Mybatis如何实现延迟加载及缓存

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

2、配置mapper文件

1、一对一

* 一方

select="com.wuxi.daos.GradeMapper.selectById">

select="com.wuxi.daos.GradeMapper.selectById">

select * from student where id = #{id}

* 另一方

select * from grade where id = #{id}

* 测试

Student student = smapper.selectStudentGradeById(4);

System.out.println(student);

// student.hashCode();

System.out.println(student.getGrade());

2、一对多

* 一方

select="com.wuxi.daos.StudentMapper.selectStudentsByGrade">

select="com.wuxi.daos.StudentMapper.selectStudentsByGrade">

select * from grade where id = #{id}

* 多方

select * from student where grade_id=#{grade_id}

* 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 = nesoMUwqew 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

3、配置mapper文件

select * from student where id = #{id}

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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