mybatis_SQL映射(4)鉴别器

 摘录自：http://blog.csdn.net/y172158950/article/details/17505739

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;
    }

}

<p>package com.yjq.entity;</p><p>public class Car extends Vehicle {

    private int doorCount;</p><p>   public Car() {
    }</p><p>    public int getDoorCount() {
       return doorCount;
    }</p><p>    public void setDoorCount(int doorCount) {
       this.doorCount = doorCount;
    }

}
</p>

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);
    }

}

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