Using HashMap dynamically for parameter mapping in mybatis

Okay so this is kinda re-posting of this question Inserting HashMap Values to a table using ibatis (but I am looking for a different way – the answer wont work for me)..

DB1GetStudentDataMapper.xml (this queries to one database)

<?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.testing.db1.DB1GetStudentDataMapper">

<select id="selectAllStudents" resultType="java.util.Map">
        SELECT STUDENT_CD, STUDENT_NM, PARENT_CD, CREATED_DATE
        FROM STUDENT
        WHERE STD_STATUS='ACT'
</select>

</mapper>

DB2InsertStudentMapper.xml (this queries to different database)

<?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.testing.db2.DB2InsertStudentMapper">
                
<insert id="insertStudent" parameterType="java.util.HashMap">
  INSERT INTO STUDENT
  <!-- dynamically select column names from hashmap -->
  (#{stdMap.keySet}) // this is not working - its coming as null
   <!-- dynamically select values for the above columns from hashmap -->
   VALUES (#{stdMap.values}) // this is not working - its coming as null
</insert>
           
</mapper>

DB2InsertStudentMapper.java

public interface TMODSBDataRefreshMapper {
    
    void insertStudent(@Param("stdMap") HashMap stdMap);
}

StudentDataProcess.java

public class Student {
    
    // I have defaultExecutorType as BATCH in my mapper config file
    
        private DB1GetStudentDataMapper db1Mapper; // Interface Mapper for first data source
        private DB2InsertStudentMapper db2Mapper; // Interface Mapper for second data source
        
        public processStudent() throws Exception {
            
            List<HashMap> rs = db1Mapper.selectAllStudents(); // Gets some 15k+ records
            for(int i =0; i < rs.size(); i++) { // so this will loop through 15k+ records
                HashMap result = rs.get(i);
                System.out.println(result.keySet()); // prints column names from select query [STUDENT_CD, STUDENT_NM, PARENT_CD, CREATED_DATE]
                System.out.println(result.values()); // prints above column values of first data set [1001, Mike, 5001, 2021-07-01]
                
                // All I am trying is to insert above 15k records into different database dynamically rather than creating POJO
                db2Mapper.insertStudent(result);
            }
            
        }
            
}

Note: Just for example I used 4 columns – I have some 150+ columns to work with..

PS: Please remember that this solution works better when you work with fewer columns – but doesn’t work well if you have bulk insert – it effects performance.

Answer

When iterating a map using <foreach />, the key and the value are assigned to the variables specified in index and item respectively.
So, your insert statement should look something like this.

<insert id="insertStudent">
  INSERT INTO STUDENT (
    <foreach collection="stdMap" index="col" separator=",">
      ${col}
    </foreach>
  ) VALUES (
    <foreach collection="stdMap" item="val" separator=",">
      #{val}
    </foreach>
  )
</insert>

• You must use ${} for column names and #{} for values. See the FAQ for the details.
• To iterates the map in the same order, you should use java.util.LinkedHashMap as the result type of <select />.