OpenCSV: How to create CSV file from POJO with custom column headers and custom column positions?

Question

I have created a MappingsBean class where all the columns of the CSV file are specified. Next I parse XML files and create a list of mappingbeans. Then I write that data int

Answer 1

I wanted to achieve bi-directional import/export - to be able to import generated CSV back to POJO and visa versa.

I was not able to use @CsvBindByPosition for this, because in this case - ColumnPositionMappingStrategy was selected automatically. Per documents: this strategy requires that the file does NOT have a header.

What I've used to achieve the goal:

HeaderColumnNameMappingStrategy
mappingStrategy.setColumnOrderOnWrite(Comparator writeOrder)

CsvUtils to read/write csv

import com.opencsv.CSVWriter;
import com.opencsv.bean.*;
import org.springframework.web.multipart.MultipartFile;

import java.io.*;
import java.util.List;

public class CsvUtils {
    private CsvUtils() {
    }

    public static  String convertToCsv(List entitiesList, MappingStrategy mappingStrategy) throws Exception {
        try (Writer writer = new StringWriter()) {
            StatefulBeanToCsv beanToCsv = new StatefulBeanToCsvBuilder(writer)
                    .withMappingStrategy(mappingStrategy)
                    .withQuotechar(CSVWriter.NO_QUOTE_CHARACTER)
                    .build();
            beanToCsv.write(entitiesList);
            return writer.toString();
        }
    }

    @SuppressWarnings("unchecked")
    public static  List convertFromCsv(MultipartFile file, Class clazz) throws IOException {
        try (Reader reader = new BufferedReader(new InputStreamReader(file.getInputStream()))) {
            CsvToBean csvToBean = new CsvToBeanBuilder(reader).withType(clazz).build();
            return csvToBean.parse();
        }
    }
}

POJO for import/export

public class LocalBusinessTrainingPairDTO {
    //this is used for CSV columns ordering on exporting LocalBusinessTrainingPairs
    public static final String[] FIELDS_ORDER = {"leftId", "leftName", "rightId", "rightName"};

    @CsvBindByName(column = "leftId")
    private int leftId;

    @CsvBindByName(column = "leftName")
    private String leftName;

    @CsvBindByName(column = "rightId")
    private int rightId;

    @CsvBindByName(column = "rightName")
    private String rightName;
    // getters/setters omitted, do not forget to add them
}

Custom comparator for predefined String ordering:

public class OrderedComparatorIgnoringCase implements Comparator {
    private List predefinedOrder;

    public OrderedComparatorIgnoringCase(String[] predefinedOrder) {
        this.predefinedOrder = new ArrayList<>();
        for (String item : predefinedOrder) {
            this.predefinedOrder.add(item.toLowerCase());
        }
    }

    @Override
    public int compare(String o1, String o2) {
        return predefinedOrder.indexOf(o1.toLowerCase()) - predefinedOrder.indexOf(o2.toLowerCase());
    }
}

Ordered writing for POJO (answer to initial question)

public static void main(String[] args) throws Exception {
     List localBusinessTrainingPairsDTO = new ArrayList<>();
     LocalBusinessTrainingPairDTO localBusinessTrainingPairDTO = new LocalBusinessTrainingPairDTO();
     localBusinessTrainingPairDTO.setLeftId(1);
     localBusinessTrainingPairDTO.setLeftName("leftName");
     localBusinessTrainingPairDTO.setRightId(2);
     localBusinessTrainingPairDTO.setRightName("rightName");

     localBusinessTrainingPairsDTO.add(localBusinessTrainingPairDTO);

     //Creating HeaderColumnNameMappingStrategy
     HeaderColumnNameMappingStrategy mappingStrategy = new HeaderColumnNameMappingStrategy<>();
     mappingStrategy.setType(LocalBusinessTrainingPairDTO.class);
     //Setting predefined order using String comparator
     mappingStrategy.setColumnOrderOnWrite(new OrderedComparatorIgnoringCase(LocalBusinessTrainingPairDTO.FIELDS_ORDER));
     String csv = convertToCsv(localBusinessTrainingPairsDTO, mappingStrategy);
     System.out.println(csv);
}

Read exported CSV back to POJO (addition to original answer)

Important: CSV can be unordered, as we are still using binding by name:

public static void main(String[] args) throws Exception {
    //omitted code from writing
    String csv = convertToCsv(localBusinessTrainingPairsDTO, mappingStrategy);

    //Exported CSV should be compatible for further import
    File temp = File.createTempFile("tempTrainingPairs", ".csv");
    temp.deleteOnExit();
    BufferedWriter bw = new BufferedWriter(new FileWriter(temp));
    bw.write(csv);
    bw.close();
    MultipartFile multipartFile = new MockMultipartFile("tempTrainingPairs.csv", new FileInputStream(temp));

    List localBusinessTrainingPairDTOList = convertFromCsv(multipartFile, LocalBusinessTrainingPairDTO.class);
}

To conclude:

1. We can read CSV to POJO, regardless of column order - because we are using @CsvBindByName
2. We can control columns order on write using custom comparator