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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

   
       
    
     org.mybatis
        
    
     mybatis
        
    
     3.5.4
    
   

2、配置数据库

mybatis-config.xml

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;    }}

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 +                '}';    }}

5、编写 SQL 语句映射文件

StudentMapper.xml

   
       
            select * from students where id = #{id}    
   

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}    }}

curd 操作

StudentMapper.xml

   
       
            select * from students where id = #{id}        
            select * from students        
    
             insert into students(name, age) values(#{name}, #{age})    
        
    
             update students set name = #{name}, age = #{age} where id = #{id}    
        
    
             delete from students where id = #{id}    
    
   

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
   
     selectAll() throws IOException {
           SqlSession session = MyBatisUtil.getSqlSession();        List
    
      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;    }}
    
   

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
   
     students = studentDao.selectAll();        for(Student student: students){
               System.out.println(student);        }    }}
   

配置文件解析

配置文件

每个数据库对应一个 SqlSessionFactory 实例

dataSource：

1. UNPOOLED 每次请求时打开和关闭连接
2. POOLED 使用连接池
3. 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

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

配置优化

执行流程

1. 读取核心配置文件
2. sqlSessionFactory 类
3. sqlSession
4. 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.Driverurl=jdbc:mysql://127.0.0.1:3306/datausername=rootpassword=123456

2、别名配置

使用别名

       
            select * from students    
   

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

       
            select id, name, age as old from students where id = #{id}    
   

2、使用结果映射类型

       
            select id, name, age from students where id = #{id}        
            
             
             
             
             
         
    
   

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）

       
            select * from students limit #{offset}, #{limit}    
   

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

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
   
     students = studentDao.selectAll(2 , 2);        for(Student student: students){
               System.out.println(student);        }    }}
   

2、使用 RowBounds

       
            select * from students    
   

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

通过打印的日志发现：

1. sql 限制起始位置和返回数量，currentPage=2, pageSize=2 时返回 2 条数据
2. 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
   
     getList();}
   

修改配置文件

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
   
     students = studentDao.getList();        for(Student student : students){
               System.out.println(student);        }    }}
   

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);

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 + '\'' +                '}';    }}

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 +                '}';    }}

3、映射文件

多对一处理方式：

（1）按结果嵌套

查询一次

       
            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        
            
             
             
             
             
                 
                  
              
         
    
   

2. 按查询嵌套

会查询 n 次，n 是 Student 数量

       
            select * from students        
            
             
                 
                  
              
         
        
            select * from teachers where id = #{id}    
   

4、引入映射文件

mybatis-config.xml

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

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
   
     students = studentDao.selectAll();        for(Student student: students){
               System.out.println(student);        }    }}
   

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'}}

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
   
     students;    public List
    
      getStudents() {
           return students;    }    public void setStudents(List
     
       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 +                '}';    }}
     
    
   

TeacherMapper

查询一次

       
            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}        
            
             
             
             
                 
                  
              
         
    
   

查询两次

       
            select *        from teachers        where id = #{id}        
            
             
             
             
         
        
            select * from students where teacher_id = #{id}    
   

mybatis-config.xml

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;    }}

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);        }    }}

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}

动态 SQL

根据不同的查询条件，生成不同的 sql

       
            select * from students                                    name = #{name}                        
   

sql:

select * from students WHERE name = ?

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

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
   
     students = studentDao.selectAllStudent("宋江");        for(Student student: students){
               System.out.println(student);        }    }}
   

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