说明：最近遇到了一个场景，功能代码开发完毕了，需要在本地测试，但是项目暂时还无法启动。又因为项目引入的依赖是Spring，而不是封装好的Spring Boot，我不知道怎么写测试类，装配Bean对象一直报NPE，百度一下说需要读取applicationContext.xml文件，不知道是项目结构复杂还是怎么，试了下也还不行。

然后，我就考虑使用JDBC来操作数据库，而不用Mapper。为此，写了一个简易的JDBC工具类，大家可能根据自己需要进行复制，可以将数据库配置写在application.yml文件里，然后在代码里用@Value注解读取。个人认为查询方法最好用，可以传入一个VO对象，然后返回封装后的对象结果集，其他三个方法一模一样，只在用途上做区分。

另外，该工具类实现了蛇形命名（如create_id）转驼峰命名（createId）。当我在写的时候，频频报错，然后一一分析解决，我逐渐意识到，这不就是DAO框架要实现的事情嘛？

import java.lang.reflect.Field;
import java.sql.*;
import java.util.ArrayList;
import java.util.List;

/**
 * @author hezhongying
 * @description jdbc工具类，用于测试
 */
public class SimpleJDBCUtils {

    private static String driver = "com.mysql.jdbc.Driver";

    private static String URL = "jdbc:mysql://127.0.01:3306/jdbc_test?useSSL=false&serverTimezone=GMT%2B8&useUnicode=true&characterEncoding=UTF-8";

    private static String DB_USER = "root";

    private static String DB_PASSWORD = "123456";

    private static Connection conn = null;

    /**
     * 创建连接
     */
    private static void createConnection() {
        try {
            Class.forName(driver);
            conn = DriverManager.getConnection(URL, DB_USER, DB_PASSWORD);
        } catch (ClassNotFoundException | SQLException e) {
            e.printStackTrace();
        }
    }

    /**
     * 关闭连接
     */
    private static void closeConnection() {
        if (conn != null) {
            try {
                conn.close();
            } catch (SQLException e) {
                e.printStackTrace();
            }
        }
    }

    /**
     * 查询数据
     *
     * @param clazz 封装的实体类，如User.class
     * @param sql  查询语句
     * @param <T> 泛型
     * @return 查询的结果集
     */
    public static <T> List<T> selectData(Class<T> clazz, String sql) {
        List<T> results = new ArrayList<>();
        createConnection();
        try {
            Statement state = conn.createStatement();
            ResultSet rs = state.executeQuery(sql);

            ResultSetMetaData metaData = rs.getMetaData();
            int columnCount = metaData.getColumnCount();
            while (rs.next()) {
                T object = clazz.newInstance();
                for (int i = 1; i <= columnCount; i++) {
                    String columnName = metaData.getColumnName(i);
                    String fieldName = convertSnakeCaseToCamelCase(columnName);
                    Field field = clazz.getDeclaredField(fieldName);
                    field.setAccessible(true);
                    field.set(object, rs.getObject(columnName));
                }
                results.add(object);
            }
            rs.close();
            state.close();
        } catch (SQLException | NoSuchFieldException | IllegalAccessException | InstantiationException e) {
            e.printStackTrace();
        } finally {
            closeConnection();
        }
        return results;
    }

    /**
     * 删除数据
     */
    public static int deleteData(String sql) {
        int count;
        createConnection();
        try {
            Statement state = conn.createStatement();
            count = state.executeUpdate(sql);
            state.close();
            conn.close();
        } catch (SQLException e) {
            throw new RuntimeException(e);
        } finally {
            closeConnection();
        }
        return count;
    }

    /**
     * 新增数据
     */
    public static int insertData(String sql) {
        int count;
        createConnection();
        try {
            Statement state = conn.createStatement();
            count = state.executeUpdate(sql);
            state.close();
            conn.close();
        } catch (SQLException e) {
            throw new RuntimeException(e);
        } finally {
            closeConnection();
        }
        return count;
    }

    /**
     * 更新数据
     */
    public static int updateData(String sql) {
        int count;
        createConnection();
        try {
            Statement state = conn.createStatement();
            count = state.executeUpdate(sql);
            state.close();
            conn.close();
        } catch (SQLException e) {
            throw new RuntimeException(e);
        } finally {
            closeConnection();
        }
        return count;
    }

    /**
     * 蛇形命名转驼峰命名
     * 如 user_name -> userName
     * @param input
     * @return
     */
    public static String convertSnakeCaseToCamelCase(String input) {
		if (!input.contains("_")) {
			return input;
		}
        
        StringBuilder result = new StringBuilder();
        boolean nextUpperCase = false;

        for (int i = 0; i < input.length(); i++) {
            char currentChar = input.charAt(i);

            if (currentChar == '_') {
                nextUpperCase = true;
            } else {
                if (nextUpperCase) {
                    result.append(Character.toUpperCase(currentChar));
                    nextUpperCase = false;
                } else {
                    result.append(Character.toLowerCase(currentChar));
                }
            }
        }
        return result.toString();
    }
}