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Mybatis—代理开发和核心配置文件深入

作者:互联网

代理开发方式介绍

采用 Mybatis 的代理开发方式实现 DAO 层的开发,这种方式是我们后面进入企业的主流。

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。

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

public interface userMapper {
    public List<User> findAll();
}

    @Test
    public void tset4() {

        InputStream resourceAsStream = null;
        try {
            resourceAsStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        } catch (IOException ioException) {
            ioException.printStackTrace();
        }
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        userMapper userMapper=sqlSession.getMapper(com.controller.userMapper.class);
        List<User> aLl = userMapper.findAll();
        System.out.println(aLl);
    }
<?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.controller.userMapper">
    <select id="findAll" resultType="user">
        select * from  user
    </select>

</mapper>
动态 SQL 之<if>

我们根据实体类的不同取值,使用不同的 SQL语句来进行查询。比如在 id如果不为空时可以根据id查询,如果username 不同空时还要加入用户名作为条件。这种情况在我们的多条件组合查询中经常会碰到。

<?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.controller.userMapper">
    <select id="findAll" resultType="user">
        select * from  user
    </select>

    <select id="findById" resultType="user" parameterType="user">
        select  * from user
        <where>
            <if test="id!=0">
            and id=#{id}
            </if>
            <if test="username!=null">
                and username=#{username}
            </if>

        </where>

    </select>
</mapper>

public interface userMapper {
    public List<User> findAll();
    public User findById(User user);
}

动态 SQL 之<foreach>

循环执行sql的拼接操作,例如:SELECT * FROM USER WHERE id IN (1,2,5)。
foreach标签的属性含义如下:

标签用于遍历集合,它的属性:

•collection:代表要遍历的集合元素,注意编写时不要写#{}

•open:代表语句的开始部分

•close:代表结束部分

•item:代表遍历集合的每个元素,生成的变量名

•sperator:代表分隔符

<?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.controller.userMapper">
    <sql id="selectUser" >select * from User</sql>
    <select id="findAll" resultType="user">
        select * from  user
    </select>

    <select id="findById" resultType="user" parameterType="user">

<include refid="selectUser"></include>
        <where>
            <if test="id!=0">
            and id=#{id}
            </if>
            <if test="username!=null">
                and username=#{username}
            </if>

        </where>

    </select>
    <select id="findByIds" parameterType="list" resultType="user">
        <include refid="selectUser"></include>
        <where>
             <foreach collection="list" open="id in (" close=")" item="id" separator=",">
                 #{id}
             </foreach>
        </where>

    </select>
</mapper>
public interface userMapper {
    public List<User> findAll();
    public User findById(User user);
    public List<User> findByIds(List<Integer> list);
}

    @Test
    public void tset4() {

        InputStream resourceAsStream = null;
        try {
            resourceAsStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        } catch (IOException ioException) {
            ioException.printStackTrace();
        }
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        userMapper userMapper=sqlSession.getMapper(com.controller.userMapper.class);
        User user = new User();user.setId(1);
        User user1 = new User();user1.setId(2);
        List<Integer> list=new ArrayList<>();
        list.add(1);
        list.add(2);
        System.out.println(userMapper.findByIds(list));
    }

2.2 SQL片段抽取

Sql 中可将重复的 sql 提取出来,使用时用 include 引用即可,最终达到 sql 重用的目的

<?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.controller.userMapper">
    <sql id="selectUser" >select * from User</sql>
    <select id="findAll" resultType="user">
        select * from  user
    </select>

    <select id="findById" resultType="user" parameterType="user">

<include refid="selectUser"></include>
        <where>
            <if test="id!=0">
            and id=#{id}
            </if>
            <if test="username!=null">
                and username=#{username}
            </if>

        </where>

    </select>
    <select id="findByIds" parameterType="list" resultType="user">
        <include refid="selectUser"></include>
        <where>
             <foreach collection="list" open="id in (" close=")" item="id" separator=",">
                 #{id}
             </foreach>
        </where>

    </select>
</mapper>

MyBatis核心配置文件深入

typeHandlers标签

无论是 MyBatis 在预处理语句(PreparedStatement)中设置一个参数时,还是从结果集中取出一个值时, 都会用类型处理器将获取的值以合适的方式转换成 Java 类型。下表描述了一些默认的类型处理器(截取部分)。

你可以重写类型处理器或创建你自己的类型处理器来处理不支持的或非标准的类型。具体做法为:实现 org.apache.ibatis.type.TypeHandler 接口, 或继承一个很便利的类 org.apache.ibatis.type.BaseTypeHandler, 然后可以选择性地将它映射到一个JDBC类型。例如需求:一个Java中的Date数据类型,我想将之存到数据库的时候存成一个1970年至今的毫秒数,取出来时转换成java的Date,即java的Date与数据库的varchar毫秒值之间转换。

开发步骤:

①定义转换类继承类BaseTypeHandler

②覆盖4个未实现的方法,其中setNonNullParameter为java程序设置数据到数据库的回调方法,getNullableResult为查询时 mysql的字符串类型转换成 java的Type类型的方法

public class MyDateTypeHandler extends BaseTypeHandler<Date> {

    @Override
    public void setNonNullParameter(PreparedStatement preparedStatement, int i, Date date, JdbcType jdbcType) throws SQLException {
        preparedStatement.setString(i,date.getTime()+"");
    }

    @Override
    public Date getNullableResult(ResultSet resultSet, String s) throws SQLException {
        return new Date(resultSet.getLong(s));
    }

    @Override
    public Date getNullableResult(ResultSet resultSet, int i) throws SQLException {
        return new Date(resultSet.getLong(i));
    }

    @Override
    public Date getNullableResult(CallableStatement callableStatement, int i) throws SQLException {
        return callableStatement.getDate(i);
    }
}

③在MyBatis核心配置文件中进行注册

<typeHandlers>

    <typeHandler handler="com.MyDateTypeHandler"/>
</typeHandlers>

测试转换是否正确

    @Test
    public void tset4() {

        InputStream resourceAsStream = null;
        try {
            resourceAsStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        } catch (IOException ioException) {
            ioException.printStackTrace();
        }
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        userMapper userMapper=sqlSession.getMapper(com.controller.userMapper.class);
        User user = new User();
        user.setUsername("xx");
        user.setPassword("123");
        user.setBirthday(new Date());
        userMapper.add(user);
    }

plugins标签

MyBatis可以使用第三方的插件来对功能进行扩展,分页助手PageHelper是将分页的复杂操作进行封装,使用简单的方式即可获得分页的相关数据

开发步骤:

①导入通用PageHelper的坐标

        <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>

②在mybatis核心配置文件中配置PageHelper插件

    <plugins>
    <plugin interceptor="com.github.pagehelper.PageHelper">
        <property name="dialect" value="mysql"/>
    </plugin>
</plugins>

③测试分页数据获取

    @Test
    public void tset4() {

        InputStream resourceAsStream = null;
        try {
            resourceAsStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        } catch (IOException ioException) {
            ioException.printStackTrace();
        }
        SqlSessionFactory sqlSessionFactory=new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession(true);
        userMapper userMapper=sqlSession.getMapper(com.controller.userMapper.class);

        PageHelper.startPage(2,2);
        List<User> all = userMapper.findAll();
        for (User user : all) {
            System.out.println(user);
        }
        PageInfo<User> pageInfo = new PageInfo<User>(all);
        System.out.println("总条数:"+pageInfo.getTotal());
        System.out.println("总页数:"+pageInfo.getPages());
        System.out.println("当前页:"+pageInfo.getPageNum());
        System.out.println("每页显示长度:"+pageInfo.getPageSize());
        System.out.println("是否第一页:"+pageInfo.isIsFirstPage());
        System.out.println("是否最后一页:"+pageInfo.isIsLastPage());
    }

标签:配置文件,userMapper,代理,resourceAsStream,user,Mybatis,new,public,User
来源: https://blog.csdn.net/weixin_44560620/article/details/113819746