Mybatis

2023-03-02  本文已影响0人  不会写诗的苏轼

原始jdbc开发存在的问题如下:
数据库连接创建、释放频繁造成系统资源浪费从而影响系统性能【资源浪费】
sql语句在代码中硬编码，造成代码不易维护，实际应用sql变化的可能较大，sql变动需要改变java代码。【耦合】
查询操作时，需要手动将结果集中的数据手动封装到实体中。插入操作时，需要手动将实体的数据设置到sql语句的占位
符位置【new Bean对象 使用set方法添加到Bean对象中】
应对上述问题给出的解决方案:
使用数据库连接池初始化连接资源【解决资源浪费】
②将sql语句抽取到xml配置文件中【解耦合】
③使用反射、内省等底层技术，自动将实体与表进行属性与字段的自动映射【使用一些方法自动将数据给Bean对象】

概念：

1.持久层的框架
2.隐藏jdbc繁杂的api,进行响应的封装。
3.只用关心sql语句。
3.实体对象和数据库表单的映射关系。

userMapper

<?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="userMapper">
    <!--查询操作-->
    <select id="findAll" resultType="com.xjbt.domain.User">
        select * from user
    </select>
    <!--增加操作-->
    <insert id="save" parameterType="com.xjbt.domain.User">
        insert into user values(#{id},#{username},#{password})
    </insert>
    <!--修改操作-->
    <insert id="update" parameterType="com.xjbt.domain.User">
        update user set username=#{username},password=#{password} where id=#{id}
    </insert>
    <!--删除操作-->
    <delete id="delete" parameterType="java.lang.Integer">
        delete from user where id=#{id}
    </delete>
</mapper>

test

public class MyBatisTest {
    @Test
    public void test1() 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回话对象
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //执行操作  参数：namespace+id
        List<User> userList = sqlSession.selectList("userMapper.findAll");
        //打印数据
        System.out.println(userList);
        //释放资源
        sqlSession.close();
    }

   @Test
        sqlSession.insert("userMapper.save",user);
        //事务提交
        sqlSession.commit();
    

    @Test
        sqlSession.update("userMapper.update",user);
        //事务提交
        sqlSession.commit();
  

    @Test
        sqlSession.update("userMapper.delete",6);
        //事务提交
        sqlSession.commit();

sqlMapConfig

<?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>
    <!--数据源环境-->
    <environments default="development">
        <environment id="development">
            <transactionManager type="JDBC"></transactionManager>
            <dataSource type="POOLED">
                <property name="driver" value="com.mysql.jdbc.Driver"/>
                <property name="url" value="jdbc:mysql://localhost:3306/test"/>
                <property name="username" value="root"/>
                <property name="password" value="123456"/>
            </dataSource>
        </environment>
    </environments>

    <!--加载映射文件-->
    <mappers>
        <mapper resource="com/xjbt/mapper/UserMapper.xml"></mapper>
    </mappers>
</configuration>
MyBatis核心配置文件概述

MyBatis常用配置解析

public class DateTypeHandler extends BaseTypeHandler<Date> {
    //将java类型转换成数据库的数据类型
    public void setNonNullParameter(PreparedStatement preparedStatement, int i, Date date, JdbcType jdbcType) throws SQLException {
        long time = date.getTime();//毫秒值
        preparedStatement.setLong(i,time);
    }

    /**
     * 将数据库的类型转换成java类型
     * @param resultSet   转换后的结果集
     * @param s  数据库要转的字段
     * @return
     * @throws SQLException
     */
    public Date getNullableResult(ResultSet resultSet, String s) throws SQLException {
        long aLong = resultSet.getLong(s);
        Date date=new Date(aLong);
        return date;
    }
    //将数据库的类型转换成java类型
    public Date getNullableResult(ResultSet resultSet, int i) throws SQLException {
        long aLong = resultSet.getLong(i);
        Date date=new Date(aLong);
        return date;
    }
    //将数据库的类型转换成java类型
    public Date getNullableResult(CallableStatement callableStatement, int i) throws SQLException {
        long aLong = callableStatement.getLong(i);
        Date date=new Date(aLong);
        return date;
    }
}

③在MyBatis核心配置文件中进行注册

 <!--注册类型转换器-->
    <typeHandlers>
        <typeHandler handler="com.xjbt.handler.DateTypeHandler"/>
    </typeHandlers>

④测试转换是否正确

 /**
     * 类型转换:插入数据库将date存成bigInt类型
     * @throws IOException
     */
    @Test
    public void testIf1() throws IOException {
        InputStream resource = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resource);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        UserMapper mapper = sqlSession.getMapper(UserMapper.class);

        User user=new User();
        user.setUsername("ceshi");
        user.setPassword("123");
        user.setBirthday(new Date());
        mapper.insertUser(user);
        sqlSession.commit();
        sqlSession.close();
    }

    /**
     * 类型转换:获取 从数据库查询出的时间戳转换成Date类型
     * @throws IOException
     */
    @Test
    public void testIf2() throws IOException {
        InputStream resource = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resource);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        User user=new User();
        List<User> all = mapper.findAll(user);
        System.out.println(all);
    }

<!--mybatis分页-->
    <dependency>
      <groupId>com.github.pagehelper</groupId>
      <artifactId>pagehelper</artifactId>
      <version>3.7.5</version>
    </dependency>
    <dependency>
      <groupId>com.github.jsqlparser</groupId>
      <artifactId>jsqlparser</artifactId>
      <version>0.9.1</version>
    </dependency>

在mybatis核心配置文件中配置

 <!--配置分页助手插件-->
    <plugins>
        <plugin interceptor="com.github.pagehelper.PageHelper">
            <property name="dialect" value="mysql"/><!--使用那种数据库语言 例如：mysql使用的是limit-->
        </plugin>
    </plugins>

PageHelper插件测试分页数据获取

    @Test
    public void testIf2() throws IOException {
        InputStream resource = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resource);
        SqlSession sqlSession = sqlSessionFactory.openSession();

        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        User user=new User();
        PageHelper.startPage(1,3);
        List<User> all = mapper.findAll(user);
        for (User u:all) {
            System.out.println(u);
        }
 //获得与分页相关参数
        PageInfo<User> pageInfo = new PageInfo<User>(all);
        System.out.println("当前页:"+pageInfo.getPageNum());
        System.out.println("每页显示条数:"+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());
    }
代理开发方式

代理开发方式介绍：可不写dao层接口的实现类，mybatis动态代理对象，实现这个接口，相当与这个实现类。
Mapper接口开发需要遵循以下规范:
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


dao层UserMapper接口 
public interface UserMapper {
    public User findById(int id);
}

UserMapper.xml

<mapper namespace="com.xjbt.dao.UserMapper">
    <!--查询操作-->
    <select id="findById" parameterType="int" resultType="user">
        select * from user where id = #{id}
    </select>
</mapper>

serviceDemo测试

public class ServiceDemo  {
    public static void main(String[] args) throws IOException {
        InputStream resourceAsStream = Resources.getResourceAsStream("sqlMapConfig.xml");
        SqlSessionFactory sessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sessionFactory.openSession();
        //mybatis动态实现dao层UserMapper接口的实现类
        UserMapper mapper = sqlSession.getMapper(UserMapper.class);
        User user = mapper.findById(1);
        System.out.println(user);
    }
}
动态sql：
  1. 动态SQL之<if>
    <select id="findAll" resultType="user" parameterType="user">
        select * from user
        <where>
            <if test="id!=0">
                and id = #{id}
            </if>
            <if test="username!=null">
                and username = #{username}
            </if>
            <if test="password!=null">
                and password = #{password}
            </if>
        </where>

    </select>
  1. 动态sql <froeach>
    <select id="findByIds" parameterType="list" resultType="user">
        select * from user
        <where>
            <foreach collection="list" open="id in (" close=")" item="id" separator=",">
                #{id}
            </foreach>
        </where>
    </select>
  1. 代码片段
<!--代码片段-->
 <sql id="selector">   
         select * from user
    </sql>
    <select id="findByIds" parameterType="list" resultType="user">
        <!--代码片段引用-->
        <include refid="selector"></include>
        <where>
            <foreach collection="list" open="id in (" close=")" item="id" separator=",">
                #{id}
            </foreach>
        </where>
    </select>
Mybatis多表查询
<mapper namespace="com.xjbt.dao.OrderMapper">
<resultMap id="orderMap" type="order">
    <!--
        column 字段名称
        property 对象的属性名称
    -->
    <id column="oid" property="id" />
    <result column="ordertime" property="datetime" />
    <result column="total" property="total"/>
    <!--<result column="uid" property="user.id"/>
    <result column="username" property="user.username"/>
    <result column="password" property="user.password"/>
    <result column="birthday" property="user.birthday"/>-->

    <!--
    property order的属性名
    javatype property的类型 这里user代表全限定名称的别名【已经在sqlMapConfig.xml中配置别名】
    -->
    <association property="user" javaType="user">
        <id column="uid" property="id"/>
        <result column="username" property="username"/>
        <result column="password" property="password"/>
        <result column="birthday" property="birthday"/>
    </association>
</resultMap>
    
    <select id="findAll" resultMap="orderMap">
        select *,o.id oid from orders o,user u WHERE o.uid=u.id;
    </select>
</mapper>

<mapper namespace="com.xjbt.dao.UserMapper">
    <!--查询操作-->
    <resultMap id="userMap" type="user">
        <id column="uid" property="id" />
        <result column="username" property="username" />
        <result column="birthday" property="birthday" />
        <collection property="orderList" ofType="order">
           <result column="ordertime" property="datetime" />
            <result property="total" column="total" />
        </collection>
    </resultMap>

   <select id="findAll" resultMap="userMap">
        select * ,o.id oid from user u,orders o where u.id=o.uid;
   </select>
</mapper>

 <resultMap id="userAndRole" type="user">
        <id column="id" property="id" />
        <result column="username" property="username" />
        <result column="birthday" property="birthday" />
        <collection property="roleList" ofType="role">
            <id column="role_id" property="id" />
            <result column="rolename" property="rolename" />
        </collection>
    </resultMap>
    <select id="findUserAndRoleAll" resultMap="userAndRole">
            SELECT * from user u left join (select * from user_role ur,role r where r.id=ur.role_Id) urr on u.id=urr.user_id

    </select>
MyBatis的常用注解

@lnsert:实现新增
@Update:实现更新
@Delete:实现删除@Select:实现查询
@Result:实现结果集封装
@Results:可以与@Result一起使用，封装多个结果集@One:实现一对一结果集封装
@Many:实现一对多结果集封装

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