[Properties]写配置文件前，必须初始化Properties（引用变量没执行有效对象，调用方法会报空指针错误）

package BaseDAO;

import com.alibaba.druid.pool.DruidDataSourceFactory;

import javax.sql.DataSource;
import java.sql.Connection;
import java.sql.SQLException;
import java.util.Properties;

//创建连接和释放连接的工具类
public class JDBCUtil {
    private static DataSource ds;
    private static Properties properties;//只是在堆里创建了个存放properties地址的空间
    static {
        try {
            properties = new Properties(); // 👈 必须先初始化！引用变量未指向有效对象，调用方法会报空指针
            properties.load(JDBCUtil.class.getClassLoader().getResourceAsStream("druid.properties"));
            ds = DruidDataSourceFactory.createDataSource(properties);
        } catch (Exception e) {
            e.printStackTrace();
            throw new ExceptionInInitializerError("Failed to initialize JDBCUtil: " + e.getMessage());
        }
    }


    public static Connection getConnection(){
        try {
            return ds.getConnection();
        } catch (SQLException e) {
            throw new RuntimeException(e);
        }
    }

    public static void close(Connection connection){
        if(connection != null){
            try {
                connection.close();//归还线程池
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }
}

如果不初始化