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Mybatis如何实现延迟加载及缓存

运维开发网 https://www.qedev.com 2020-12-25 17:54 出处:网络 作者: 一路繁花似锦绣前程
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-->

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

以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持我们。

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