javaweb基础：ServletContext

本质：ServletContext是servlet的中间件，为servlet保存一些东西

　　注：ServletContext是邻驾与servlet之上的

 

1.ServletContext概述：

　　web容器在启动时，它会为每一个web程序都创建一个ServletContext对象，它代表当前的web应用

 

 

1.1共享数据

　　在此Servlet中保存数据，可在另一个servlet中拿到；

package com.king.servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class HelloServlet extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        //this.getInitParameter(); servlet参数
        //this.getServletConfig(); servlet配置
        //this.getServletContext(); servlet上下文
        ServletContext context = this.getServletContext();

        String username="king";//数据
        context.setAttribute("username",username);//将一个数据保存在ServletContext中，名字为：username

    }

}

package com.king.servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class GetServlet extends HttpServlet {
    @Override
    protected void doGet(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);

    }

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

 

web.xml配置

<?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"
         metadata-complete="true">

    <!--配置一些web应用的初始化参数-->
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql//localhost:3306/mybaits</param-value>
    </context-param>

    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.king.servlet.HelloServlet</servlet-class>
    </servlet>
    <!--Servlet的请求路径-->
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/king</url-pattern>
    </servlet-mapping>

    <servlet>
        <servlet-name>getc</servlet-name>
        <servlet-class>com.king.servlet.GetServlet</servlet-class>
    </servlet>
    <!--Servlet的请求路径-->
    <servlet-mapping>
        <servlet-name>getc</servlet-name>
        <url-pattern>/getc</url-pattern>
    </servlet-mapping>


    <servlet>
        <servlet-name>gp</servlet-name>
        <servlet-class>com.king.servlet.ServletDemo03</servlet-class>
    </servlet>
    <!--Servlet的请求路径-->
    <servlet-mapping>
        <servlet-name>gp</servlet-name>
        <url-pattern>/gp</url-pattern>
    </servlet-mapping>


    <servlet>
        <servlet-name>sd4</servlet-name>
        <servlet-class>com.king.servlet.ServletDemo04</servlet-class>
    </servlet>
    <!--Servlet的请求路径-->
    <servlet-mapping>
        <servlet-name>sd4</servlet-name>
        <url-pattern>/sd4</url-pattern>
    </servlet-mapping>

</web-app>

 

转发的是实现（servlet配置在上面web.xml）

package com.king.servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;

public class ServletDemo04 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        System.out.println("进入了servlet——sd4");
        //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);
        //结果显示的是gp的数据，但url是当前url，
        //总结：转发不会改变路径，
        //转发本质：gp的数据传到sd4，sd4传给结果

    }

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

 转发的本质原理图