Why opencsv capitalizing csv headers while writing to file

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();