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Java学习路线-59：MyBatis数据持久层框架

彭世瑜发表于 2021/08/14 01:22:22 2021/08/14

【摘要】 MyBatis 第一个程序 MyBatis 是基于 Java 的数据持久层框架持久化：数据从瞬时状态变为持久状态持久层：完成持久化工作的代码块 DAO 简而言之： MyBatis 将数据存入数据库中，从数据库中取数据通过框架可以减少重复代码，提高开发效率 MyBatis 是一个半自动化的 ORM 框架 Object Relationship Mappi...

MyBatis 第一个程序

MyBatis 是基于 Java 的数据持久层框架

持久化：数据从瞬时状态变为持久状态
持久层：完成持久化工作的代码块 DAO

简而言之：

MyBatis 将数据存入数据库中，从数据库中取数据

通过框架可以减少重复代码，提高开发效率

MyBatis 是一个半自动化的 ORM 框架
Object Relationship Mapping

文档：
https://mybatis.org/mybatis-3/zh/index.html

1、依赖

<!-- https://mvnrepository.com/artifact/org.mybatis/mybatis -->
<dependency> <groupId>org.mybatis</groupId> <artifactId>mybatis</artifactId> <version>3.5.4</version>
</dependency>

  
 
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2、配置数据库

mybatis-config.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">

<configuration> <environments default="development"> <environment id="development"> <transactionManager type="JDBC"/> <dataSource type="POOLED"> <property name="driver" value="com.mysql.cj.jdbc.Driver"/> <property name="url" value="jdbc:mysql://127.0.0.1:3306/data"/> <property name="username" value="root"/> <property name="password" value="123456"/> </dataSource> </environment> </environments> <mappers> <mapper resource="StudentMapper.xml"/> </mappers>
</configuration>

  
 
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3、SQL 工厂类

package com.pengshiyu.mybatis.util;


import org.apache.ibatis.io.Resources;
import org.apache.ibatis.session.SqlSession;
import org.apache.ibatis.session.SqlSessionFactory;
import org.apache.ibatis.session.SqlSessionFactoryBuilder;

import java.io.IOException;
import java.io.InputStream;

public class MyBatisUtil { public static SqlSessionFactory getSqlSessionFactory() throws IOException { String resource = "mybatis-config.xml"; InputStream inputStream = Resources.getResourceAsStream(resource); SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream); return sqlSessionFactory; } public static SqlSession getSqlSession() throws IOException { SqlSession session = getSqlSessionFactory().openSession(); return session; }
}


  
 
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4、创建实体类

package com.pengshiyu.mybatis.entity;

public class Student { private int id; private String name; private int age; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } @Override public String toString() { return "Student{" + "id=" + id + ", name='" + name + '\'' + ", age=" + age + '}'; }
}


  
 
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5、编写 SQL 语句映射文件

StudentMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.pengshiyu.mybatis.entity.StudentMapper"> <select id="selectStudent" resultType="com.pengshiyu.mybatis.entity.Student"> select * from students where id = #{id} </select>
</mapper>

  
 
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6、测试

package com.pengshiyu.mybatis.test;

import com.pengshiyu.mybatis.entity.Student;
import com.pengshiyu.mybatis.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.io.IOException;

public class Demo { public static void main(String[] args) throws IOException { SqlSession session = MyBatisUtil.getSqlSession(); Student student = session.selectOne("com.pengshiyu.mybatis.entity.StudentMapper.selectStudent", 3); System.out.println(student); session.close(); // Student{id=3, name='李白', age=30} }
}


  
 
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curd 操作

StudentMapper.xml

<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN" "http://mybatis.org/dtd/mybatis-3-mapper.dtd">

<mapper namespace="com.pengshiyu.mybatis.entity.StudentMapper"> <select id="selectStudent" resultType="com.pengshiyu.mybatis.entity.Student"> select * from students where id = #{id} </select> <select id="selectAllStudent" resultType="com.pengshiyu.mybatis.entity.Student"> select * from students </select> <insert id="insertStudent" parameterType="com.pengshiyu.mybatis.entity.Student"> insert into students(name, age) values(#{name}, #{age}) </insert> <update id="updateStudent" parameterType="com.pengshiyu.mybatis.entity.Student"> update students set name = #{name}, age = #{age} where id = #{id} </update> <delete id="deleteStudent"> delete from students where id = #{id} </delete>
</mapper>

  
 
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package com.pengshiyu.mybatis.dao;

import com.pengshiyu.mybatis.entity.Student;
import com.pengshiyu.mybatis.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.io.IOException;
import java.util.List;

public class StudentDao { public Student select(int id) throws IOException { SqlSession session = MyBatisUtil.getSqlSession(); Student student = session.selectOne("com.pengshiyu.mybatis.entity.StudentMapper.selectStudent", id); session.close(); return student; } public List<Student> selectAll() throws IOException { SqlSession session = MyBatisUtil.getSqlSession(); List<Student> students = session.selectList("com.pengshiyu.mybatis.entity.StudentMapper.selectAllStudent"); session.close(); return students; } public int insert(Student student) throws IOException { SqlSession session = MyBatisUtil.getSqlSession(); int result = session.insert("com.pengshiyu.mybatis.entity.StudentMapper.insertStudent", student); session.commit(); session.close(); return result; } public int update(Student student) throws IOException { SqlSession session = MyBatisUtil.getSqlSession(); int result = session.update("com.pengshiyu.mybatis.entity.StudentMapper.updateStudent", student); session.commit(); session.close(); return result; } public int delete(int id) throws IOException { SqlSession session = MyBatisUtil.getSqlSession(); int result = session.delete("com.pengshiyu.mybatis.entity.StudentMapper.deleteStudent", id); session.commit(); session.close(); return result; }
}


  
 
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package com.pengshiyu.mybatis.test;

import com.pengshiyu.mybatis.dao.StudentDao;
import com.pengshiyu.mybatis.entity.Student;

import java.io.IOException;
import java.util.List;

public class Demo { public static void main(String[] args) throws IOException { StudentDao studentDao = new StudentDao(); // 查询 Student student = studentDao.select(3); System.out.println(student); // Student{id=3, name='李白', age=30} // 写入 Student student = new Student(); student.setName("Jack"); student.setAge(23); System.out.println(studentDao.inset(student)); // 1 // 更新 Student student = studentDao.select(16); student.setAge(33); student.setName("Tom"); System.out.println(studentDao.update(student)); // 1 // 删除数据 System.out.println(studentDao.delete(12)); // 1 // 查询多条数据 List<Student> students = studentDao.selectAll(); for(Student student: students){ System.out.println(student); } }
}


  
 
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配置文件解析

配置文件

每个数据库对应一个 SqlSessionFactory 实例

dataSource：

UNPOOLED 每次请求时打开和关闭连接
POOLED 使用连接池
JNDI 能在如 EJB 或应用服务器这类容器中使用

mapper 文件

namespace 命名规则：
包名+类名/包名+mapper 文件名

parameterType 参数类型
resultType 返回结果类型
useGeneratedKeys=“true” 使用自增主键

配置优化

执行流程

读取核心配置文件
sqlSessionFactory 类
sqlSession
执行相关操作

1、可以将数据库配置单独放在一个文件里边

db.properties

driver=com.mysql.cj.jdbc.Driver
url=jdbc:mysql://127.0.0.1:3306/data
username=root
password=123456

  
 
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<?xml version="1.0" encoding="UTF-8" ?>
<!DOCTYPE configuration PUBLIC "-//mybatis.org//DTD Config 3.0//EN" "http://mybatis.org/dtd/mybatis-3-config.dtd">

<configuration> <!-- 加载数据库配置 --> <properties resource="db.properties" /> <settings> <!-- 打印sql日志 --> <setting name="logImpl" value="STDOUT_LOGGING"/> </settings> <environments default="development"> <environment id="development"> <transactionManager type="JDBC"/> <dataSource type="POOLED"> <property name="driver" value="${driver}"/> <property name="url" value="${url}"/> <property name="username" value="${username}"/> <property name="password" value="${password}"/> </dataSource> </environment> </environments> <mappers> <mapper resource="StudentMapper.xml"/> </mappers>

</configuration>

  
 
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2、别名配置

<configuration> <typeAliases> <!-- 指定单个类的别名 --> <typeAlias type="com.pengshiyu.mybatis.entity.Student" alias="Student"/> <!-- 指定整个包下的类都是别名 --> <package name="com.pengshiyu.mybatis.entity"/> </typeAliases>

</configuration>

  
 
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使用别名


<mapper namespace="com.pengshiyu.mybatis.entity.StudentMapper"> <select id="selectAllStudent" resultType="Student"> select * from students </select>
</mapper>

  
 
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属性名和列名不一致

MyBatis 会根据列名取赋值，会将列名转为小写

1、为列名指定别名

<mapper namespace="com.pengshiyu.mybatis.entity.StudentMapper"> <select id="selectStudent" resultType="Student"> select id, name, age as old from students where id = #{id} </select>
</mapper>

  
 
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2、使用结果映射类型

<mapper namespace="com.pengshiyu.mybatis.entity.StudentMapper"> <select id="selectStudent" resultMap="StudentMap"> select id, name, age from students where id = #{id} </select> <resultMap id="StudentMap" type="Student"> <!-- id为主键 --> <id column="id" property="id" /> <!-- column是数据库表的列名，property是实体类属性名 --> <result column="name" property="name"/> <result column="age" property="age"/> </resultMap>
</mapper>

  
 
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分页的实现

1、sql 中实现
如果将数据看做下标从 0 开始，那么就是数据切片 [startIndex, pageSize）

<mapper namespace="com.pengshiyu.mybatis.entity.StudentMapper"> <select id="selectAllStudent" parameterType="Map" resultType="Student"> select * from students limit #{offset}, #{limit} </select>
</mapper>

  
 
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public class StudentDao { public List<Student> selectAll(int currentPage, int pageSize) throws IOException { SqlSession session = MyBatisUtil.getSqlSession(); Map<String, Integer> map = new HashMap<String, Integer>(); map.put("offset", (currentPage - 1) * pageSize); map.put("limit", pageSize); List<Student> students = session.selectList( "com.pengshiyu.mybatis.entity.StudentMapper.selectAllStudent", map); session.close(); return students; }

}

  
 
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public class Demo { public static void main(String[] args) throws IOException { StudentDao studentDao = new StudentDao(); // 查询第二页的数据，每页2条 List<Student> students = studentDao.selectAll(2 , 2); for(Student student: students){ System.out.println(student); } }
}


  
 
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2、使用 RowBounds

<mapper namespace="com.pengshiyu.mybatis.entity.StudentMapper"> <select id="selectAllStudent"  resultType="Student"> select * from students </select>
</mapper>

  
 
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import org.apache.ibatis.session.RowBounds;

public class StudentDao { public List<Student> selectAll(int currentPage, int pageSize) throws IOException { SqlSession session = MyBatisUtil.getSqlSession(); RowBounds rowBounds = new RowBounds((currentPage - 1) * pageSize, pageSize); List<Student> students = session.selectList( "com.pengshiyu.mybatis.entity.StudentMapper.selectAllStudent", null, rowBounds); session.close(); return students; }

}


  
 
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通过打印的日志发现：

sql 限制起始位置和返回数量，currentPage=2, pageSize=2 时返回 2 条数据
RowBounds 不限制起始位置，currentPage=2, pageSize=2 时返回 4 条数据

注解开发

面向接口编程

扩展性好，分层开发中，上层不用管具体实现，

大家都遵循共同的实现，开发变得容易，规范性更好

DAO 接口

package com.pengshiyu.mybatis.dao;

import com.pengshiyu.mybatis.entity.Student;
import org.apache.ibatis.annotations.Select;

import java.util.List;

public interface IStudentDao { @Select("select * from students") public List<Student> getList();
}


  
 
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修改配置文件

<configuration> <mappers>
<!-- <mapper resource="StudentMapper.xml"/>--> <mapper class="com.pengshiyu.mybatis.dao.IStudentDao"/> </mappers>
</configuration>

  
 
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测试使用

package com.pengshiyu.mybatis.test;

import com.pengshiyu.mybatis.dao.IStudentDao;
import com.pengshiyu.mybatis.dao.StudentDao;
import com.pengshiyu.mybatis.entity.Student;
import com.pengshiyu.mybatis.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.io.IOException;
import java.util.List;

public class Demo { public static void main(String[] args) throws IOException { SqlSession session =  MyBatisUtil.getSqlSession(); IStudentDao studentDao = session.getMapper(IStudentDao.class); List<Student> students = studentDao.getList(); for(Student student : students){ System.out.println(student); } }
}


  
 
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多对一的处理

多个学生 student 对一个老师 teacher

1、数据库表设计

create table teachers(
	id int PRIMARY key auto_increment,
	name varchar(10)
);

create table students(
	id int PRIMARY key auto_increment,
	name varchar(10),
	teacher_id int
);

insert into teachers(name) values("王老师");
insert into teachers(name) values("李老师");
insert into teachers(name) values("赵老师");

insert into students(name, teacher_id) values("宋江", 1);
insert into students(name, teacher_id) values("李逵", 1);
insert into students(name, teacher_id) values("鲁智深", 2);
insert into students(name, teacher_id) values("林冲", 3);
insert into students(name, teacher_id) values("高俅", 3);

  
 
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2、实体类

Teacher

package com.pengshiyu.mybatis.entity;

public class Teacher { private int id; private String name; public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "Teacher{" + "id=" + id + ", name='" + name + '\'' + '}'; }
}


  
 
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Student

package com.pengshiyu.mybatis.entity;

public class Student { private int id; private String name; private Teacher teacher; public Teacher getTeacher() { return teacher; } public void setTeacher(Teacher teacher) { this.teacher = teacher; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "Student{" + "id=" + id + ", name='" + name + '\'' + ", teacher=" + teacher + '}'; }
}


  
 
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3、映射文件

多对一处理方式：

（1）按结果嵌套

查询一次

<mapper namespace="com.pengshiyu.mybatis.entity.StudentMapper"> <select id="selectAllStudent" resultMap="StudentMap"> select s.id sid, s.name sname, t.id tid, t.name tname from students as s left join teachers as t on s.teacher_id = t.id </select> <resultMap id="StudentMap" type="Student"> <!-- 主键 --> <id column="sid" property="id"/> <result column="sname" property="name"/> <!-- 关联对象--> <association property="teacher" javaType="Teacher"> <id column="tid" property="id"/> <result column="tname" property="name"/> </association> </resultMap>


</mapper>

  
 
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按查询嵌套

会查询 n 次，n 是 Student 数量

<mapper namespace="com.pengshiyu.mybatis.entity.StudentMapper"> <select id="selectAllStudent" resultMap="StudentTeacher"> select * from students </select> <resultMap id="StudentTeacher" type="Student"> <!-- 关联对象--> <association  property="teacher"  column="teacher_id" javaType="Teacher" select="getTeacher"> <id column="tid" property="id"/> <result column="tname" property="name"/> </association> </resultMap> <select id="getTeacher" resultType="Teacher"> select * from teachers where id = #{id} </select>
</mapper>

  
 
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4、引入映射文件

mybatis-config.xml

<configuration> <typeAliases> <package name="com.pengshiyu.mybatis.entity"/> </typeAliases> <mappers> <mapper resource="StudentMapper.xml"/> </mappers>

</configuration>

  
 
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5、Dao 编写

package com.pengshiyu.mybatis.dao;

import com.pengshiyu.mybatis.entity.Student;
import com.pengshiyu.mybatis.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.io.IOException;
import java.util.List;

public class StudentDao { public List<Student> selectAll() throws IOException { SqlSession session = MyBatisUtil.getSqlSession(); List<Student> students = session.selectList( "com.pengshiyu.mybatis.entity.StudentMapper.selectAllStudent"); session.close(); return students; }

}


  
 
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6、测试类

package com.pengshiyu.mybatis.test;

import com.pengshiyu.mybatis.dao.StudentDao;
import com.pengshiyu.mybatis.entity.Student;

import java.io.IOException;
import java.util.List;

public class Demo { public static void main(String[] args) throws IOException { StudentDao studentDao = new StudentDao(); List<Student> students = studentDao.selectAll(); for(Student student: students){ System.out.println(student); } }
}


  
 
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查询结果

Student{id=1, name='宋江', teacher=Teacher{id=1, name='王老师'}}
Student{id=2, name='李逵', teacher=Teacher{id=1, name='王老师'}}
Student{id=3, name='鲁智深', teacher=Teacher{id=2, name='李老师'}}
Student{id=4, name='林冲', teacher=Teacher{id=3, name='赵老师'}}
Student{id=5, name='高俅', teacher=Teacher{id=3, name='赵老师'}}

  
 
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一对多关系

Teacher

package com.pengshiyu.mybatis.entity;

import java.util.List;

public class Teacher { private int id; private String name; private List<Student> students; public List<Student> getStudents() { return students; } public void setStudents(List<Student> students) { this.students = students; } public int getId() { return id; } public void setId(int id) { this.id = id; } public String getName() { return name; } public void setName(String name) { this.name = name; } @Override public String toString() { return "Teacher{" + "id=" + id + ", name='" + name + '\'' + ", students=" + students + '}'; }
}


  
 
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TeacherMapper

查询一次


<mapper namespace="com.pengshiyu.mybatis.entity.TeacherMapper"> <select id="selectOneTeacher" resultMap="TeacherStudent"> select t.id tid, t.name tname, s.id sid, s.name sname from teachers t left join students s on t.id = s.teacher_id where t.id = #{id} </select> <resultMap id="TeacherStudent" type="Teacher"> <id column="tid" property="id"/> <result column="tname" property="name"/> <!-- 关联集合 --> <collection property="students" ofType="Student"> <id column="sid" property="id"/> <result column="sname" property="name"/> </collection> </resultMap>

</mapper>

  
 
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查询两次


<mapper namespace="com.pengshiyu.mybatis.entity.TeacherMapper"> <select id="selectOneTeacher" resultMap="TeacherStudent"> select * from teachers where id = #{id} </select> <resultMap id="TeacherStudent" type="Teacher"> <!-- 关联集合 --> <!-- column 是外键 --> <collection property="students" column="id" ofType="Student" select="getStudentByTeacherId"> </collection> </resultMap> <select id="getStudentByTeacherId" resultType="Student"> select * from students where teacher_id = #{id} </select>
</mapper>

  
 
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mybatis-config.xml


<configuration> <typeAliases> <package name="com.pengshiyu.mybatis.entity"/> </typeAliases> <mappers> <mapper resource="TeacherMapper.xml"/> </mappers>
</configuration>

  
 
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TeacherDao

package com.pengshiyu.mybatis.dao;

import com.pengshiyu.mybatis.entity.Student;
import com.pengshiyu.mybatis.entity.Teacher;
import com.pengshiyu.mybatis.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.io.IOException;
import java.util.List;

public class TeacherDao { public Teacher selectOne(int id) throws IOException { SqlSession session = MyBatisUtil.getSqlSession(); Teacher teacher = session.selectOne( "com.pengshiyu.mybatis.entity.TeacherMapper.selectOneTeacher", id); session.close(); return teacher; }

}


  
 
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package com.pengshiyu.mybatis.test;

import com.pengshiyu.mybatis.dao.StudentDao;
import com.pengshiyu.mybatis.dao.TeacherDao;
import com.pengshiyu.mybatis.entity.Student;
import com.pengshiyu.mybatis.entity.Teacher;

import java.io.IOException;
import java.util.List;

public class Demo { public static void main(String[] args) throws IOException { TeacherDao teacherDao = new TeacherDao(); Teacher teacher = teacherDao.selectOne(1); System.out.println(teacher); for(Student student: teacher.getStudents()){ System.out.println(student); } }
}


  
 
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输出

Teacher{id=1, name='王老师', students=[ Student{id=1, name='宋江', teacher=null}, Student{id=2, name='李逵', teacher=null} ]
}
Student{id=1, name='宋江', teacher=null}
Student{id=2, name='李逵', teacher=null}

  
 
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动态 SQL

根据不同的查询条件，生成不同的 sql

<mapper namespace="com.pengshiyu.mybatis.entity.StudentMapper"> <select id="selectAllStudent" resultType="Student"> select * from students <where> <if test="name != null"> name = #{name} </if> </where> </select>
</mapper>

  
 
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sql:

select * from students WHERE name = ?

  
 
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package com.pengshiyu.mybatis.dao;

import com.pengshiyu.mybatis.entity.Student;
import com.pengshiyu.mybatis.util.MyBatisUtil;
import org.apache.ibatis.session.SqlSession;

import java.io.IOException;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

public class StudentDao { public List<Student> selectAllStudent(String name) throws IOException { SqlSession session = MyBatisUtil.getSqlSession(); Map<String, String> map = new HashMap<>(); map.put("name", name); List<Student> students = session.selectList( "com.pengshiyu.mybatis.entity.StudentMapper.selectAllStudent", map ); session.close(); return students; }

}


  
 
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package com.pengshiyu.mybatis.test;

import com.pengshiyu.mybatis.dao.StudentDao;
import com.pengshiyu.mybatis.entity.Student;

import java.io.IOException;
import java.util.List;

public class Demo { public static void main(String[] args) throws IOException { StudentDao studentDao = new StudentDao(); List<Student> students = studentDao.selectAllStudent("宋江"); for(Student student: students){ System.out.println(student); } }
}


  
 
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文章来源: pengshiyu.blog.csdn.net，作者：彭世瑜，版权归原作者所有，如需转载，请联系作者。

原文链接：pengshiyu.blog.csdn.net/article/details/106889222

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Java学习路线-59：MyBatis数据持久层框架

MyBatis 第一个程序

curd 操作

配置文件解析

配置优化

属性名和列名不一致

分页的实现

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一对多关系

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Java学习路线-59：MyBatis数据持久层框架

MyBatis 第一个程序

curd 操作

配置文件解析

配置优化

属性名和列名不一致

分页的实现

注解开发

多对一的处理

一对多关系

动态 SQL

全部回复

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关于作者

目录

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