ServletContext的应用(其一)★★★

ServletContext应用

Q:什么叫ServletContext对象

A: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

1、共享数据

在这个Servlet中保存的数据，可以在另外一个servlet中拿到

1.1 代码

1.用于保存数据的HelloServlet代码

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上下文环境
        ServletContext context = this.getServletContext();

        String username = "xy"; //数据
        context.setAttribute("username",username); //将一个数据保存在了ServletContext中，名字 username 值 username

        System.out.println("Hello");
    }
}

2.用于测试获取数据的GetServlet代码

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");//获得ServletContext中存储的数据username(xy)

        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);
    }
}

3.web.xml代码

<?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">
    <servlet>
        <servlet-name>hello</servlet-name>
        <servlet-class>com.xy.servlet.HelloServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>hello</servlet-name>
        <url-pattern>/hello</url-pattern>
    </servlet-mapping>
    <servlet>
        <servlet-name>getc</servlet-name>
        <servlet-class>com.xy.servlet.GetServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>getc</servlet-name>
        <url-pattern>/getc</url-pattern>
    </servlet-mapping>
</web-app>

1.2 代码测试

测试的时候发现了一个问题，如下图所示，获得的数据为null

这说明共享没有成功，这是为什么呢？

原来是因为：这个共享数据还没有被分享出去

换句话说：null的原因是没有setAtturibute 所以应该先访问HelloServlet

也就是说：这个数据是在HelloServlet程序内写入的，我光顾着抓紧测试了，还没有走HelloServlet程序

也就是说还没有共享数据，当然也就没法在测试网页获得数据了

重新测试一下，测试结果为数据共享成功

2、获取初始化参数

2.1 代码

1.用于获取初始化参数的ServletDemo03代码

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;

/**
 * 获取初始化参数练习
 */
public class ServletDemo03 extends HttpServlet{
    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        ServletContext context = this.getServletContext();
        String url = context.getInitParameter("url");
        resp.getWriter().print(url);
    }
}

2.web.xml代码

<!--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>

<!--注册映射路径-->
<servlet>
    <servlet-name>gp</servlet-name>
    <servlet-class>com.xy.servlet.ServletDemo03</servlet-class>
</servlet>
<servlet-mapping>
    <servlet-name>gp</servlet-name>
    <url-pattern>/gp</url-pattern>
</servlet-mapping>