jdbc-mysql基础 ResultSetMetaData getColumnCount 得到结果集的列数

package com.jizuiku;

/**
 * 这个类很重要，即完成了抛异常的动作、通过编译，又可以使数据逻辑层的接口保持简洁。
 * 
 * @author 博客园-给最苦
 * @version V17.11.08
 */
public class DaoException extends RuntimeException {

	/**
	 * 
	 */
	private static final long serialVersionUID = 1L;

	public DaoException() {
		// TODO Auto-generated constructor stub
	}

	public DaoException(String message) {
		super(message);
		// TODO Auto-generated constructor stub
	}

	public DaoException(Throwable cause) {
		super(cause);
		// TODO Auto-generated constructor stub
	}

	public DaoException(String message, Throwable cause) {
		super(message, cause);
		// TODO Auto-generated constructor stub
	}

	public DaoException(String message, Throwable cause, boolean enableSuppression, boolean writableStackTrace) {
		super(message, cause, enableSuppression, writableStackTrace);
		// TODO Auto-generated constructor stub
	}

}

package com.jizuiku;

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.SQLException;

/**
 * 
 * 
 * @author 给最苦
 * @version V17.11.07
 */
public final class JDBCUtils {

	/**
	 * url格式 -> jdbc:子协议:子名称//主机名:端口号/数据库的名字?属性名=属性值&属性名=属性值
	 * configString变量中有多个参数，需要深入地去研究它们的具体含义
	 */
	private static String configString = "?useUnicode=true&characterEncoding=utf8&useSSL=true";
	private static String url = "jdbc:mysql://localhost:3306/jdbcforjava" + configString;
	// 本地的mysql数据库(无子名称) 端口号3306 数据库jdbcforjava

	private static String user = "root";
	private static String password = "";

	// 工具类，直接使用，不可生对象
	private JDBCUtils() {
	}

	// 注册驱动,这里应用的是static代码块只执行一次的特点
	static {
		try {
			Class.forName("com.mysql.jdbc.Driver");
		} catch (ClassNotFoundException e) {
			// TODO Auto-generated catch block
			throw new ExceptionInInitializerError(e);
		}
	}

	/**
	 * 获取与指定数据库的链接
	 * 
	 */
	public static Connection getConnection() throws SQLException {
		return DriverManager.getConnection(url, user, password);
	}

	/**
	 * 释放三种资源ResultSet PreparedStatement Connection
	 * 
	 */
	public static void free(ResultSet rs, PreparedStatement ps, Connection con) {
		try {
			if (rs != null) {
				rs.close();
			}
		} catch (SQLException e) {
			// TODO Auto-generated catch block
			throw new DaoException(e.getMessage(), e);
		} finally {
			try {
				if (ps != null) {
					ps.close();
				}
			} catch (SQLException e) {
				// TODO Auto-generated catch block
				throw new DaoException(e.getMessage(), e);
			} finally {
				try {
					if (con != null) {
						con.close();
					}
				} catch (SQLException e) {
					// TODO Auto-generated catch block
					throw new DaoException(e.getMessage(), e);
				}
			}
		}
	}
}

package com.jizuiku;

import java.sql.Connection;
import java.sql.PreparedStatement;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;

/**
 * 
 * 
 * @author 博客园-给最苦
 * @version V17.11.09
 */
public class Demo {

	public static void main(String[] args) {
		test();
	}

	public static void test() {
		Connection con = null;
		PreparedStatement ps = null;
		ResultSet rs = null;

		try {

			// 注册驱动并建立连接
			con = JDBCUtils.getConnection();

			// 创建语句
			String sql = "select id,name,quantity,time,price from book";
			ps = con.prepareStatement(sql);

			// 执行语句
			rs = ps.executeQuery();
		
			/* 
			 * 这五列分别是id,name,quantity,time,price
			 * 为什么 给最苦 感这么肯定 这五列呢？
			 * 因为 sql语句中写了呀！
			 * String sql = "select id,name,quantity,time,price from book";
			 * 
			 */			
			System.out.println("列数:" + rs.getMetaData().getColumnCount());

		} catch (SQLException e) {
			throw new DaoException(e);

		} finally {
			// 释放资源
			JDBCUtils.free(rs, ps, con);

		}
	}
}