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 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 resultMap='studentGradeById'>select * from student where id = #{id} </select>

* 另一方

<select 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 resultMap='gradeStudents'>select * from grade where id = #{id} </select>

* 多方

<select 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 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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