本文节选自《Spring 5核心原理》

1 实现思路概述 1.1 从ResultSet说起
说到ResultSet，有Java开发经验的“小伙伴”自然最熟悉不过了，不过我相信对于大多数人来说也算是“最熟悉的陌生人”。从ResultSet取值操作大家都会，比如：

private static List select(String sql) { List result = new ArrayList<>(); Connection con = null; PreparedStatement pstm = null; ResultSet rs = null; try { //1. 加载驱动类 Class.forName("com.mysql.jdbc.Driver"); //2. 建立连接 con = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/gp-vip-spring-db-demo", "root","123456"); //3. 创建语句集 pstm =con.prepareStatement(sql); //4. 执行语句集 rs = pstm.executeQuery(); while (rs.next()){ Member instance = new Member(); instance.setId(rs.getLong("id")); instance.setName(rs.getString("name")); instance.setAge(rs.getInt("age")); instance.setAddr(rs.getString("addr")); result.add(instance); } //5. 获取结果集 }catch (Exception e){ e.printStackTrace(); } //6. 关闭结果集、关闭语句集、关闭连接 finally { try { rs.close(); pstm.close(); con.close(); }catch (Exception e){ e.printStackTrace(); } } return result; }

以上我们在没有使用框架以前的常规操作。随着业务和开发量的增加，在数据持久层这样的重复代码出现频次非常高。因此，我们就想到将非功能性代码和业务代码进行分离。我们首先想到将ResultSet封装数据的代码逻辑分离，增加一个mapperRow()方法，专门处理对结果的封装，代码如下：

private static List select(String sql) { List result = new ArrayList<>(); Connection con = null; PreparedStatement pstm = null; ResultSet rs = null; try { //1. 加载驱动类 Class.forName("com.mysql.jdbc.Driver"); //2. 建立连接 con = DriverManager.getConnection("jdbc:mysql://127.0.0.1:3306/gp-vip-spring-db-demo", "root","123456"); //3. 创建语句集 pstm =con.prepareStatement(sql); //4. 执行语句集 rs = pstm.executeQuery(); while (rs.next()){ Member instance = mapperRow(rs,rs.getRow()); result.add(instance); } //5. 获取结果集 }catch (Exception e){ e.printStackTrace(); } //6. 关闭结果集、关闭语句集、关闭连接 finally { try { rs.close(); pstm.close(); con.close(); }catch (Exception e){ e.printStackTrace(); } } return result; }private static Member mapperRow(ResultSet rs, int i) throws Exception { Member instance = new Member(); instance.setId(rs.getLong("id")); instance.setName(rs.getString("name")); instance.setAge(rs.getInt("age")); instance.setAddr(rs.getString("addr")); return instance; }

但在真实的业务场景中，这样的代码逻辑重复率实在太高，上面的改造只能应用Member类，换一个实体类又要重新封装，聪明的程序员肯定不会通过纯体力劳动给每一个实体类写一个mapperRow()方法，一定会想到代码复用方案。我们不妨来做这样一个改造。
先创建Member类：

package com.gupaoedu.vip.orm.demo.entity; import lombok.Data; import javax.persistence.Entity; import javax.persistence.Id; import javax.persistence.Table; import java.io.Serializable; @Entity @Table(name="t_member") @Data public class Member implements Serializable { @Id private Long id; private String name; private String addr; private Integer age; @Override public String toString() { return "Member{" + "id=" + id + ", name='" + name + '\'' + ", addr='" + addr + '\'' + ", age=" + age + '}'; } }

优化JDBC操作：

public static void main(String[] args) { Member condition = new Member(); condition.setName("Tom"); condition.setAge(19); List result =select(condition); System.out.println(Arrays.toString(result.toArray())); }private static List select(Object condition) {List