一、延遲加載

1、在mybatis.xml配置文件中，開啟延遲加載

  <settings>
    <!--開啟延遲加載-->
    <setting name="lazyLoadingEnabled" value="true"></setting>
    <setting name="aggressiveLazyLoading" value="false"></setting>
    <!--延遲加載觸發(fā)方法，equals、hashCode、toString都會觸發(fā)加載-->
    <setting name="lazyLoadTriggerMethods" value="hashCode"></setting>
    <!--數據庫下劃線（_）命名轉駝峰命名-->
    <setting name="mapUnderscoreToCamelCase" value="true"/>
  </settings>

2、配置mapper文件

1、一對一

* 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>　　　　　　　　　 <!--關閉延遲加載會做兩次查詢-->
        <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>

* 測試

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

2、一對多

* 一方

      <resultMap type="Grade" id="gradeStudents">
        <id column="id" property="id"></id>
        <result column="name" property="name"></result>　　　　　　　　　 <!--關閉延遲加載會做兩次查詢-->
        <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>

* 測試

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

二、緩存

1、一級緩存

1、概念

一級緩存是SqlSession范圍的緩存，當調用SqlSession的修改，添加，刪除，commit()，close()等方法時，就會清空一級緩存。

2、測試

// 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("杜蘭特");
student.setAge(28);
student.setSex(1);
smapper.insertStudent(student);
Student student2 = smapper.selectStudentGradeById(1);
System.out.println(student1 == student2); // false

2、二級緩存

1、開啟二級緩存

1、對象需要實現Serializable接口

2、在mybatis.xml配置文件中，開啟二級緩存

<settings>
<!--開啟二級緩存-->
<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、測試

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();
// 只查詢了一次數據庫。二級緩存存儲的是數據，并不是對象
System.out.println(student1 == student2); // false

以上就是本文的全部內容，希望對大家的學習有所幫助，也希望大家多多支持腳本之家。

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