Why opencsv capitalizing csv headers while writing to file

Actually, HeaderColumnNameMappingStrategy uses toUpperCase() for storing and retrieving the field names. In order to use custom field name you have to annotate you field with @CsvBindByName

@CsvBindByName(column = "Partner Code" )
private String partnerCode;

By default it will be capitalized to PARTNER CODE because of the above reason. so, in order to take control over it we have to write a class implementing HeaderColumnNameTranslateMappingStrategy. With csv 5.0 and java8 i have implemented like this

import java.lang.reflect.Field;
import java.util.HashMap;
import java.util.Map;

import com.opencsv.bean.CsvBindByName;
import com.opencsv.bean.HeaderColumnNameTranslateMappingStrategy;
import com.opencsv.exceptions.CsvRequiredFieldEmptyException;

public class AnnotationStrategy<T> extends HeaderColumnNameTranslateMappingStrategy<T> {
    Map<String, String> columnMap = new HashMap<>();
    public AnnotationStrategy(Class<? extends T> clazz) {

        for (Field field : clazz.getDeclaredFields()) {
            CsvBindByName annotation = field.getAnnotation(CsvBindByName.class);
            if (annotation != null) {

                    columnMap.put(field.getName().toUpperCase(), annotation.column());
            }
        }
        setType(clazz);      
    }

    @Override
    public String getColumnName(int col) {
        String name = headerIndex.getByPosition(col);
        return name;
    }

    public String getColumnName1(int col) {
        String name = headerIndex.getByPosition(col);
        if(name != null) {
            name = columnMap.get(name);
        }
        return name;
    }
    @Override
    public String[] generateHeader(T bean) throws CsvRequiredFieldEmptyException {
        String[] result = super.generateHeader(bean);
        for (int i = 0; i < result.length; i++) {
            result[i] = getColumnName1(i);
        }
        return result;
    }
}

It happens, because the code in HeaderColumnNameMappingStrategy uses toUpperCase() for storing and retrieving the field names.

You could use the HeaderColumnNameTranslateMappingStrategy instead and create the mapping by reflection.

    
    public class AnnotationStrategy extends HeaderColumnNameTranslateMappingStrategy
    {
        public AnnotationStrategy(Class<?> clazz)
        {
            Map<String,String> map=new HashMap<>();
            //To prevent the column sorting
            List<String> originalFieldOrder=new ArrayList<>();
            for(Field field:clazz.getDeclaredFields())
            {
                CsvBindByName annotation = field.getAnnotation(CsvBindByName.class);
                if(annotation!=null)
                {
                    map.put(annotation.column(),annotation.column());
                    originalFieldOrder.add(annotation.column());
                }
            }
            setType(clazz);
            setColumnMapping(map);
            //Order the columns as they were created
            setColumnOrderOnWrite((a,b) -> Integer.compare(originalFieldOrder.indexOf(a), originalFieldOrder.indexOf(b)));
        }
        
        @Override
        public String[] generateHeader(Object bean) throws CsvRequiredFieldEmptyException
        {
            String[] result=super.generateHeader(bean);
            for(int i=0;i<result.length;i++)
            {
                result[i]=getColumnName(i);
            }
            return result;
        }
    }

And, assuming that there is only one class of items (and always at least one item), the creation of beanWriter has to be expanded:

StatefulBeanToCsv beanWriter = builder.withSeparator(';')
    .withMappingStrategy(new AnnotationStrategy(projectInfos.iterator().next().getClass()))
    .build();