mybatis多对一一对多的关联及拼接操作以及缓存处理
1、多对一：简单的说，就是将查询出来的多个结果分别赋予另一个结果的三个属性
多对一关联：
①创建对象，添加属性（这里不再展示config.xml文件连接数据库，这是每一个项目必备的）
@Data
public class Emp {private Integer empNo;private String ename;private String job;private Integer mgr;private String hireDate1;private Double sal;private Double comm;private Integer deptNo;public Integer state;//描述多对一的关系private Dept dept;
}
@Data
public class Dept {private Integer deptNo;private String dname;private String loc;
}
②建立接口，创建方法
public interface EmpMapper {//描述多对一的关联Emp findById1(int id);
}③xml文件书写SQL查询语句
<!--多对一的关联操作--><resultMap id="map2" type="com.jiazhong.mybatis.m2.bean.Emp" autoMapping="true"><id property="empNo" column="empno"/><result property="hireDate1" column="hiredate"/><!--描述多对一简单的说：就是将查询出来的deptno,dname,loc分别赋予dept的三个属性--><association property="dept" javaType="com.jiazhong.mybatis.m2.bean.Dept" autoMapping="true"><id property="deptNo" column="deptno"/><result property="dname" column="dname"/><result property="loc" column="loc"/></association></resultMap><select id="findById1" resultMap="map2">select * from emp e left join dept d on e.deptno=d.deptno where state=1 and empno=#{id}</select>④ 实现多对一关联
private static void b1(){InputStream inputStream = Main.class.getClassLoader().getResourceAsStream("config.xml");SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);SqlSession sqlSession = sqlSessionFactory.openSession();EmpMapper empMapper = sqlSession.getMapper(EmpMapper.class);Emp emp = empMapper.findById1(7980);System.out.println(emp);sqlSession.close();}多对一拼接：
①建立接口，创建方法
public interface EmpMapper {//描述多对一的拼接操作Emp findById2(int id);
}
public interface DeptMapper {//多对一的，查询出来的部门信息Dept findById(int id);
}②xml文件书写SQL查询语句
<select id="findById" resultType="com.jiazhong.mybatis.m2.bean.Dept">select * from dept where DEPTNO = #{id}</select>③实现多对一拼接
private static void b2(){InputStream inputStream = Main.class.getClassLoader().getResourceAsStream("config.xml");SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);SqlSession sqlSession = sqlSessionFactory.openSession();EmpMapper empMapper = sqlSession.getMapper(EmpMapper.class);//只能获取到员工信息Emp emp = empMapper.findById2(7980);//只能查询到部门信息DeptMapper deptMapper = sqlSession.getMapper(DeptMapper.class);Dept dept = deptMapper.findById(emp.getDeptNo());//组装emp.setDept(dept);System.out.println(emp);sqlSession.close();}2、一对多：相当于一个部门拥有多个员工
一对多关联：
①创建对象，添加属性
@Data
public class Dept {private Integer deptNo;private String dname;private String loc;//描述一对多关系private Set<Emp> emp;
}
@Data
public class Emp {private Integer empNo;private String ename;private String job;private Integer mgr;private String hireDate1;private Double sal;private Double comm;private Integer deptNo;public Integer state;
}②建立接口，创建方法
public interface DeptMapper {//一对多的关联写法Dept findById1(int id);
}③xml文件书写SQL查询语句
<!--一对多的关联写法--><resultMap id="map3" type="com.jiazhong.mybatis.m2.bean.Dept" autoMapping="true"><id property="deptNo" column="deptno"/><!--描述这个部门的员工--><collection property="emp" ofType="com.jiazhong.mybatis.m2.bean.Emp" autoMapping="true"/></resultMap><select id="findById1" resultMap="map3">select * from dept d left join emp e on d.deptno = e.deptno where d.deptno = #{id}</select>④ 实现一对多关联
private static void c1(){InputStream inputStream = Main.class.getClassLoader().getResourceAsStream("config.xml");SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);SqlSession sqlSession = sqlSessionFactory.openSession();DeptMapper deptMapper = sqlSession.getMapper(DeptMapper.class);Dept dept = deptMapper.findById1(10);System.out.println(dept);sqlSession.close();}一对多拼接：
①建立接口，创建方法
public interface DeptMapper {//一对多的拼接写法Dept findById2(int id);
}
public interface EmpMapper {//描述一对多的拼接操作Set<Emp> findByDeptNo(int id);
}
②xml文件书写SQL查询语句
<select id="findById2" resultType="com.jiazhong.mybatis.m2.bean.Dept">select * from dept where deptno = #{id}</select>
<select id="findByDeptNo" resultType="com.jiazhong.mybatis.m2.bean.Emp">select * from emp where state=1 and empno=#{id}</select>③实现一对多拼接
private static void c2(){InputStream inputStream = Main.class.getClassLoader().getResourceAsStream("config.xml");SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);SqlSession sqlSession = sqlSessionFactory.openSession();//查询部门DeptMapper deptMapper = sqlSession.getMapper(DeptMapper.class);Dept dept = deptMapper.findById2(10);//查询员工信息EmpMapper empMapper = sqlSession.getMapper(EmpMapper.class);Set<Emp> emps = empMapper.findByDeptNo(10);//组装dept.setEmp(emps);System.out.println(dept);sqlSession.close();}3、一级缓存只在一个会话间起作用，二级缓存可以解决一级缓存中不是一个会话、手动调用了清空缓存的方法、当执行更新操作（insert,update和delete）导致缓存失效等问题
一级缓存：
①创建对象，添加属性
@Data
public class Emp{private Integer empNo;private String ename;private String job;private Integer mgr;private String hireDate1;private Double sal;private Double comm;private Integer deptNo;public Integer state;
}②建立接口，创建方法
public interface EmpMapper {Emp findById(int id);
}③xml文件书写SQL查询语句
<select id="findById" resultType="com.jiazhong.mybatis.m3.bean.Emp">select * from emp where empno = #{id}</select>④实现一级缓存处理
private static void a(){InputStream inputStream = Main.class.getClassLoader().getResourceAsStream("config.xml");SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);SqlSession sqlSession = sqlSessionFactory.openSession();EmpMapper empMapper = sqlSession.getMapper(EmpMapper.class);//查询7908的员工Emp emp1 = empMapper.findById(7908);System.out.println(emp1);//又查询了7908的员工，这次不需要连接数据库了 使用了缓存（一级缓存）Emp emp2 = empMapper.findById(7908);System.out.println(emp2);sqlSession.close();}二级缓存：
①在config.xml文件中开启二级缓存 ------>cacheEnabled
<!--① 开启二级缓存--><setting name="cacheEnabled" value="true"/>②让实体类序列化 ------>implements Serializable
@Data
public class Emp implements Serializable {private Integer empNo;private String ename;private String job;private Integer mgr;private String hireDate1;private Double sal;private Double comm;private Integer deptNo;public Integer state;
}③在书写SQL语句的xml文件中使当前mapper参与缓存
<!--③ 当前mapper参与缓存--><cache/>二级缓存效果范围更大