Appending to a workbook using poi sxssfworkbook

I need to append rows to a sheet of a workbook. I am using org.apache.poi.xssf.streaming.SXSSFWorkbook but I am not able to achieve a low memory footprint. Following is the code:

import java.io.BufferedInputStream;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;

import org.apache.poi.ss.usermodel.Cell;
import org.apache.poi.ss.usermodel.Row;
import org.apache.poi.ss.usermodel.Sheet;
import org.apache.poi.xssf.streaming.SXSSFWorkbook;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;

public class ExcelHelper {
    public static void createExcelFileWithLowMemFootprint(
            ArrayList<HashMap<String, Object>> data,
            ArrayList<String> fieldNames, String fileName, int rowNum) {
        try {
            if (rowNum == 0) {
                // Creating a new workbook and writing the top heading here
                SXSSFWorkbook workbook = new SXSSFWorkbook(1000);
                Sheet worksheet = workbook.createSheet("Sheet 1");
                int i = 0;
                Iterator<String> it0 = fieldNames.iterator();
                Row row = worksheet.createRow(i);
                int j = 0;
                while (it0.hasNext()) {
                    Cell cell = row.createCell(j);
                    String fieldName = it0.next();
                    cell.setCellValue(fieldName);
                    j++;
                }
                rowNum++;
                FileOutputStream fileOut = new FileOutputStream(fileName);
                workbook.write(fileOut);
                fileOut.flush();
                fileOut.close();
            }
            InputStream fileIn = new BufferedInputStream(new FileInputStream(
                    fileName), 1000);
            SXSSFWorkbook workbook = new SXSSFWorkbook(
                    new XSSFWorkbook(fileIn), 1000);
            Sheet worksheet = workbook.getSheetAt(0);
            Iterator<HashMap<String, Object>> it = data.iterator();
            int i = rowNum;
            while (it.hasNext()) {
                Row row = worksheet.createRow(i);
                int j = 0;
                HashMap<String, Object> rowContent = it.next();
                Iterator<String> it1 = fieldNames.iterator();
                while (it1.hasNext()) {
                    Cell cell = row.createCell(j);
                    String key = it1.next();
                    Object o = rowContent.get(key);
                    if (o instanceof String) {
                        cell.setCellValue((String) o);
                    } else if (o instanceof Double) {
                        cell.setCellType(cell.CELL_TYPE_NUMERIC);
                        cell.setCellValue((Double) o);
                    }
                    j++;
                }
                i++;
            }
            fileIn.close();
            FileOutputStream fileOut = new FileOutputStream(fileName);
            workbook.write(fileOut);
            fileOut.flush();
            fileOut.close();
        } catch (FileNotFoundException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

I am appending to the file by passing the content in batches(so as to save on the jvm memory) and by incrementing the variable rowNum.

As per my understanding, when I am re-opening the file with

SXSSFWorkbook workbook = new SXSSFWorkbook(new XSSFWorkbook(fileIn),1000);

the constructor for XSSWorkbook reloads the complete file in memory, result in gc limit exceeded.

I went through http://poi.apache.org/spreadsheet/how-to.html but was unable to find a suitable solution to my usecase.

Can you guys please suggest how to fix this to achieve a low memory footprint for appending rows to the workbook?

SXSSFWorkbook doesn't need to be output then loaded back in for good memory management. Just write all of the data at once. If you try loading a whole workbook it stores that in memory, when writing it at once it uses storage space instead. Also 1000 is a lot to put in the constructor on some computers. If you want, try putting 100 or some other lower number in the constructor instead of 1000.