MyBatis进阶

一.映射Mapper

• 接口方法映射到对应的SQL
• Mapper.xml的命名空间名称就是Maper接口的全限定名
• Mapper接口上也可以写SQL(不建议这么做)

1.employeeMapper.xml

<mapper namespace="cn.wing._02_mapper.mapper.EmployeeMapper">
    
    <insert id="save" parameterType="employee">
      ...
    insert>

    <select id="findAll" resultType="employee">
      ...
    select>
mapper>

2.EmployeeMapper

package cn.wing._02_mapper.mapper;
public interface EmployeeMapper {

    void save(Employee employee);
//    @Select("select * from employee")
    List<Employee> findAll();
}

3.调用 Mapper的方法

SqlSession session = MyBatisUtil.openSession();
EmployeeMapper mapper = session.getMapper(EmployeeMapper.class);

mapper.findAll().forEach(e -> System.out.println(e));

二.高级查询

• 准备一个Query对象(封装所有条件)
• 模糊查询 concat("%",#{name},"%")
• 遇到特殊符号 1.转义 < 2.CDATA段
• 使用where标签(第一个and变成where)
• if中有多个条件使用 and/or 进行关联
• 如果出现相就的代码,可以单独抽取sql标签,引用include即可

<select id="findByQuery" parameterType="employeeQuery" resultType="employee">
        select * from employee <include refid="whereSql" />
select>


<sql id="whereSql">
    <where>
        <if test="name!=null and name!=''">
            and name like concat("%",#{name},"%")
        if>
        <if test="minAge!=null">
            and age >= #{minAge}
        if>
        <if test="maxAge!=null">
            
        if>
    where>
sql>

三 批量删除,添加,动态修改

3.1 批量删除

• 首先知道sql delete from 表名 where id in (?,?,…)
  collection="":代表你要循环的是什么? array/list
  如果传过来的是数据,写array(集合就写list)
  item:循环的每一个数据
  open:拼接字符串以什么开始
  close:拼接字符串以什么结尾
  separator:拼接的时候每个值使用,隔开
  index:遍历的下标

3.1.1 传数组的方法

 <delete id="batchDelete" parameterType="long[]">
        delete from employee where id in
        <foreach collection="array" item="v" open="(" close=")" separator=",">
            #{v}
        foreach>
delete>

3.1.2 传集合的方式

 <delete id="batchDelete" parameterType="list">
        delete from employee where id in
        <foreach collection="list" item="v" open="(" close=")" separator=",">
            #{v}
        foreach>
    delete>

3.1 批量添加

首先知道sql `insert into 表名 (p1,p2,…) values (#{p1},#{p2}),(#{p1},#{p2}),…

<insert id="batchSave" parameterType="list">
    insert into employee (name,age,sex) values
    <foreach collection="list" item="emp" separator=",">
        (#{emp.name},#{emp.age},#{emp.sex})
    foreach>
insert>

3.2 动态修改

咱们修改一个对象,咱们只能修改有数据的部分

<update id="update" parameterType="cn.wing._02_mapper.domain.Employee">
    UPDATE employee
    <set>
        <if test="name!=null and name!=''">
            name = #{name},
        if>
        <if test="age!=null">
            age=#{age},
        if>
    set>
    WHERE id=#{id}
update>

四 多对一

• 准备(两张表employee,department,两个domain,两个mapper.xml)
• 查询(嵌套结果[一条sql],嵌套查询[n+1条sql])

4.1 准备domain

• Employee

public class Employee {

    private Long id;
    private String name;
    private Integer age;
    private Boolean sex;

    private Department dept;
    //... getter,setter与toString
}

• Department

public class Department {
    private Long id;
    private String name;
    //... getter,setter与toString
}

4.2 嵌套结果

• 查询的sql要关连多张表(一定要取别名,不然有些名称会产生冲突)
• 当我们使用了association 后默认的映射失败,需要自己手动完成映射

<select id="findAll" resultMap="employeeMap">
    select e.id eid,e.name ename,e.age,e.sex,d.id did,d.name dname
      from employee e join department d on d.id = e.dept_id
select>

<resultMap id="employeeMap" type="employee">
    
    <id property="id" column="eid" />
    <result property="name" column="ename" />
    <result property="sex" column="sex" />
    <result property="age" column="age" />
    
    <association property="dept" javaType="department">
        <id property="id" column="did" />
        <result property="name" column="dname" />
    association>
resultMap>

##4.2 嵌套查询

• 会产生n+1条sql
• 需要去找到对应的那条sql并且执行
• 保证MyBatis能找到这两个xml

employeeMapper.xml

<select id="findAll" resultMap="employeeMap">
    SELECT * FROM employee
select>

<resultMap id="employeeMap" type="employee">
    <id property="id" column="id" />
    <result property="name" column="name" />
    <result property="sex" column="sex" />
    <result property="age" column="age" />

    <association property="dept" javaType="department"
             column="dept_id" select="cn.wing._04_many2one.DepartmentMapper.findById" />
resultMap>

departmentMapper.xml

<mapper namespace="cn.wing._04_many2one.DepartmentMapper">
    <select id="findById" parameterType="long" resultType="department">
        select * from department where id = #{id}
    select>
mapper>

五 一对多

5.1 准备domain

• Employee

public class Employee {

    private Long id;
    private String name;
    private Integer age;
    private Boolean sex;
     //... getter,setter与toString
}

• Department

public class Department {
    private Long id;
    private String name;

    private List<Employee> employees = new ArrayList<>();

六.缓存

• 自带一级级联
• 二级缓存需要加上标签
• 二级缓存的对象必需是序列化对象 … implements Serializable