第一次，没调用getProperties()方法，代码如下：

package util;

import java.io.IOException;
import java.io.InputStream;
import java.sql.Connection;
import java.sql.DriverManager;
import java.util.Properties;
	
public class DBUtil {
	private static String driverClassName;
	private static String url;
	private static String username;
	private static String password;
	public static Properties ps = new Properties();
	
	public static void getProperties() throws IOException{
		InputStream ips= DBUtil.class.getClassLoader()
						.getResourceAsStream("db.properties");
		ps.load(ips);
		driverClassName = ps.getProperty("driverClassName");
		url = ps.getProperty("url");
		username = ps.getProperty("username");
		password = ps.getProperty("password");
	}
	public static Connection getConnection() throws Exception{
		/*//配置文件方式*/
		getProperties();  //这里没有调用，导致获取不到参数值，纠结了半小时才调试出来
		Class.forName(driverClassName);
		Connection conn = DriverManager.getConnection(url,username,password);
		return conn; 
	}                
	
	public static void close(Connection conn){
	  if(conn!=null){
		try {
			conn.close();
		} catch (Exception e) { 
			e.printStackTrace();
		}
	  }
	}
	 
}

====DAOFactory.java文件的代码====

public class DAOFactory {
  DBUtil.getConnection();//这里没有调用，导致获取不到参数值，纠结了2个小时，且不能在 DBUtil.getValue(type)前直接先调用
  String className = DBUtil.getValue(type);  //一直获取不到className的值
  ...
}

====DBUtil.java文件的代码====
    
public class DBUtil {
    private static String driverClassName;
    private static String url;
    private static String username;
    private static String password;
    public static Properties ps = new Properties();
    
    public static void getProperties() throws IOException{
        InputStream ips= DBUtil.class.getClassLoader()
                        .getResourceAsStream("daoconfig.properties"); 
        ps.load(ips);
        driverClassName = ps.getProperty("driverClassName");
        url = ps.getProperty("url");
        username = ps.getProperty("username");
        password = ps.getProperty("password");
    } 
    public static Connection getConnection() throws Exception{ 
        /*//配置文件方式*/
        getProperties();
        Class.forName(driverClassName);
        Connection conn = DriverManager.getConnection(url,username,password);
        return conn; 
    }                 
    public static void close(Connection conn){
      if(conn!=null){
        try {
            conn.close();
        } catch (Exception e) { 
            e.printStackTrace();
        }
      }
    } 
    /*根据参数key，获取配置文件的参数值value*/
    public static String getValue(String key){
        return ps.getProperty(key);
    }
 
    /*调试*/ 
    public static void main(String[] args) throws Exception{