Mybatis如何实现延迟加载及缓存

一、延迟加载

1、在mybatis.xml配置文件中，开启延迟加载

  <settings>
    <!--开启延迟加载-->
    <setting name="lazyLoadingEnabled" value="true"></setting>
    <setting name="aggressiveLazyLoading" value="false"></setting>
    <!--延迟加载触发方法，equals、hashCode、toString都会触发加载-->
    <setting name="lazyLoadTriggerMethods" value="hashCode"></setting>
    <!--数据库下划线（_）命名转驼峰命名-->
    <setting name="mapUnderscoreToCamelCase" value="true"/>
  </settings>

2、配置mapper文件

1、一对一

* 一方

      <resultMap id="studentGradeById" type="Student">
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>
        <result column="age" property="age"></result>
        <result column="sex" property="sex"></result>　　　　　　　　　 <!--关闭延迟加载会做两次查询-->
        <association column="grade_id" property="grade" javaType="Grade"
               select="com.wuxi.daos.GradeMapper.selectById"></association>
      </resultMap>
      <select id="selectStudentGradeById" resultMap="studentGradeById">
        select * from student where id = #{id}
      </select>

* 另一方

      <select id="selectById" resultType="Grade">
        select * from grade where id = #{id}
      </select>

* 测试

Student student = smapper.selectStudentGradeById(4);
System.out.println(student);
// student.hashCode();
System.out.println(student.getGrade());

2、一对多

* 一方

      <resultMap type="Grade" id="gradeStudents">
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>　　　　　　　　　 <!--关闭延迟加载会做两次查询-->
        <collection property="students" ofType="Student" column="id"
              select="com.wuxi.daos.StudentMapper.selectStudentsByGrade"></collection>
      </resultMap>
      <select id="selectById" resultMap="gradeStudents">
        select * from grade where id = #{id}
      </select>

* 多方

      <select id="selectStudentsByGrade" resultType="Student">
        select * from student where grade_id=#{grade_id}
      </select>

* 测试

Grade grade = gmapper.selectById(1);
System.out.println(grade);
// student.hashCode();
System.out.println(grade.getStudents());

二、缓存

1、一级缓存

1、概念

一级缓存是SqlSession范围的缓存，当调用SqlSession的修改，添加，删除，commit()，close()等方法时，就会清空一级缓存。

2、测试

// Student student1 = smapper.selectStudentGradeById(1);
// Student student2 = smapper.selectStudentGradeById(1);
// System.out.println(student1 == student2); // true
// ********************************
Student student1 = smapper.selectStudentGradeById(1);
Student student = new Student();
student.setName("杜兰特");
student.setAge(28);
student.setSex(1);
smapper.insertStudent(student);
Student student2 = smapper.selectStudentGradeById(1);
System.out.println(student1 == student2); // false

2、二级缓存

1、开启二级缓存

1、对象需要实现Serializable接口

2、在mybatis.xml配置文件中，开启二级缓存

<settings>
<!--开启二级缓存-->
<setting name="cacheEnabled" value="true"/>
</settings>

3、配置mapper文件

<cache/>
<select id="selectStudentGradeById" resultMap="studentGradeById" useCache="true">
select * from student where id = #{id}
</select>

2、测试

SqlSession sqlSession1 = sqlSessionFactory.openSession();
StudentMapper mapper1 = sqlSession1.getMapper(StudentMapper.class);
Student student1 = mapper1.selectStudentGradeById(1);
sqlSession1.close();
SqlSession sqlSession2 = sqlSessionFactory.openSession();
StudentMapper mapper2 = sqlSession2.getMapper(StudentMapper.class);
Student student2 = mapper2.selectStudentGradeById(1);
sqlSession2.close();
// 只查询了一次数据库。二级缓存存储的是数据，并不是对象
System.out.println(student1 == student2); // false

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