30个类手写Spring核心原理之自定义ORM（上）（6）

本文节选自《Spring 5核心原理》

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