理解Servlet和ServletContext对象的关系

• 是什么

  ServletContext是javax.servlet包下的一个接口，又称上下文对象，是配置对象也是一个域对象；

  当服务器启动时，会为服务器中的每一个web应用程序创建一个ServletContext对象；

  在web应用中的servlet要想实现资源的共享，可以通过ServletContext来完成；

• 如何获取这个对象:

public ServletContext getServletContext() {
    //获取ServletContext对象
    return this.getServletConfig().getServletContext();
}

• 这个对象里面也有很多方法：

getInitParameter() //获取指定参数名称的全局参数值
getRealPath(String path) //获得当前项目的服务器磁盘路径
getContextPath() //获取项目的根路径
getAttribute(String parameterName) //获取ServletContext域中指定名称的参数值；
setAttribute(String paramterName,Object parameterValue) //存储参数到ServletContext域中；
removeAttribute(String parameterNam) //将ServletContext域中指定名称的参数移除；

• 需求

获取servlet里面定义的参数

    <context-param>
        <param-name>username</param-name>
        <param-value>root</param-value>
    </context-param>
    <context-param>
        <param-name>password</param-name>
        <param-value>root456</param-value>
    </context-param>

package com.by.servlet;

import javax.servlet.*;
import javax.servlet.http.HttpServlet;

public class ServletContextServlet extends HttpServlet {

    @Override
    public void service(ServletRequest servletRequest, 
                        ServletResponse servletResponse) throws ServletException {
        //获取ServletContext对象
        ServletContext servletContext = getServletContext();
        //1.获取全局初始化参数
        String username = servletContext.getInitParameter("username");
        System.out.println(username);
        String password = servletContext.getInitParameter("password");
        System.out.println(password);
        //2.获取服务器真实路径
        String upload = servletContext.getRealPath("");
        System.out.println(upload);
        //3.获取项目的根路径
        String contextPath = servletContext.getContextPath();
        System.out.println(contextPath);

        //4.往ServletContext域中，存储一个名称为msg的属性，值为"hello"
        String str = "hello";
        servletContext.setAttribute("msg",str);
    }
}

思考：param参数可不可以在另外的Servlet中获取? 不能

package com.by.servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;

public class ServletContextServlet2 extends HttpServlet {

    @Override
    public void doGet(ServletRequest servletRequest, 
                      ServletResponse servletResponse) throws ServletException {
        //从ServletContext域中取出msg的值
        ServletContext servletContext = getServletContext();
         //1.获取全局初始化参数
        String username = servletContext.getInitParameter("username");
        System.out.println(username);
        String password = servletContext.getInitParameter("password");
        System.out.println(password);
        
        //2.获得msg属性
        Object msg = servletContext.getAttribute("msg");//能获取
        System.out.println(msg);
    }
}

package com.by.servlet;

import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.ServletRequest;
import javax.servlet.ServletResponse;
import javax.servlet.http.HttpServlet;

public class ServletContextServlet3 extends HttpServlet {

    @Override
    public void service(ServletRequest servletRequest, ServletResponse servletResponse) throws ServletException {
        //1.获取ServletContext对象
        ServletContext servletContext = getServletContext();
        //将ServletContext域中的msg参数移除
        servletContext.removeAttribute("msg");
    }
}

    <servlet>
        <servlet-name>context3=</servlet-name>
        <servlet-class>com.by.servlet.ServletContextServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>context3=</servlet-name>
        <url-pattern>/context=</url-pattern>
    </servlet-mapping>
    

    <servlet>
        <servlet-name>context2</servlet-name>
        <servlet-class>com.by.servlet.ServletContextServlet2</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>context2</servlet-name>
        <url-pattern>/context2</url-pattern>
    </servlet-mapping>
    

    <servlet>
        <servlet-name>context3</servlet-name>
        <servlet-class>com.by.servlet.ServletContextServlet3</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>context3</servlet-name>
        <url-pattern>/context3</url-pattern>
    </servlet-mapping>