从配置文件读取连接数据库的配置信息
在webapps下新建Web应用程序的目录为conntest：
其目录结构如下：
webapps:
|-conntext
|  |-src
|  |  |-conntest
|  |       |-DBConfig.java
|  |       |-ConnDBServlet.java
|  |
|  |-WEB-INF
|     |-classes
|     |    |-conntest
|     |         |-database.properties
|     |         |-DBConfig.class
|     |         |-ConnDBServlet.class
|     |
|     |-lib
|     |  |-mysql-connector-java-5.1.26-bin.jar
|     |
|     |-web.xml

1.database.properties

#driverClass(数据库驱动类的类名)
#driverClass=oracle.jdbc.OracleDriver
driverClass=com.mysql.jdbc.Driver

#url(要连接数据库的地址)
#url=jdbc:oracle:thin:@192.168.1.180:1521:orcl
#url=jdbc:mysql://127.0.0.1:3306/test
url=jdbc:mysql://127.0.0.1:3306

#user(数据库用户)
#user=scott
user=root

#password(密码)
#password=tiger
password=root

2.web.xml

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app xmlns="http://java.sun.com/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee
                      http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"
  version="3.0"
  metadata-complete="true">
  <context-param>
      <param-name>driverClass</param-name>
      <param-value>com.mysql.jdbc.Driver</param-value>
  </context-param>
  <context-param>
      <param-name>url</param-name>
      <param-value>jdbc:mysql://localhost:3306/test</param-value>
  </context-param>
  <context-param>
      <param-name>user</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>ConnDBServlet</servlet-name>
    <servlet-class>conntest.ConnDBServlet</servlet-class>
    <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</param-value>
    </init-param>
    <init-param>
        <param-name>user</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>ConnDBServlet</servlet-name>
    <url-pattern>/conndb</url-pattern>
  </servlet-mapping>
  
</web-app>

3.DBConfig.java

package conntest;

public class DBConfig{
    private String driverClass;
    private String url;
    private String user;
    private String password;
    public DBConfig(String driverClass, String url, String user, String password){
        this.driverClass = driverClass;
        this.url = url;
        this.user = user;
        this.password = password;
    }
    
    public String getDriverClass(){
        return this.driverClass;
    }
    
    public String getUrl(){
        return this.url;
    }
    
    public String getUser(){