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【三层架构及JDBC总结】
2022-07-05 08:19:00 【今夜鸣风】
在学习JDBC的过程中,需要注意的是工具类必须和查询字段对应,即使查询方法是通用的,但在面对不同表格的时候也得注意工具类的声明属性是否和查询语句查询时候用到的属性对应,不然无法查询到查询语句所需要的对象信息,也不会有正确的结果。
还有比较关键的一点就是由于表的字符集可能存在差异,为了能正常查询和增删改数据,要与现在properties配置文件里面添加关于字符集的语句说明,如下所示:
url=jdbc:mysql://localhost:3306/homework3?useUnicode=true&characterEncoding=UTF-8&rewriteBatchedStatements=true
关于多可变参数:
除了工具类需要对应之外,为了提高查询方法的通用性引入了可变参数【args……】来代指SQL语句所需要的参数。
当存在可变参数时,args相当于一个数组, 底层系统会先判断它的长度然后创建一个同等长度的数组,之后获取参数就遍历一下数组即可
/**
* 查询
* @param sql
* @param args
* @throws SQLException
* @throws ClassNotFoundException
*/
public static List<SelectTools> select1(String sql, Object... args) {
Connection conn = null;
PreparedStatement pre = null;
ResultSet resultSet=null;
try {
conn = JDBCUtils.getConn1();
pre = conn.prepareStatement(sql);
for (int i = 0; i < args.length; i++){
pre.setObject(i +1,args[i]);
}//当存在可变参数时,args相当于一个数组,
// 底层系统会先判断它的长度然后创建一个同等长度的数组,之后获取参数就遍历一下数组即可。
resultSet =pre.executeQuery();
// while (resultSet.next()){
// System.out.println(resultSet.getObject(1) + " " + resultSet.getObject(2)); ;
// }
//查询多行数据使用迭代器while(resultSet.next())
ResultSetMetaData rsmd=resultSet.getMetaData();
// System.out.println(rsmd);
int columnCount=rsmd.getColumnCount();
List<SelectTools> list = new ArrayList<>();
while (resultSet.next()){
SelectTools se=new SelectTools();
for (int i = 0; i <columnCount; i++) {
Object columnValue= resultSet.getObject(i+1);
String columnName=rsmd.getColumnLabel(i+1);
Field file= SelectTools.class.getDeclaredField(columnName);
file.setAccessible(true);
file.set(se,columnValue);
}
list.add(se);
}
return list;
} catch (SQLException e) {
e.printStackTrace();
} catch (NoSuchFieldException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} finally {
JDBCUtils.closeResource1(conn,pre,resultSet);
}
return null;
}
JDBC的六个步骤:
大致分为六大步,分别是:
①加载驱动
②和数据库建立链接
③通过连接创建一个SQL命令发送器Statement(更好用的避免SQL注入的现在使用prepareStatement)
④使用SQL命令发送器向数据库发送SQL命令。
⑤处理结果
⑥关闭资源
三层结构各层之间的关系如下:
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