Mybatis的理解与应用

简介

mybatis 是一个优秀的基于java的持久层框架，它内部封装了jdbc，使开发者只需要关注sql语句本身，而不需要花费精力去处理加载驱动、创建连接、创建statement等繁杂的过程。

mybatis通过xml或注解的方式将要执行的各种 statement配置起来，并通过java对象和statement中sql的动态参数进行映射生成最终执行的sql语句。

最后mybatis框架执行sql并将结果映射为java对象并返回。采用ORM思想解决了实体和数据库映射的问题，对jdbc 进行了封装，屏蔽了jdbc api 底层访问细节，使我们不用与jdbc api 打交道，就可以完成对数据库的持久化操作。

应用

传统方式

userMapper.xml

<mapper namespace="userMapper">
    <!--查询所有-->
    <select id="select" resultType="user">
    	select * from User
    </select>

    <insert id="insert" parameterType="user">
    	insert into user values (null ,#{username},#{password},null)
    </insert>


    <delete id="delete" parameterType="int">
    	delete from user where id=#{id}
    </delete>

    <update id="update" parameterType="int">
    	update user set username='wangwu' where id=#{id}
    </update>
</mapper>

sqlMapConfig.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>
    <!--起别名-->
    <typeAliases>
        <typeAlias type="com.heqiang.domain.User" alias="user"></typeAlias>
    </typeAliases>


    <!--配置数据源环境-->
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"/>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql:///maven"/>
                <property name="username" value="root"/>
                <property name="password" value="myroot"/>
            </dataSource>
        </environment>
    </environments>

    <!--加载映射文件-->
    <mappers>
        <mapper resource="com/com.heqiang/dao/userMapper.xml"/>
    </mappers>
</configuration>

User

@Data
@NoArgsConstructor
@AllArgsConstructor
public class User {
    private int id;
    private String username;
    private String password;
}

测试

 @Test
//删除操作
public void test4() throws IOException {

    //获得核心配置文件
    InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
    
    //获得session工厂对象
    SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
    
    //获得session会话对象
    SqlSession sqlSession = sqlSessionFactory.openSession();
    
    //执行操作  参数：namespace+id
    sqlSession.delete("userMapper.delete",1);

    //mybatis执行更新操作  需要手动提交事务
    sqlSession.commit();

    //释放资源
    sqlSession.close();
}

面向接口代理开发方式

介绍

Mapper 接口开发方法只需要程序员编写Mapper 接口（相当于Dao 接口），由Mybatis 框架根据接口定义创建接口的动态代理对象，代理对象的方法体同上边Dao接口实现类方法。

遵循四个原则

1) Mapper.xml文件中的namespace与mapper接口的全限定名相同

2) Mapper接口方法名和Mapper.xml中定义的每个statement的id相同

3) Mapper接口方法的输入参数类型和mapper.xml中定义的每个sql的parameterType的类型相同

4) Mapper接口方法的输出参数类型和mapper.xml中定义的每个sql的resultType的类型相同

PhgeHelp插件

1.导入坐标
<dependency>
        <groupId>com.github.pagehelper</groupId>
        <artifactId>pagehelper-spring-boot-starter</artifactId>
        <version>1.2.4</version>
</dependency>

<dependency>
        <groupId>com.github.jsqlparser</groupId>
        <artifactId>jsqlparser</artifactId>
        <version>1.0</version>
</dependency>

2.配置文件(properties)
pagehelper.helperDialect=mysql
pagehelper.reasonable=true
pagehelper.supportMethodsArguments=true
pagehelper.params=count=countSql

3.使用方式
ublic List<User> findAll() {
    PageHelper.startPage(1,5);
    List<User> list = dao.findAll();
    //获得与分页相关参数
    PageInfo<User> pageInfo = new PageInfo<User>(list);
    System.out.println("当前页数：" + pageInfo.getPageNum());
    System.out.println("每页条数：" + pageInfo.getPageSize());
    System.out.println("总条数：" + pageInfo.getTotal());
    System.out.println("总页数：" + pageInfo.getPages());
    System.out.println("上一页：" + pageInfo.getPrePage());
    System.out.println("下一页：" + pageInfo.getNextPage());
    System.out.println("是否是第一个：" + pageInfo.isIsFirstPage());
    System.out.println("是否是最后一个：" + pageInfo.isIsLastPage());
    return list;
}

多表查询

配置文件方式

一对一

<resultMap id="orderMap" type="order">
    <!--主键单独有一个id-->
    <id column="oid" property="id"></id>
    <result column="ordertime" property="ordertime"></result>
    <result column="total" property="total"></result>
    <!--一对一第一种封装方式-->
    <!--<result column="uid" property="user.id"></result>-->
    <!--<result column="username" property="user.username"></result>-->
    <!--<result column="password" property="user.password"></result>-->
    <!--<result column="birthday" property="user.birthday"></result>-->

    <!--一对一第二种封装方式-->
    <!--property: 当前实体(order)中的属性名称(private User user)-->
    <!--javaType: 当前实体(order)中的属性的类型(User)-->
    <association property="user" javaType="user">
        <id column="uid" property="id"></id>
        <result column="username" property="username"></result>
        <result column="password" property="password"></result>
        <result column="birthday" property="birthday"></result>
    </association>
</resultMap>

<select id="findAll" resultMap="orderMap">
     SELECT *,o.id oid FROM orders o,USER u WHERE o.uid=u.id
</select>

一对多

<resultMap id="userMap" type="user">
    <id column="uid" property="id"></id>
    <result column="username" property="username"></result>
    <result column="password" property="password"></result>
    <result column="birthday" property="birthday"></result>
    <!--配置集合信息
        property:集合名称
        ofType：当前集合中的数据类型
    -->
    <collection property="orderList" ofType="order">
        <!--封装order的数据-->
        <id column="oid" property="id"></id>
        <result column="ordertime" property="ordertime"></result>
        <result column="total" property="total"></result>
    </collection>
</resultMap>

<select id="findAll" resultMap="userMap">
    SELECT *,o.id oid FROM USER u,orders o WHERE u.id=o.uid
</select>

多对多

 <resultMap id="userRoleMap" type="user">
    <!--user的信息-->
    <id column="userId" property="id"></id>
    <result column="username" property="username"></result>
    <result column="password" property="password"></result>
    <result column="birthday" property="birthday"></result>
    <!--user内部的roleList信息-->
    <collection property="roleList" ofType="role">
        <id column="roleId" property="id"></id>
        <result column="roleName" property="roleName"></result>
        <result column="roleDesc" property="roleDesc"></result>
    </collection>
</resultMap>

<select id="findUserAndRoleAll" resultMap="userRoleMap">
    SELECT * FROM USER u,sys_user_role ur,sys_role r WHERE u.id=ur.userId AND ur.roleId=r.id
</select>

注解方式

@Insert("insert into user values(#{id},#{username},#{password},#{birthday})")
public void save(User user);

@Delete("delete from user where id=#{id}")
public void delete(int id);

@Update("update user set username=#{username},password=#{password} where id=#{id}")
public void update(User user);

@Select("select * from user where id=#{id}")
public User findById(int id);

一对一

@Select("select * from orders")
@Results({
        @Result(column = "id", property = "id"),
        @Result(column = "ordertime", property = "ordertime"),
        @Result(column = "total", property = "total"),
        @Result(
                property = "user", //要封装的属性名称
                column = "uid", //根据该字段去查询user表的数据
                javaType = User.class, //要封装的实体类型
                //select属性 代表查询那个接口的方法获得数据
                one = @One(select = "com.itheima.dao.UserMapper.findById")
        )
})
public List<Order> findAll();

一对多

@Select("select * from user")
@Results({
        @Result(id = true, column = "id", property = "id"),
        @Result(column = "username", property = "username"),
        @Result(column = "password", property = "password"),
        @Result(
                property = "orderList",//封装User中的的属性名
                column = "id",//根据什么再次查询
                javaType = List.class,//
                many = @Many(select = "com.itheima.dao.OrderMapper.findByUid")
        )
})
public List<User> findUserAndOrderAll();

多对多

@Select("SELECT * FROM USER")
@Results({
        @Result(id = true, column = "id", property = "id"),
        @Result(column = "username", property = "username"),
        @Result(column = "password", property = "password"),
        @Result(
                property = "roleList",
                column = "id",
                javaType = List.class,
                many = @Many(select = "com.itheima.dao.RoleMapper.findByUid")
        )
})
public List<User> findUserAndRoleAll();

注意事项

mybatis执行更新操作  需要手动提交事务