用ServletContext获得Web.xml中子元素param-value的值异常

今天在servlet中加载JDBC驱动时，在init（）时获得配置文件中的driverClass时由于用ServletContext sc=getServletContext();
     driverClass=sc.getInitParameter("driverClass");导致驱动无法加载。详细见下面代码

package com.amaker.servletcontext;

import java.io.IOException;

import java.io.PrintWriter;

import javax.servlet.ServletContext;

import javax.servlet.ServletException;

import javax.servlet.annotation.WebServlet;

import javax.servlet.http.HttpServlet;

import javax.servlet.http.HttpServletRequest;

import javax.servlet.http.HttpServletResponse;

/**

 * Servlet implementation class ServletConfigTest

 */

@WebServlet("/ServletConfigTest")

public class ServletConfigTest extends HttpServlet {

    private static final long serialVersionUID = 1L;

    String driverClass=null;

    /**

     * Default constructor.

     */

    public ServletConfigTest() {

        // TODO Auto-generated constructor stub

        

    }

    public void init() throws ServletException{
        ServletContext sc=getServletContext();  //这里这样写其实也不会导致读取不了配置文件中的参数值。假如这样的情况，外加配置文件中

     driverClass=sc.getInitParameter("driverClass");  //红色字体的那几条语句，是肯定读取不了的。

        try {

            //Class.forName("oracle.jdbc.driver.OracleDriver");

            Class.forName("driverClass");

            System.out.println(driverClass);

        } catch (ClassNotFoundException e) {

            // TODO Auto-generated catch block

            e.printStackTrace();

        }

    }

    /**

     * @see HttpServlet#doGet(HttpServletRequest request, HttpServletResponse response)

     */

    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {

        // TODO Auto-generated method stub

        resp.setCharacterEncoding("gb2312");

        PrintWriter out=resp.getWriter();

        out.println(driverClass);

        out.println("驱动加载成功！");

        out.close();

    }

    /**

     * @see HttpServlet#doPost(HttpServletRequest request, HttpServletResponse response)

     */

    /*protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {

        // TODO Auto-generated method stub

    }*/

}

web.xml配置文件中代码：

<?xml version="1.0" encoding="GBK"?>

<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">

<servlet>

            <servlet-name>ServletConfigTest</servlet-name>

            <servlet-class>com.amaker.servletcontext.ServletConfigTest</servlet-class>

  <init-param>

                <param-name>driverClass</param-name>

                <param-value>oracle.jdbc.driver.OracleDriver</param-value>

            </init-param>

</servlet>

<servlet-mapping>

            <servlet-name>ServletConfigTest</servlet-name>

            <url-pattern>/test</url-pattern>

</servlet-mapping>

</web-app>

我做了实验，源文件中红色字体的语句和配置文件中红色字体的语句结合，导致无法读取对应的param-value

解决方法：

第一种：将 ServletContext sc=getServletContext(); 

            driverClass=sc.getInitParameter("driverClass");


     改为ServletConfig Config=getServletConfig();

       ServletContext sc=[b]Config.getServletContext(); 

     driverClass=sc.getInitParameter("driverClass"); [/b]

第二种：修改配置文件

               将  <init-param>

                <param-name>driverClass</param-name>

                <param-value>oracle.jdbc.driver.OracleDriver</param-value>

            </init-param>去掉

加上<context-param>

    <param-name>driverClass</param-name>

    <param-value>oracle.jdbc.driver.OracleDriver</param-value>

</context-param>

 配置文件就变为：

<?xml version="1.0" encoding="GBK"?>

<web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns="http://java.sun.com/xml/ns/javaee" xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd" id="WebApp_ID" version="3.0">
<context-param>

    <param-name>driverClass</param-name>

    <param-value>oracle.jdbc.driver.OracleDriver</param-value>

</context-param>

<servlet>

            <servlet-name>ServletConfigTest</servlet-name>

            <servlet-class>com.amaker.servletcontext.ServletConfigTest</servlet-class>

</servlet>

<servlet-mapping>

            <servlet-name>ServletConfigTest</servlet-name>

            <url-pattern>/test</url-pattern>

</servlet-mapping>

</web-app>