MyBatis-star-dao

MyBatis generic CRUD DAO based on ResultMapping only. Could be useful when DB structure is actively changed e.g. on project start.

Introduction

MyBatis is a great ORM flexible and powerful but the flexibility requires additional efforts. Normally you define mapping and define CRUD operations in the mapping INSERT/UPDATE/DELETE statements.
INSERT and UPDATE normally looks like this:

<insert id="insert">
  INSERT INTO USERS (ID, LOGIN, EMAIL, ...)
  VALUES (#{id},#{name},#{email}, ...)
</insert>

<update id="update">
  UPDATE USER SET
    LOGIN = #{name},
    EMAIL = #{email},
    ...
  WHERE ID = #{id}
</update>

Every time when you need to add a new field/column you have to go to the INSERT and UPDATE statements and manually add the new COLUMN and property for the column.
That's really annoying. To fix this the GenericDAO was created (yeah, one more:).

With the Generic DAO extension no need to manually add new column to INSERT and UPDATE statements. It is enough to define property/column mapping and INSERT/UPDATE statements get the new property on fly.

How to use

Initially MyBatis setup is the same. The mybatis-config.xml should be created and list of and should be defined.
Let's consider we have a User entity POJO and corresponding table in DB. The User class is a simple one with getters/setters.

public class User {
    private Long id;
    private String name;
    private String email;

    //getters/setters there
}

and now we need a mapping for the class. The mapping is almost the same for all entities (see below).

<mapper namespace="User">

    <!--a reference to parent base-mapper.xml where CRUD logic mapping is done-->
    <cache-ref namespace="GenericDAO" />

    <resultMap id="defaultResultMap" type="User">
        <id property="id" column="ID"/>
        <result property="name" column="LOGIN"/>
        <result property="email" column="EMAIL"/>
    </resultMap>

    <!--customize DB table name there to be used in CRUD-->
    <sql id="TableName">USERS</sql>

    <!--we cannot move selectAll to base-mapper.xml because User.defaultResultMap is not available there-->
    <select id="selectAll" resultMap="defaultResultMap">
        SELECT * FROM ${tableName}
    </select>

    <!--we cannot move selectById to base-mapper.xml because User.defaultResultMap is not available there-->
    <select id="selectById" resultMap="defaultResultMap">
        SELECT * FROM ${tableName}
        WHERE ID=#{entity}
    </select>

</mapper>

The mapping needs to define two things - table name sql and defaultResultMap where POJO properties and DB table columns are mapped.
The last step is to define UserDAO class and all the CRUD operations are available.

public class UserDao extends GenericDAO<User, Long> {

    public UserDao(org.apache.ibatis.session.SqlSession session, String mappingName) {
        super(session, mappingName);
    }
}

The dao can be used this way (See also UserDaoTest).

public class App {

    public static void main(String[] args) {
        InputStream inputStream = App.class.getResourceAsStream("/org/mybatis/mybatis-config.xml");
        SqlSessionFactory sqlSessionFactory = new SqlSessionFactoryBuilder().build(inputStream);
        SqlSession session = sqlSessionFactory.openSession();
        //initDB(session.getConnection());

        User user = new User(u -> {
            u.setLogin("login2");
            u.setEmail("login2@test.test");
        });

        UserDao dao = new UserDao(session, "User");
        int insertedCount = dao.insert(user);
        System.out.println("insertedCount=" + insertedCount);

        user = dao.getById(user.getId());

        user.setLogin("UPD:login2");
        user.setEmail("UPD:login2@test.test");
        int updatedCount = dao.update(user);
        System.out.println("updatedCount=" + updatedCount);

        int deletedCount = dao.delete(user);
        System.out.println("deletedCount=" + deletedCount);

        List<User> users = dao.getAll();
        System.out.println(users);
    }
}