MyBatis的三层级联和二层缓存

       我们这里说的三层级联和二级缓存其实也是MyBatis映射器的知识点，只是因为比较难理解，所以单独拿出来讲解，下面是三层级联的内容；

       我们知道，在数据库中包含着一对一，一对多，多对多的关系，例如，我们希望在查出学生表的时候，能够连着查出学生证表的信息，这就是一对一的关系查询，我们又希望，能够在查出学生表的时候，能够连着查出你每一个学生的多门课程成绩，而课程成绩又与课程相关，所以呢，我们先用学生表与课程表实现一对多的关系，然后，再利用课程表与课程成绩实现一对一的查询，这样的话，你就实现了一对多的关系查询了。最后，我们说了，学生表中有男女性别的区别，那么，如果我要根据性别来分别查询男女的生理特征的话，那么这就是多对多的关系了。下面我们先来讲讲一对一；对了，这三种级联分别对应了三种元素，分别是association,collection,discriminator；

        一对一的关系：association；

        首先，我们按上面的学生表来做例子，Student代表学生，StudentSelfcard代表学生证；思路就是我们先做一个查询学生证的resultMap，然后我们再做一个查询学生表的resultMap，然后我们把查询学生证的resultMap注册到查询学生表的resultMap中去；

<?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.learn.chapter4.mapper.StudentSelfcardMapper">
    <resultMap type = "com.learn.chapter4.po.StudentSelfcardBean">
    <id property = "id" column = "id"/>
    <id property = "studentId" column = "student_id"/>
    <id property = "native_" column = "native"/>
    <id property = "issueDate" column = "issue_date"/>
    <id property = "endDate" column = "end_date"/>
    <id property = "note" column = "note"/>
</resultMap>
<select id = "findStudentSelfcardByStudentId" parameterType = "int" resultMap = "studentSelfcardMap">
    select id,student_id,native,issue_date,end_date,note from t_student_selfcard where student_id = #{studentId}
</select>
</mapper>

　　这个呢，就是我们的学生证查询resultMap定义了，接下来是我们的学生表查询；

<?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.learn.chapter4.mapper.StudentMapper">
    <resultMap id = "studentMap" type = "com.learn.chpater4.po.StudentBean">
        <id property = "id" column = "id"/>
        <result property = "cnname" column = "cnname"/>
        <result property = "sex" column = "sex" jdbcType = "INTEGER" javaType = "com.learn.chapter4.enums.SexEnum"
        typeHandler = "com.learn.chapter4.typehandler.SexTypeHandler"/>
        <result property = "note" column = "note"/>
        <association property = "studentSelfcard" column = "id" select = "com.learn.chapter4.mapper.StudentSelfcardMapper.findStudentSelfcardByStudentId"/>
        </resultMap>
        <select id = "getStudent" parameterType = "int" resultMap = "studentMap">
            select id, cnname, sex, note from t_student where id = #{id}
        </select>
    </mapper>

　　最后，做一下测试就可以了；

SqlSession sqlSession = null;
try{
    sqlSession = SqlSessionFactoryUtil.openSqlSession();
    StduentMapper stuMapper = sqlSession.getMapper(StudentMapper.class);
    StudentBean stu = stuMapper.getStudent(1);
}finally{
    if(sqlSession != null){
        sqlSession.close();
    }
}

接下来是collection一对多级联；

这个时候我们就要先建立一个LectureBean 的POJO来记录课程，而学生课程表则建立一个StudentLectureBean来记录成绩，里面有一个类型为LectureBean属性的lecture，用来记录学生成绩；毕竟学生课程表里面要有多个课程，所以我们建立了一个类型为lectureBean属性的lecture；

下面是LectureBean和StudentLectureBean设计；

public class LectureBean{
    private Integer id;
    private String lectureName;
    private String note;
    ...setter and getter...
}
public class StudentLectureBean{
    private int id;
    private Integer studentId;
    private LectureBean lecture;
    private BigDecimal grade;
    private String nte;
}

　　接下来我们做一对多的级联；

<?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.learn.chpater4.mapper.StudentMapper">
    <resultMap id = "studentMap" type = "com.learn.chpater4.po.StudentBean">
        <id property = "id" column = "id"/>
        <result property = "cnname" column = "cnname"/>
        <result property = "sex" column = "sex" jdbcType = "INTEGER"
            javaType = "com.learn.chapter4.enums.SexEnum"
            jdbcType = "com.learn.chapter4.typehandler.SexTypeHandler"/>
        <result property = "note" column = "note"/>
        <association property = "studentSelfcard" column = "id" select = "com.learn.chapter4.mapper.StudentSelfcardMapper.findStudentSelfcardByStudentId"/>
        <collection property = "studentLectureList" column = "id" select = "com.learn.chapter4.mapper.StudentLectureMapper.findStudentLectureByStudentId"/>
    </resultMap>
    <select id = "getStudent" parameterType = "int" resultMap = "studentMap">
        select id, cnname, sex, note from t_student where id = #{id}
    </select>
</mapper>

　　   StudentLectureMapper.xml

<?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.learn.chapter4.mapper.StudentLectureMapper">
    <resultMap id = "studentLectureMap" type = "com.learn.chpater4.po.StudentLectureBena">
        <id property = "id" column = "id"/>
        <id property = "studentId" column = "student_id"/>
        <id property = "grade" column = "grade"/>
        <id property = "note" column = "note"/>
        <association property = "lecture" column = "lecture_id" select = "com.learn.chpater4.mapper.LectureMapper.getLecture"/>
    </resultMap>
    <select id = "findStudentLectureByStuId" parameterType = "int" resultMap = "studetntLectureMap">
        select id, student_id,lecture_id,grade,note from t_student_lecture where student_id = #{id}
    </select>
</mapper>

　　       LectureMapper.xml

<?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.learn.chapter4.mapper.LectureMapper">
    <select id = "getLecture" parameterType = "int" resultTYpe = "com.learn.chapter4.po.LectureBean">
        select id,lecture_name as lectureName,note from t_lecture where id = #{id}
    </select>
</mapper>

　　下面是一对多的测试类；

Logger logger = Logger.getLogger(Chpater4Main.class);
SqlSession sqlSession = null;
try{
    sqlSession = SqlSessionFactoryUtil.openSqlSession();
    StudentMapper studentMapper = sqlSession.getMapper(StudentMapper.class);
    StudentBean student = studentMapper/getStudent(1);
    logger.info(student.getStudentSelfcard().getNative_());
    StudentLectureBean studentLecture = student.getStudentLectureList().get(0);
    LectureBean lecture = studentLecture.getLecture();
    logger.info(student.getCnname() + "	" + lecture.getLectureName())
}finally{
    if(sqlSession != null){
        sqlSession.close();
    }
}

discriminator鉴别器级联；

先来看个设计；

public class MaleStudentBean extends StudentBean{
    privaate List<StudentHealthMaleBean> studentHealthMaleList = null;
}
public class FemaleStudentBean extends StudentBean{
    private List<StudentHealthFemaleBean> studentHealthFemaleList = null;
}

　　下面是鉴别器的内容；

<?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.learn.chpater4.mapper.StudentMapper">
    <resultMap id = "studentMap" type = "com.learn.chapter4.po.StudentBean">
        <id property = "id" column = "id"/>
        <result property = "cnname" column = "cnname"/>
        <result property = "sex" column = "sex" jdbcType = "INTEGER" typeHandler = "com.learn.chpater4.typeHandler.SexTypeHandler"/>
        <result property = "note" column = "note"/>
        <association property = "studentSelfcard" column = "id" select = "com.learn.chapter4.mapper.StudentSelfcardMapper.findStudentSelfcardByStudentId"/>
        <collection property = "studentLectureList" column = "id" select = "com.learn.chpater4.mapper.StudentLectureMapper.findStudentLectureByStuId"/>
        <discriminator javaType = "int" column = "sex">
            <case value = "1" resultMap = "maleStudentMap"/>
            <case value = "2" resultMap = "femaleStudentMap"/>
        </discriminator>
    </resultMap>
    <select id = "getStudent" parameterType = "int" resultMap = "studentMap">
        select id,cnname,sex,note form t_student where id = #{id}
    </select>
    <resultMap id = "maleStudentMap" type = "com.learn.chpater4.po.MaleStudentBean" extends = "studentMap">
        <collection property = "studentHealthMaleList" select = "com.learn.chapter4.mapper.StudentHealthMaleMapper.findStudentHealthMaleByStuId" column = "id"/>
    </resultMap>
    <resultMap id = "femaleStudentMap" type = "com.learn.chapter4.po.FemaleStudentBean" extends = "studentMap">
        <collection property = "studentHealthFemaleList" select = "com.learn.chapter4.mapper.StudentHealthFemaleMapper.findStudentHealthFemaleByStuId" column = "id"/>
    </resultMap>
</mapper>

　　

一个是鉴别器的关键代码；

        <discriminator javaType = "int" column = "sex">

接下来是继承类

    extends = "studentMap"

    接下来是关于延迟加载的问题，我们既然，学了关于级联的相关操作就知道，我们如果只是想要查找某一个对象的话，如果是通过级联，我们可能会不经意地把相关的数据也给查了出来，这样就会造成性能的问题，所以我们提出来两个概念，延迟加载LazyLoadingEnabled和完整加载aggressiveLazyLoading；其中，如果你将延迟加载设置为true，即表示可以按需加载，不然的话就是即时加载，而完整加载的意思是你是按照层次来加载呢？还是一次性全部加载，如果说你想找的课程成绩，而查找课程成绩，你就会相关地查找到学生证信息，但这不是我们想要的，所以，我们这个时候就要按照层次加载了，我们要将aggressiveLazyLoading设置为false,即关闭掉完整加载；

    上面的这两个加载都是通过mybatisXML文件来设置加载的；

<settings>
    <setting naem = "lazyLoadingEnabled" value = "true"/>
</settings>

　　这是开启延迟加载的；

<setting name = "aggressiveLazyLoading" value = "false"/>

这个是关闭掉完整加载，实现层次加载；

但是现在又有一个问题出现了，我们在xml文件中设置的相当于全局变量，而我们有时候有些文件需要实现延迟和层次，而有些又不用，这个时候，我们可以使用局部变量设置的方式来实现；

代码如下；

<association property = "studentSelfcard" column = "id" fetchType = "lazy"
    select = "com.learn.chapter4.mapper.StudentSelfcardMapper.findStudentSelfcardByStudentId"/>
    <collection property = "studentLectureList" column = "id" fetchType = "eager" 
    select = "com.learn.chapter4.mapper.StudentLectureMapper.findStudentLectureByStuId"/>

这个时候，你把aggressiveLazyLoading设置为false,这样的话，完整加载关闭，层次加载实现，而设置为lazy的为延迟层次加载，eager的为即时完整加载了；

    讲完了三层吉级联，我们来讲讲二层缓存；概念较为简单，所谓一级缓存是指建立在SqlSession的层面上的，如果在你的SqlSession还没有commit提交之前的话，用SqlSession所创建出来的对象可以进行多次查询，而二级缓存则是建立在SqlSessionFactory的层面上，能够实现SqlSession的对象共享；

而默认二级缓存是关闭的，需要手动开启；

开启的方式也很简单，只需要在映射XML文件配置就可以了；

<cache/>