[Java知识库]ServletContext

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前言

web容器在启动的时候，它会为每个web程序都创建一个对应的ServletContext对象，它代表了当前的web应用

一、共享数据

在这个servlet中共享数据，可以在另一个servlet中拿到

设置servlet中的属性值的类

package com.xxx.servlet01;

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()  初始化参数
        //this.getServletConfig()   servlet配置
      //  this.getServletContext()   servlet上下文
        ServletContext servletContext = this.getServletContext();
        String username="去噗";//数据
        servletContext.setAttribute("username",username); //将一个数据保存在了ServletContext中，名字为:username,值为username
        System.out.println("hello");
    }
}

web.xml配置映射

  <servlet>
        <servlet-name>hello01</servlet-name>
        <servlet-class>com.xxx.servlet01.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello01</servlet-name>
        <url-pattern>/he</url-pattern>
    </servlet-mapping>

获取servlet中保存的属性值的类

package com.xxx.servlet01;

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 doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    }

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext = this.getServletContext();
        String username =(String) servletContext.getAttribute("username");

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

web.xml配置映射

 <servlet>
    <servlet-name>get</servlet-name>
    <servlet-class>com.xxx.servlet01.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>get</servlet-name>
        <url-pattern>/he1</url-pattern>
    </servlet-mapping>

注：先运行第一个Java类（路径为/he）先将数据保存在ServletContext
再运行第二个Java类（路径为/he1)获取ServletContext中保存的内容

运行结果

二、获取参数

1.在web.xml中设置参数

<context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/mybatis</param-value>
    </context-param>

2.获取参数

package com.xxx.servlet01;

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 ServletDemo01 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        String url = this.getServletContext().getInitParameter("url");//获取参数
        resp.getWriter().print(url);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

    }
}

三、请求转发

getRequestDispatche为请求转发的意思

package com.xxx.servlet01;

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 ServletDemo02 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext servletContext=this.getServletContext();
        //System.out.println("转法到he2页面");
        servletContext.getRequestDispatcher("/he2").forward(req,resp);//请求转发到he2页面
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
    }
}

四、资源配置文件的获取

在resource目录创建db.properties资源配置文件

username=root
password=123456

将资源配置文件打印到浏览器上

package com.xxx.servlet01;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.IOException;
import java.io.InputStream;
import java.util.Properties;

public class ServletDemo03 extends HttpServlet {
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
              InputStream is= this.getServletContext().getResourceAsStream("/WEB-INF/classes/db.properties");
              Properties properties=new Properties();
              properties.load(is);
              String user=properties.getProperty("username");
              String pwd=properties.getProperty("password");
              resp.getWriter().print(user+":"+pwd);
    }
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        super.doPost(req, resp);
    }
}