mybatis_SQL映射(4)_鉴别器

鉴别器：有时一个单独的数据库查询也许返回很多不同（但是希望有些关联）数据类型的结果集。鉴别器元素就是被设计来处理这个情况的，还有包括类的继承层次结构。[抄了一个定义，不是很理解，还是看例子吧]

1. 交通工具表vehicle

create table test.vehicle ( id bigint(10) primary key AUTO_INCREMENT, vin varchar(10), year date, color varchar(10), vendor varchar(10), vehicle_type int,    //类型：1表示car, 2表示boat door_count int,      //车门数量，car独有属性 quant varchar(10)    //船桨，boat独有属性);

2. java对应的实体类Vehicle,Car,Boat

package com.yjq.entity;import java.sql.Date;public class Vehicle {private int id;private String vin;  //交通登记号码private Date year;private String color;private String vendor;private int vehicleType;public Vehicle() {}public int getId() {return id;}public void setId(int id) {this.id = id;}public String getVin() {return vin;}public void setVin(String vin) {this.vin = vin;}public Date getYear() {return year;}public void setYear(Date year) {this.year = year;}public String getColor() {return color;}public void setColor(String color) {this.color = color;}public String getVendor() {return vendor;}public void setVendor(String vendor) {this.vendor = vendor;}public int getVehicleType() {return vehicleType;}public void setVehicleType(int vehicleType) {this.vehicleType = vehicleType;}}

package com.yjq.entity;
public class Car extends Vehicle { private int doorCount;
public Car() {}
public int getDoorCount() { return doorCount;}
public void setDoorCount(int doorCount) { this.doorCount = doorCount;} }

package com.yjq.entity;public class Boat extends Vehicle {private String quant;  //船桨public Boat() {}public String getQuant() {return quant;}public void setQuant(String quant) {this.quant = quant;}}

3. 如何将查询结果映射为不同的对象呢？鉴别器登场

<?xml version="1.0" encoding="UTF-8"?><!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd"><mapper namespace="com.yjq.entity.Vehicle"><resultMap id="vehicleResult" type="Vehicle"><id property="id" column="id" /><result property="vin" column="vin"/><result property="year" column="year"/><result property="vendor" column="vendor"/><result property="color" column="color"/><result property="vehicleType" column="vehicle_type"/><discriminator javaType="int" column="vehicle_type"><case value="1" resultMap="carResult"/><case value="2" resultMap="boatResult"/></discriminator></resultMap><resultMap id="carResult" type="Car"><result property="vehicleType" column="vehicle_type"/><result property="doorCount" column="door_count" /></resultMap><resultMap id="boatResult" type="Boat"><result property="vehicleType" column="vehicle_type"/><result property="quant" column="quant" /></resultMap><select id="selectVehicle" parameterType="int" resultMap="vehicleResult">select * from vehicle where id =#{id};</select></mapper>

4. 表中的数据

5. dao代码，看看查询效果

package com.yjq.dao;import org.apache.ibatis.session.SqlSession;import com.yjq.db.DbFactory;import com.yjq.entity.Boat;import com.yjq.entity.Car;import com.yjq.entity.Vehicle;public class VehicleDao {public Vehicle selectVehicleById(int id) {SqlSession session = DbFactory.getInstance().openSession();Vehicle vehicle = (Vehicle) session.selectOne("com.yjq.entity.Vehicle.selectVehicle", id);session.commit();session.close();return vehicle;}public static void print(Vehicle v) {if(v instanceof Car) {Car c = (Car)v;System.out.println("Car: [id=" + c.getId() + ", vehicleType=" + c.getVehicleType() + ", doorCount=" + c.getDoorCount() + "]");} else if (v instanceof Boat) {Boat b = (Boat)v;System.out.println("Boat: [id=" + b.getId() + ", vehicleType="+ b.getVehicleType() + ", quant=" + b.getQuant() + "]");} else {System.out.println("Vehicle: [id=" + v.getId() + ", vehicleType="+ v.getVehicleType() + "]");}}public static void main(String[] args) {VehicleDao dao = new VehicleDao();Vehicle v1 = dao.selectVehicleById(1);Vehicle v2 = dao.selectVehicleById(2);Vehicle v3 = dao.selectVehicleById(3);VehicleDao.print(v1);VehicleDao.print(v2);VehicleDao.print(v3);}}

//outputCar: [id=1, vehicleType=1, doorCount=4]Boat: [id=2, vehicleType=2, quant=lxj]Vehicle: [id=3, vehicleType=3]

6. 蛋疼的地方

a) 重复配置：为了在Car,Boat,Vehicle类中能获取vehicleType属性,<result property="vehicleType" column="vehicle_type"/>配置了三次。

b) 多个resultMap：

i. 将查询语句的resultMap修改下，再看看输出：将所有的查询结果强行映射为Car了

<select id="selectVehicle" parameterType="int" resultMap="carResult">select * from vehicle where id =#{id};</select>

//outputCar: [id=1, vehicleType=1, doorCount=4]Car: [id=2, vehicleType=2, doorCount=0]Car: [id=3, vehicleType=3, doorCount=0]

7. 鉴别器的另外一种写法，试了下，好像没起作用，先不管了

<resultMap id="vehicleResult" type="Vehicle"><id property="id" column="id" /><result property="vin" column="vin"/><result property="year" column="year"/><result property="vendor" column="vendor"/><result property="color" column="color"/><result property="vehicleType" column="vehicle_type"/><discriminator javaType="int" column="vehicle_type"><case value="1" resultMap="carResult" type="Car"><result property="doorCount" column="door_count" /></case><case value="2" resultMap="boatResult" ><result property="quant" column="quant" /></case></discriminator></resultMap>