JDBC备忘录3_常用的JDBC单例类
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要点：
1、类设计
2、单例模式
3、resultSet，Statement，Connection的关闭顺序以及异常捕获
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1、工具类，不让其他类继承，所以是final；url，password，driver，class等常量信息
2、单例模式，饿汉模式和懒汉模式的区别，双重加锁
（1）单例模式三要素：静态私有的成员变量，私有构造方法，防止外部实例化，公共的获取方法
（2）饿汉模式，饥所以及时加载；懒汉模式，懒所以延迟加载；
3、打开顺序：Connection --> Statement(PreparedStatement) --> Result
关闭顺序： Result --> Statement --> Connection
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package com.zhaoyp.jdbc.util;import java.sql.Connection;import java.sql.DriverManager;import java.sql.ResultSet;import java.sql.SQLException;import java.sql.Statement;public final class JdbcUtils {private static final String URL = "jdbc:mysql://localhost:3306/jdbc";private static final String USERNAME = "root";private static final String PASSWORD = "123";private static final String DRIVER = "com.mysql.jdbc.Driver";//延迟加载，需要new的时候才去new//懒汉模式，懒所以延迟加载//饿汉模式，饿所以及时加载//private static JdbcUtils instance = new JdbcUtils();private static JdbcUtils instance = null;private JdbcUtils() {}//public static synchronized JdbcUtils getInstance() {//if(instance == null) {//instance = new JdbcUtils();//}//return instance;//}//double-checkpublic static JdbcUtils getInstance() {if(instance == null) {//class级别锁synchronized (JdbcUtils.class) {if(instance == null) {instance = new JdbcUtils();}}}return instance;}static {try {Class.forName(DRIVER);} catch (ClassNotFoundException e) {// TODO Auto-generated catch blocke.printStackTrace();}}public static Connection getConnection() throws SQLException {return DriverManager.getConnection(URL, USERNAME, PASSWORD);}public static void free(ResultSet rs, Statement st, Connection conn) {try {if(rs!=null && !rs.isClosed())rs.close();} catch (Exception e) {System.out.println("异常。。");} finally {try {if(st != null && !st.isClosed())st.close();} catch (Exception e) {System.out.println("st 异常");} finally {try {if(conn != null && !conn.isClosed()) conn.close();} catch (Exception e) {System.out.println("conn 异常");} finally {System.out.println("程序结束");}}}}}?