mybatis 级联

　　级联是一个数据库实体的概念。一对多的级联，一对多的级联，在MyBatis中还有一种被称为鉴别器的级联，它是一种可以选择具体实现类的级联。
　　级联不是必须的，级联的好处是获取关联数据十分便捷，但是级联过多会增加系统的复杂度，同时降低系统的性能，此增彼减，所以当级联的层级超过3层时，就不要考虑使用级联了，因为这样会造成多个对象的关联，导致系统的耦合、复杂和难以维护。在现实的使用过程中，要根据实际情况判断是否需要使用级联。

MyBatis中的级联

MyBatis的级联分为3种。
•鉴别器（discriminator）：它是一个根据某些条件决定采用具体实现类级联的方案，比如体检表要根据性别去区分。
•一对一（association）：比如学生证和学生就是一种一对一的级联，雇员和工牌表也是一种一对一的级联。
•一对多（collection）：比如班主任和学生就是一种一对多的级联。
值得注意的是，MyBatis没有多对多级联，因为多对多级联比较复杂，使用困难，而且可以通过两个一对多级联进行替换，所以MyBatis不支持多对多级联了。

　　为了更好地阐述级联，先给出一个雇员级联模型，如图所示。

分析雇员级联模型：
•该模型是以雇员表为中心的。
•雇员表和工牌表是一对一的级联关系。
•雇员表和员工任务表是一对多的级联关系。
•员工任务表和任务表是一对一的级联关系。
•每个雇员都会有一个体检表，随着雇员表字段性别取值的不同，会有不同的关联表。
据此给出级联模型建表SQL，如代码清单所示。

DROP TABLE
IF EXISTS t_female_health_form;

DROP TABLE
IF EXISTS t_male_health_form;

DROP TABLE
IF EXISTS t_task;

DROP TABLE
IF EXISTS t_work_card;

DROP TABLE
IF EXISTS t_employee_task;

DROP TABLE
IF EXISTS t_employee;

/*==============================================================*/
/* Table: t_employee                                            */
/*==============================================================*/
CREATE TABLE t_employee (
    id INT (12) NOT NULL AUTO_INCREMENT,
    real_name VARCHAR (60) NOT NULL,
    sex INT (2) NOT NULL COMMENT '1 - 男              0 -女',
    birthday DATE NOT NULL,
    mobile VARCHAR (20) NOT NULL,
    email VARCHAR (60) NOT NULL,
    POSITION VARCHAR (20) NOT NULL,
    note VARCHAR (256),
    PRIMARY KEY (id)
);

/*==============================================================*/
/* Table: t_employee_task                                       */
/*==============================================================*/
CREATE TABLE t_employee_task (
    id INT (12) NOT NULL,
    emp_id INT (12) NOT NULL,
    task_id INT (12) NOT NULL,
    task_name VARCHAR (60) NOT NULL,
    note VARCHAR (256),
    PRIMARY KEY (id)
);

/*==============================================================*/
/* Table: t_female_health_form                                  */
/*==============================================================*/
CREATE TABLE t_female_health_form (
    id INT (12) NOT NULL AUTO_INCREMENT,
    emp_id INT (12) NOT NULL,
    heart VARCHAR (64) NOT NULL,
    liver VARCHAR (64) NOT NULL,
    spleen VARCHAR (64) NOT NULL,
    lung VARCHAR (64) NOT NULL,
    kidney VARCHAR (64) NOT NULL,
    uterus VARCHAR (64) NOT NULL,
    note VARCHAR (256),
    PRIMARY KEY (id)
);

/*==============================================================*/
/* Table: t_male_health_form                                    */
/*==============================================================*/
CREATE TABLE t_male_health_form (
    id INT (12) NOT NULL AUTO_INCREMENT,
    emp_id INT (12) NOT NULL,
    heart VARCHAR (64) NOT NULL,
    liver VARCHAR (64) NOT NULL,
    spleen VARCHAR (64) NOT NULL,
    lung VARCHAR (64) NOT NULL,
    kidney VARCHAR (64) NOT NULL,
    prostate VARCHAR (64) NOT NULL,
    note VARCHAR (256),
    PRIMARY KEY (id)
);

/*==============================================================*/
/* Table: t_task                                                */
/*==============================================================*/
CREATE TABLE t_task (
    id INT (12) NOT NULL,
    title VARCHAR (60) NOT NULL,
    context VARCHAR (256) NOT NULL,
    note VARCHAR (256),
    PRIMARY KEY (id)
);

/*==============================================================*/
/* Table: t_work_card                                           */
/*==============================================================*/
CREATE TABLE t_work_card (
    id INT (12) NOT NULL AUTO_INCREMENT,
    emp_id INT (12) NOT NULL,
    real_name VARCHAR (60) NOT NULL,
    department VARCHAR (20) NOT NULL,
    mobile VARCHAR (20) NOT NULL,
    POSITION VARCHAR (30) NOT NULL,
    note VARCHAR (256),
    PRIMARY KEY (id)
);

ALTER TABLE t_employee_task ADD CONSTRAINT FK_Reference_4 FOREIGN KEY (emp_id) REFERENCES t_employee (id)
ON DELETE RESTRICT ON UPDATE RESTRICT;

ALTER TABLE t_employee_task ADD CONSTRAINT FK_Reference_8 FOREIGN KEY (task_id) REFERENCES t_task (id)
ON DELETE RESTRICT ON UPDATE RESTRICT;

ALTER TABLE t_female_health_form ADD CONSTRAINT FK_Reference_5 FOREIGN KEY (emp_id) REFERENCES t_employee (id)
ON DELETE RESTRICT ON UPDATE RESTRICT;

ALTER TABLE t_male_health_form ADD CONSTRAINT FK_Reference_6 FOREIGN KEY (emp_id) REFERENCES t_employee (id)
ON DELETE RESTRICT ON UPDATE RESTRICT;

ALTER TABLE t_work_card ADD CONSTRAINT FK_Reference_7 FOREIGN KEY (emp_id) REFERENCES t_employee (id)
ON DELETE RESTRICT ON UPDATE RESTRICT;

建立POJO

　　根据设计模型建立对应的POJO。首先看体检表，由于男性和女性的体检表有多个字段重复，于是可以先设计一个父类，然后通过继承的方式来完成POJO，体检表设计类图如图所示。

如图所示，其中MaleHealthForm和FemaleHealthForm是HealthForm的子类，由此可得3个关于体检表的POJO，如代码清单所示。

//体检表父类
public abstract class HealthForm {
    private Long id;
    private Long empId;
    private String heart;
    private String liver;
    private String spleen;
    private String lung;
    private String kidney;
    private String note;
}

//女性体检表
public class FemaleHealthForm extends HealthForm {
    private String uterus;
}

//男性体检表
public class MaleHealthForm extends HealthForm {
    private String prostate;
}

接下来设计员工表、工牌表和任务表的POJO，它们是以员工表作为核心的，先完成工牌表和任务表的POJO，如代码清单所示。

//工牌
public class WorkCard {
    private Long id;
    private Long empId;
    private String realName;
    private String department;
    private String mobile;
    private String position;
    private String note;
}

//任务
public class Task {
    private Long id;
    private String title;
    private String context;
    private String note;
}

还剩雇员表和雇员任务表，它们有一定的关联。先从雇员任务表入手，雇员任务表是通过任务编号（task_id）和任务进行一对一关联的，这里只考虑其自身和任务编号的关联，而雇员对它的关联则由雇员去维护，这样就可以得到雇员任务POJO，如代码清单所示。

//雇员任务
public class EmployeeTask {
    private Long id;
    private Long empId;
    private Task task = null;
    private String taskName;
    private String note;
}

属性task是一个Task类对象，由它进行关联任务信息。设置雇员表是关键。雇员根据性别分为男雇员和女雇员，他们会有不同的体检表记录，但是无论男、女都是雇员，所以先建立一个雇员类（Employee）。它有两个子类：男雇员（MaleEmployee）和女雇员（FemaleEmployee）。在MyBatis中，这就是一个鉴别器，通过雇员类的字段性别（sex）来决定使用哪个具体的子类（MaleEmployee或者FemaleEmployee）初始化对象（Employee）。它与工牌表是一对一的关联关系，对于雇员任务表是一对多的关联关系，这样就可以得到3个类，如代码清单所示。

//雇员父类
public class Employee {
    private Long id;
    private String realName;
    private SexEnum sex = null;
    private Date birthday;
    private String mobile;
    private String email;
    private String position;
    private String note;
    //工牌按一对一级联
    private WorkCard workCard;
    // 雇员任务，一对多级联
    private List<EmployeeTask> emplyeeTaskList = null;
}

//男雇员类
public class MaleEmployee extends Employee {
    private MaleHealthForm maleHealthForm = null;
}

//女雇员类
public class FemaleEmployee extends Employee {
    private FemaleHealthForm femaleHealthForm = null;
}

MaleEmployee和FemaleEmployee都继承了Employee类，有着不同的体检表。Employee类是通过employeeTaskList属性和多个雇员任务进行一对多关联的，而工牌表是通过WorkCard进行一对一关联的，这样就完成了所有POJO的设计。

配置映射文件

<?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.xc.mapper.employee.TaskMapper">

    <select id="getTask" parameterType="long" resultType="com.xc.pojo.employee.Task">
        select id, title, context, note
        from t_task
        where id = #{id}
    </select>

</mapper>

<?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.xc.mapper.employee.WorkCardMapper">

    <select id="getWorkCardByEmpId" parameterType="long" resultType="com.xc.pojo.employee.WorkCard">
        SELECT id, emp_id as empId, real_name as realName, department, mobile, position, note
        FROM t_work_card
        where emp_id = #{empId}
    </select>

</mapper>

<?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.xc.mapper.employee.EmployeeTaskMapper">

    <resultMap type="com.xc.pojo.employee.EmployeeTask" id="EmployeeTaskMap">
        <id column="id" property="id"/>
        <result column="emp_id" property="empId"/>
        <result column="task_name" property="taskName"/>
        <result column="note" property="note"/>
        <association property="task" column="task_id" select="com.xc.mapper.employee.TaskMapper.getTask"/>
    </resultMap>

    <select id="getEmployeeTaskByEmpId" parameterType="long" resultMap="EmployeeTaskMap">
        select id, emp_id, task_name, task_id, note
        from t_employee_task
        where emp_id = #{empId}
    </select>

</mapper>

<?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.xc.mapper.employee.MaleHealthFormMapper">

    <select id="getMaleHealthForm" parameterType="long" resultType="com.xc.pojo.employee.MaleHealthForm">
        select id,
               heart,
               liver,
               spleen,
               lung,
               kidney,
               prostate,
               note
        from t_male_health_form
        where emp_id = #{id}
    </select>

</mapper>

<?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.xc.mapper.employee.FemaleHealthFormMapper">

    <select id="getFemaleHealthForm" parameterType="long" resultType="com.xc.pojo.employee.FemaleHealthForm">
        select id,
               heart,
               liver,
               spleen,
               lung,
               kidney,
               uterus,
               note
        from t_female_health_form
        where emp_id = #{id}
    </select>

</mapper>

<?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.xc.mapper.employee.EmployeeMapper">

    <resultMap type="com.xc.pojo.employee.Employee" id="employee">
        <id column="id" property="id"/>
        <result column="real_name" property="realName"/>
        <result column="sex" property="sex" typeHandler="com.xc.util.typehandler.SexEnumTypeHandler"/>
        <result column="birthday" property="birthday"/>
        <result column="mobile" property="mobile"/>
        <result column="email" property="email"/>
        <result column="position" property="position"/>
        <result column="note" property="note"/>
        <!--association元素，对工牌进行一对一级联-->
        <association property="workCard" column="id" select="com.xc.mapper.employee.WorkCardMapper.getWorkCardByEmpId"/>
        <!--collection元素，一对多级联-->
        <collection property="employeeTaskList" column="id"
        select="com.xc.mapper.employee.EmployeeTaskMapper.getEmployeeTaskByEmpId"/>
        <!--discriminator元素，鉴别器-->
        <discriminator javaType="long" column="sex">
            <case value="1" resultMap="maleHealthFormMapper"/>
            <case value="2" resultMap="femaleHealthFormMapper"/>
        </discriminator>
    </resultMap>

    <resultMap type="com.xc.pojo.employee.FemaleEmployee" id="femaleHealthFormMapper" extends="employee">
        <association property="femaleHealthForm" column="id"
        select="com.xc.mapper.employee.FemaleHealthFormMapper.getFemaleHealthForm"/>
    </resultMap>

    <resultMap type="com.xc.pojo.employee.MaleEmployee" id="maleHealthFormMapper" extends="employee">
        <association property="maleHealthForm" column="id" select="com.xc.mapper.employee.MaleHealthFormMapper.getMaleHealthForm"/>
    </resultMap>

    <select id="getEmployee" parameterType="long" resultMap="employee">
        select id,
               real_name as realName,
               sex,
               birthday,
               mobile,
               email,
               position,
               note
        from t_employee
        where id = #{id}
    </select>

</mapper>

另一种级联

MyBatis还提供了另一种级联方式，它是基于SQL表连接的基础上，进行再次设计的

   <!--MyBatis还提供了另一种级联方式，它是基于SQL表连接的基础上，进行再次设计的-->
    <resultMap id="employee2" type="com.xc.pojo.employee.Employee">
        <id column="id" property="id"/>
        <result column="real_name" property="realName"/>
        <result column="sex" property="sex" typeHandler="com.xc.util.typehandler.SexEnumTypeHandler"/>
        <result column="birthday" property="birthday"/>
        <result column="mobile" property="mobile"/>
        <result column="email" property="email"/>
        <result column="position" property="position"/>
        <association property="workCard" javaType="com.xc.pojo.employee.WorkCard" column="id">
            <id column="wc_id" property="id"/>
            <result column="id" property="empId"/>
            <result column="wc_real_name" property="realName"/>
            <result column="wc_department" property="department"/>
            <result column="wc_mobile" property="mobile"/>
            <result column="wc_position" property="position"/>
            <result column="wc_note" property="note"/>
        </association>
        <collection property="employeeTaskList" ofType="com.xc.pojo.employee.EmployeeTask" column="id">
            <id column="et_id" property="id"/>
            <result column="id" property="empId"/>
            <result column="task_name" property="taskName"/>
            <result column="note" property="note"/>
            <association property="task" javaType="com.xc.pojo.employee.Task" column="et_task_id">
                <id column="t_id" property="id"/>
                <result column="t_title" property="title"/>
                <result column="t_context" property="context"/>
                <result column="t_note" property="note"/>
            </association>
        </collection>
        <discriminator javaType="int" column="sex">
            <case value="1" resultMap="maleHealthFormMapper2"/>
            <case value="2" resultMap="femaleHealthFormMapper2"/>
        </discriminator>
    </resultMap>
    <resultMap type="com.xc.pojo.employee.MaleEmployee" id="maleHealthFormMapper2" extends="employee2">
        <association property="maleHealthForm" column="id" javaType="com.xc.pojo.employee.MaleHealthForm">
            <id column="h_id" property="id"/>
            <result column="h_heart" property="heart"/>
            <result column="h_liver" property="liver"/>
            <result column="h_spleen" property="spleen"/>
            <result column="h_lung" property="lung"/>
            <result column="h_kidney" property="kidney"/>
            <result column="h_prostate" property="prostate"/>
            <result column="h_note" property="note"/>
        </association>
    </resultMap>
    <resultMap type="com.xc.pojo.employee.FemaleEmployee" id="femaleHealthFormMapper2" extends="employee">
        <association property="femaleHealthForm" column="id" javaType="com.xc.pojo.employee.FemaleHealthForm">
            <id column="h_id" property="id"/>
            <result column="h_heart" property="heart"/>
            <result column="h_liver" property="liver"/>
            <result column="h_spleen" property="spleen"/>
            <result column="h_lung" property="lung"/>
            <result column="h_kidney" property="kidney"/>
            <result column="h_uterus" property="uterus"/>
            <result column="h_note" property="note"/>
        </association>
    </resultMap>

    <select id="getEmployee2" parameterType="long" resultMap="employee2">
        select emp.id,
               emp.real_name,
               emp.sex,
               emp.birthday,
               emp.mobile,
               emp.email,
               emp.position,
               emp.note,
               et.id                                   as et_id,
               et.task_id                              as et_task_id,
               et.task_name                            as et_task_name,
               et.note                                 as et_note,
               if(emp.sex = 1, mhf.id, fhf.id)         as h_id,
               if(emp.sex = 1, mhf.heart, fhf.heart)   as h_heart,
               if(emp.sex = 1, mhf.liver, fhf.liver)   as h_liver,
               if(emp.sex = 1, mhf.spleen, fhf.spleen) as h_spleen,
               if(emp.sex = 1, mhf.lung, fhf.lung)     as h_lung,
               if(emp.sex = 1, mhf.kidney, fhf.kidney) as h_kidney,
               if(emp.sex = 1, mhf.note, fhf.note)     as h_note,
               mhf.prostate                            as h_prostate,
               fhf.uterus                              as h_uterus,
               wc.id                                      wc_id,
               wc.real_name                               wc_real_name,
               wc.department                              wc_department,
               wc.mobile                                  wc_mobile,
               wc.position                                wc_position,
               wc.note                                 as wc_note,
               t.id                                    as t_id,
               t.title                                 as t_title,
               t.context                               as t_context,
               t.note                                  as t_note
        from t_employee emp
                 left join t_employee_task et on emp.id = et.emp_id
                 left join t_female_health_form fhf on emp.id = fhf.emp_id
                 left join t_male_health_form mhf on emp.id = mhf.emp_id
                 left join t_work_card wc on emp.id = wc.emp_id
                 left join t_task t on et.task_id = t.id
        where emp.id = #{id}
    </select>

也会引发其他问题：首先，SQL会比较复杂；其次，所需要的配置比之前复杂得多；再次，一次性将所有的数据取出会造成内存的浪费。这样的复杂SQL，同时也会给日后的维护工作带来一定的困难，所以使用这样的级联，一般用于那些比较简单且关联不多的场景下。