ResultSetMetaData using MyBatis - mybatis

Question

How can I get ResultSetMetaData using MyBatis. I can't use INFORMATION_SCHEMA.columns as I have a complex dynamic query joins with multiple tables. For example, I need Number of records(this one I can get based on list size), List of Columns and data type of each column. I browsed a lot and haven't got the right ways to use it.
Please suggest the right way to get ResultSetMetaData either with TypeHandler or some other options with small example ?
I have 2 scenarios.
Scenario 1:
My Query reads a file using Abinitio QueryIt service (It consider files similar to table and can use SQL query to read). Those files are received from various up streams and not fixed set of columns. It can be N number of columns and any data type. Once my query reads the data from file, it will be sent to UI to display in Grid with formatted output based on data type (Integer, Double, String, Date) for user view. To build the Grid model, I need to know number of columns and datatype as well (for formatting). Until I know Number of Columns and Data Type of each column, I can’t build the grid model. Return type of my mapper will be List<Map<String, Object>>.
Scenario 2(for different process): Similar to above method, query reads data from multiple tables instead of reading from file based on criteria selection from UI. Here also number of columns are dynamic for grid based on selection from UI. Hence, I need number of columns and data type of each column. Return type is same as above type.
Thanks in advance.

Answer 1

For complicated queries, you can add custom mappers. I typically create extended mappers in separate directories so they don't get replaced when you generate again. Also, this keeps all custom SQL together.
<?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.company.core.app.db.mapper.custom.SomethingExtendedMapper">
<resultMap id="SomethingMap" type="com.company.core.app.db.Something">
<result column="ID" jdbcType="INTEGER" property="somethingId" />
<result column="LAST_CHANGE_DATE" jdbcType="DATE" property="lastChangeDate"
...
</resultMap>
<select id="getSomething" resultMap="SomethingMap" parameterType="Integer">
select
*
from
something
join something else...
WHERE SOMETHING_ID = #{id}
</select>
</mapper>
Then this would be the interface:
public interface SomethingExtendedMapper {
public List<Something> getSomething(#Param("id") Integer id);
}

Answer 2

You can write a custom TypeHanlder that extends BaseTypeHandler, which would give you access to ResultSetMetaData. I have previously written such a handler to convert a Date to a LocalDate. This is a bit of a hack to your answer because the mapper would not directly return a List<Map<String, Object>>, but that could be a property on the object that you are returning.
#MappedTypes(LocalDate.class)
public class DbLocalDateTypeHandler extends BaseTypeHandler<LocalDate> {
#Override
public void setNonNullParameter(PreparedStatement ps, int i, LocalDate parameter, JdbcType jdbcType) throws SQLException {
if (parameter == null) {
ps.setDate(i, null);
} else {
ps.setDate(i, Date.valueOf(parameter));
}
}
#Override
public LocalDate getNullableResult(ResultSet rs, String columnName) throws SQLException {
Date date = rs.getDate(columnName);
if (date != null) {
return date.toLocalDate();
}
return null;
}
#Override
public LocalDate getNullableResult(ResultSet rs, int columnIndex) throws SQLException {
Date date = rs.getDate(columnIndex);
if (date != null) {
return date.toLocalDate();
}
return null;
}
#Override
public LocalDate getNullableResult(CallableStatement cs, int columnIndex) throws SQLException {
Date date = cs.getDate(columnIndex);
if (date != null) {
return date.toLocalDate();
}
return null;
}
}
Then in your resultMap you just need to reference that handler:
<result column="CREATE_DATE" jdbcType="DATE" property="createDate" typeHandler="com.company.core.framework.db.DbLocalDateTypeHandler"/>
Lastly, if you do not need the ResultSetMetaData you could look into creating a custom ObjectFactory.
e.g.
How to return an Optional from MyBatis query
Helpful MyBatis Docs:
http://www.mybatis.org/mybatis-3/configuration.html#typeHandlers
http://www.mybatis.org/mybatis-3/configuration.html#objectFactory