servletContext1 = request.getServletContext();

        //3. adopt session Object acquisition 
        HttpSession session = request.getSession();
        ServletContext servletContext2 = session.getServletContext();

        System.out.println(servletContext);
        System.out.println(servletContext1);
        System.out.println(servletContext2);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}

<?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_3_1.xsd"
  version="3.1"
  metadata-complete="true">

  <servlet>
    <servlet-name>ctx</servlet-name>
    <servlet-class>com.ha.controller.ServletContextController</servlet-class>
  </servlet>

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


</web-app>

1.3 ServletContext effect

1.3.1 Get the real path of the project

Get the real path of the current project published on the server

String realpath=servletContext.getRealPath("/"); 

1.3.2 Get the project context path

Gets the current project context path ( Application name )

System.out.println(servletContext.getContextPath());  // Context path ( Application name )

System.out.println(request.getContextPath());

 

1.3.3 Global container

ServletContext Have scope , Data can be stored in a global container

Store the data : servletContext.setAttribute("name",value);

get data : servletContext.getAttribute("name"); 

Remove data : servletContext.removeAttribute("name");

1.4 ServletContext characteristic

Uniqueness ： One application corresponds to one individual ServletContext.

Life cycle ： As long as the container is not closed or the application is not uninstalled ,ServletContext Just one Direct existence .

1.5 ServletContext Application scenarios

ServletContext Count the number of visits to the current project

package com.qf.counter;

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(name = "CounterController",value = "/counterController")
public class CounterController extends HttpServlet {
    protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //1. obtain ServletContext object 
        ServletContext servletContext = request.getServletContext();

        //2. Get counter 
        Integer counter = (Integer) servletContext.getAttribute("counter");

        if(counter == null){
            counter = 1;
            servletContext.setAttribute("counter",counter);
        }else{
            counter++;
            servletContext.setAttribute("counter",counter);
        }
        System.out.println("counter:"+counter);
    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        doPost(request, response);
    }
}