Use Spring JdbcTemplate to easily read data, without Spring Data JPA, and map columns using Column DTO mappings, using a secondary database connection

ColumnRowMapper.java

/** Based nearly completely on this same code from https://stackoverflow.com/a/52534584/1469525 */
@Slf4j
public class ColumnRowMapper<T> extends BeanPropertyRowMapper<T> {

  private ColumnRowMapper(final Class<T> mappedClass) { super(mappedClass); }

  @Override
  protected String underscoreName(final String name) {
    final Column annotation;
    final String columnName;
    Field declaredField = null;

    try {
      declaredField = getMappedClass().getDeclaredField(name);
    }
    catch (NoSuchFieldException | SecurityException e) {
      log.warn("field «{}» not found in «{}».", name, getMappedClass());
    }

    if (declaredField == null || (annotation = declaredField.getAnnotation(Column.class)) == null
        || StringUtils.isEmpty(columnName = annotation.name())) {
      return super.underscoreName(name);
    }

    return StringUtils.lowerCase(columnName);
  }
}

SecondaryDBConfig.java

@Configuration
public class SecondaryDBConfig {
  @ConfigurationProperties(prefix = "secondary-db.datasource")
  @Bean
  public DataSource secondaryDataSource() { return DataSourceBuilder.create().build(); }
  
  @Bean
  public NamedParameterJdbcTemplate secondaryJdbcTemplate() {
    return new NamedParameterJdbcTemplate(secondaryDataSource());
  }
}

UserInfoDto.java

@Data
public class UserInfoDto {
  @Column("usr_name_sys")
  private String username;
  
  @Column("dis_first_last_name")
  private String displayName;
  
  @Column("cityInState")
  private String city;
}

UserSQLRepository.java

@Repository
public class SecondarySQLRepository {
  private final NamedParameterJdbcTemplate secondaryJdbcTemplate;
  
  public SecondarySQLRepository(@Qualifier("secondaryJdbcTemplate") NamedParameterJdbcTemplate secondaryJdbcTemplate) {
    this.secondaryJdbcTemplate = secondaryJdbcTemplate;
  }
  
  public List<UserInfoDto> findUsersByCity(String state) {
    String sql = "SELECT usr_name_sys, dis_first_last_name, cityInState FROM user_info WHERE state = :state;";
    Map<String, Object> params = Map.of("state", state);
    return secondaryJdbcTemplate.query(sql, params, new ColumnRowMapper<>(UserInfoDto.class));
  }

zz-application.properties

secondary-db.datasource.driver-class-name=oracle.rdb.jdbc.rdbThin.Driver
secondary-db.datasource.jdbc-url=${DB2_URL:jdbc:rdbThin://example.com:1777/RDB_JDBC_BLAH}
secondary-db.datasource.username=${DB2_USER:dev_user}
secondary-db.datasource.password=${DB2_PASS:dev_pass}

#(only named zz to push to bottom of the gist list)