MyBatis安装、配置

1 简介

1.1 概述

MyBatis是一个半自动化的持久化层框架

1.2 为什么要使用MyBatis

对于开发人员而言，核心SQL还是需要自己优化
SQL和Java编码分开，功能边界清晰，一个专注业务，一个专注数据

1.2.1 JDBC

SQL夹在Java代码块里，耦合度高导致硬编码内伤
维护不易，且实际开发需求中SQL频繁修改的情况是多见的

1.2.2 Hibernate

长难SQL对于Hibernate而言处理也不容易
内部自动产生的SQL，不容易做特殊优化
基于全映射的全自动框架，大量自动的POJO进行部分映射时比较困难，导致数据库性能下降

2 idea导入MyBatis

使用 Maven 来构建项目，则需将下面的依赖代码置于 pom.xml 文件中：

<dependency>
  <groupId>org.mybatis</groupId>
  <artifactId>mybatis</artifactId>
  <version>x.x.x</version>
</dependency>

“x.x.x”替换成对应的版本号

3 Hello world

3.1 创建MySQL数据表、添加数据

create table employees (
    id int(11) primary key auto_increment,
    last_name varchar(255),
    gender char(1),
    email varchar(255)
)

insert into employees (id, last_name, gender, email) values (1, "tom", 0, "136xxxx@qq.com")

select * from employees e 
id|last_name|gender|email         |
--+---------+------+--------------+
 1|tom      |0     |136xxxx@qq.com|

3.2 从 XML 中构建 SqlSessionFactory

package com.lnhoo;

public class Employee {
    private Integer id;
    private String lastName;
    private char gender;
    private String email;

    public void setId(Integer id) {
        this.id = id;
    }

    public void setLastName(String lastName) {
        this.lastName = lastName;
    }

    public void setGender(char gender) {
        this.gender = gender;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    @Override
    public String toString() {
        return "Employee{" +
                "id=" + id +
                ", lastName='" + lastName + '\'' +
                ", gender=" + gender +
                ", email='" + email + '\'' +
                '}';
    }
}
// 测试类
public class MyBatisTest {
    @Test
    public void testMapper() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        try (SqlSession session = sqlSessionFactory.openSession()) {
            Employee employee = (Employee) session.selectOne("com.lnhoo.EmployeeMapper.selectEmployee", 1);
            System.out.println(employee);
        }
    }
}

每个基于 MyBatis 的应用都是以一个 SqlSessionFactory 的实例为核心的。SqlSessionFactory 的实例可以通过 SqlSessionFactoryBuilder 获得。而 SqlSessionFactoryBuilder 则可以从 XML 配置文件或一个预先配置的 Configuration 实例来构建出 SqlSessionFactory 实例。

3.3 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://localhost:3306/mybatis"/>
                <property name="username" value="root"/>
                <property name="password" value="******"/>
            </dataSource>
        </environment>
    </environments>
    <mappers>
        <mapper resource="EmployeeMapper.xml"/>
    </mappers>
</configuration>

XML 配置文件中包含了对 MyBatis 系统的核心设置，包括获取数据库连接实例的数据源（DataSource）以及决定事务作用域和控制方式的事务管理器（TransactionManager）

3.4 Mapper配置文件

<?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.lnhoo.EmployeeMapper">
    <select id="selectEmployee" resultType="com.lnhoo.Employee">
        select id id, last_name lastName, gender gender, email email from employees where id = #{id}
    </select>
</mapper>

在数据库表中的字段last_name对应于Employee类中的lastName属性，因为命名不一致，所以需要在SQL语句中将last_name字段起别名为lastName

3.5 接口式编程

3.5.1 定义接口

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public interface EmployeeMapper {
    Employee getEmployeeById(Integer id);
}

在接口中定义方法的参数和返回值

3.5.2 定义Mapper

<?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.lnhoo.dao.EmployeeMapper">
    <select id="getEmployeeById" resultType="com.lnhoo.Employee">
        select id id, last_name lastName, gender gender, email email from employees where id = #{id}
    </select>
</mapper>

通过xml文件配置接口和SQL语句的绑定关系
mapper标签的namespace属性必须和Mapper接口的全路径一致
select标签的id属性必须和接口中定义的方法名一致，resultType属性填返回对象所属类的全路径

3.5.3 调用方法

public class MyBatisTest {
    @Test
    public void testMapper() throws IOException {
        String resource = "mybatis-config.xml";
        InputStream inputStream = Resources.getResourceAsStream(resource);
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);

        try (SqlSession session = sqlSessionFactory.openSession()) {
            EmployeeMapper employeeMapper = session.getMapper(EmployeeMapper.class);
            Employee employee = employeeMapper.getEmployeeById(1);
            System.out.println(employee);
        }
    }
}

将接口和xml配置文件绑定后，MyBatis会自动为接口创建动态代理对象，由代理对象去执行增删改查