Mybatis系列之——dao层实现和动态SQL

1.9、Mybatis的dao层实现

1.9.1、传统开发方式

编写UserDao接口

public interface UserDao {
    
	List<User> findAll() throws IOException; 
}

编写UserDaoImpl实现

public class UserDaoImpl implements UserDao {
    public List<User> findAll() throws IOException{
        InputStream resourceAsStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        List<User> userList = sqlSession.selectList("userMapper.findAll");
        sqlSession.close();
        return userList; 
    } 
}

测试手动对dao进行实现

@Test
public void testTraditionDao() throws IOException {
    //手动对dao进行实现
    UserDao userDao = new UserDaoImpl();
    List<User> all = userDao.findAll();
    System.out.println(all);
}

1.9.2、代理开发方式

代理开发介绍：

采用Mybatis的代理开发实现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框架根据定义接口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

测试代理dao

public class UserService {
    public static void main(String[] args) throws IOException {
        //测试代理dao
        InputStream resourceAsStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();
        //获得MyBatis框架生成的UserMapper接口的实现类
        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        User user = userMapper.findById(1);
        System.out.println(user);
        sqlSession.close();
        
    }
}

1.10、MyBatis映射文件深入

1.10.1、动态sql语句

Mybatis 的映射文件中，前面我们的 SQL 都是比较简单的，有些时候业务逻辑复杂时，我们的 SQL是动态变化的此时在前面的学习中我们的 SQL 就不能满足要求了。

参考的官方文档，描述如下

动态 SQL 之

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。

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

当查询条件id和username都存在时，控制台打印的sql语句如下：

public class UserService {
    public static void main(String[] args) throws IOException {

        InputStream resourceAsStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();


        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
        User condition = new User();
        
        condition.setId(1);
        condition.setUsername("[email protected]");
        //condition.setPassword("123456");
        
        User user = userMapper.findByCondition(condition);

        System.out.println(user);
        sqlSession.close();

    }
}

动态SQL之

循环执行sql的拼接操作，例如: select *from user where id in (1,2,5)

<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>

public class UserService {
    public static void main(String[] args) throws IOException {

        InputStream resourceAsStream = Resources.getResourceAsStream("SqlMapConfig.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(resourceAsStream);
        SqlSession sqlSession = sqlSessionFactory.openSession();


        UserMapper userMapper = sqlSession.getMapper(UserMapper.class);
//        User condition = new User();
//
//        condition.setId(1);
//        condition.setUsername("[email protected]");
//        //condition.setPassword("123456");
//
//        User user = userMapper.findByCondition(condition);
//
//        System.out.println(user);

        List<Integer> ids = new ArrayList<Integer>();
        ids.add(1);
        ids.add(2);
        
        List<User> userList = userMapper.findByIds(ids);
        System.out.println(userList);

        sqlSession.close();

    }
}

SQL片段抽取

<sql id="selectUser">selecet * from usersql>

<select id="findByCondition" parameterType="user" resultType="user">
    <include refid="selectUser">include>
    <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>

<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>