【web后端(十二)】jsp、servlet_servlet初始化参数、servelt上下文参数

Servlet初始化参数

我们要在这里取到servlet自己的初始化参数，并在页面展示。这在web.xml文件的配置上面有改动——配置了<init-param>（初始化参数）。

web.xml

<?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">
   
    <servlet>
        <servlet-name>ParamServlet</servlet-name>
        <servlet-class>edu.xalead.web.ParamServlet</servlet-class>
        <!--配初始化参数：只能在ParamServlet的配置里用，
        其他的servelt用不了。-->
        <init-param>
            <param-name>driverClass</param-name>
            <param-value>com.mysql.jdbc.Driver</param-value>
        </init-param>
        <init-param>
            <param-name>url</param-name>
            <param-value>jdbc:mysql://localhost:3306/mysql</param-value>
        </init-param>
        <init-param>
            <param-name>username</param-name>
            <param-value>root</param-value>
        </init-param>
        <init-param>
            <param-name>password</param-name>
            <param-value>root</param-value>
        </init-param>
    </servlet>
   
    <servlet-mapping>
        <servlet-name>ParamServlet</servlet-name>
        <url-pattern>/pas</url-pattern>
    </servlet-mapping>
</web-app>

ParamServlet

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

//@WebServlet(name = "paramServlet", urlPatterns = "aaa")
public class ParamServlet extends HttpServlet {

    //数据库的四个参数
    private String driverClass = null;
    private String url = null;
    private String username = null;
    private String password = null;

    public void init() {

    }

    protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
        //取servlet自己的配置信息，
        //servlet把他自己的配置信息封装到一个servlet对象里边，
        //然后也放到servlet里边取引用了。
        //只要我们拿到了getServletConfig()，
        //就能取初始化参数了。
        ServletConfig config = this.getServletConfig();

        //取Servelt自己的参数
        this.driverClass = config.getInitParameter("driverClass");
        this.url = config.getInitParameter("url");
        this.username = config.getInitParameter("username");
        this.password = config.getInitParameter("password");
        //给客户端输出
        response.getWriter().println(driverClass + ":" + url + ":" + username + ":" + password);
    }
}

Servelt上下文参数

上下文参数不需要在某个特定的Servlet中配置参数，而是配置到web.xml中，代表ServletContext的参数，所有的Servlet都可以使用

<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">
    <!-- 上下文参数 -->
    <context-param>
        <param-name>url</param-name>
        <param-value>jdbc:mysql://localhost:3306/tt</param-value>
    </context-param>
    <context-param>
        <param-name>username</param-name>
        <param-value>root</param-value>
    </context-param>
    <context-param>
        <param-name>password</param-name>
        <param-value>root</param-value>
    </context-param>
    <servlet>
        <servlet-name>addd</servlet-name>
        <servlet-class>edu.xalead.ServeltContextServlet</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>addd</servlet-name>
        <url-pattern>/context</url-pattern>
    </servlet-mapping>
   
</web-app>

访问上下文参数的代码

String url = this.getServletContext().getInitParameter("url");
String username = this.getServletContext().getInitParameter("username");
String password = this.getServletContext().getInitParameter("password");
response.getWriter().println(url + ":" + username + ":" + password);

context-param配置的上下文参数是所有Servlet都可以使用的参数