How to insert a row between two rows in an existing excel with HSSF (Apache POI)

Helper function to copy rows shamelessly adapted from here

import org.apache.poi.hssf.usermodel.*;
import org.apache.poi.ss.usermodel.Cell;
import org.apache.poi.ss.util.CellRangeAddress;

import java.io.FileInputStream;
import java.io.FileOutputStream;

public class RowCopy {

    public static void main(String[] args) throws Exception{
        HSSFWorkbook workbook = new HSSFWorkbook(new FileInputStream("c:/input.xls"));
        HSSFSheet sheet = workbook.getSheet("Sheet1");
        copyRow(workbook, sheet, 0, 1);
        FileOutputStream out = new FileOutputStream("c:/output.xls");
        workbook.write(out);
        out.close();
    }

    private static void copyRow(HSSFWorkbook workbook, HSSFSheet worksheet, int sourceRowNum, int destinationRowNum) {
        // Get the source / new row
        HSSFRow newRow = worksheet.getRow(destinationRowNum);
        HSSFRow sourceRow = worksheet.getRow(sourceRowNum);

        // If the row exist in destination, push down all rows by 1 else create a new row
        if (newRow != null) {
            worksheet.shiftRows(destinationRowNum, worksheet.getLastRowNum(), 1);
        } else {
            newRow = worksheet.createRow(destinationRowNum);
        }

        // Loop through source columns to add to new row
        for (int i = 0; i < sourceRow.getLastCellNum(); i++) {
            // Grab a copy of the old/new cell
            HSSFCell oldCell = sourceRow.getCell(i);
            HSSFCell newCell = newRow.createCell(i);

            // If the old cell is null jump to next cell
            if (oldCell == null) {
                newCell = null;
                continue;
            }

            // Copy style from old cell and apply to new cell
            HSSFCellStyle newCellStyle = workbook.createCellStyle();
            newCellStyle.cloneStyleFrom(oldCell.getCellStyle());
            ;
            newCell.setCellStyle(newCellStyle);

            // If there is a cell comment, copy
            if (oldCell.getCellComment() != null) {
                newCell.setCellComment(oldCell.getCellComment());
            }

            // If there is a cell hyperlink, copy
            if (oldCell.getHyperlink() != null) {
                newCell.setHyperlink(oldCell.getHyperlink());
            }

            // Set the cell data type
            newCell.setCellType(oldCell.getCellType());

            // Set the cell data value
            switch (oldCell.getCellType()) {
                case Cell.CELL_TYPE_BLANK:
                    newCell.setCellValue(oldCell.getStringCellValue());
                    break;
                case Cell.CELL_TYPE_BOOLEAN:
                    newCell.setCellValue(oldCell.getBooleanCellValue());
                    break;
                case Cell.CELL_TYPE_ERROR:
                    newCell.setCellErrorValue(oldCell.getErrorCellValue());
                    break;
                case Cell.CELL_TYPE_FORMULA:
                    newCell.setCellFormula(oldCell.getCellFormula());
                    break;
                case Cell.CELL_TYPE_NUMERIC:
                    newCell.setCellValue(oldCell.getNumericCellValue());
                    break;
                case Cell.CELL_TYPE_STRING:
                    newCell.setCellValue(oldCell.getRichStringCellValue());
                    break;
            }
        }

        // If there are are any merged regions in the source row, copy to new row
        for (int i = 0; i < worksheet.getNumMergedRegions(); i++) {
            CellRangeAddress cellRangeAddress = worksheet.getMergedRegion(i);
            if (cellRangeAddress.getFirstRow() == sourceRow.getRowNum()) {
                CellRangeAddress newCellRangeAddress = new CellRangeAddress(newRow.getRowNum(),
                        (newRow.getRowNum() +
                                (cellRangeAddress.getLastRow() - cellRangeAddress.getFirstRow()
                                        )),
                        cellRangeAddress.getFirstColumn(),
                        cellRangeAddress.getLastColumn());
                worksheet.addMergedRegion(newCellRangeAddress);
            }
        }
    }
}

For people who are looking to insert a row between two rows in an existing excel with XSSF (Apache POI), there is already a method "copyRows" implemented in the XSSFSheet.

import org.apache.poi.ss.usermodel.CellCopyPolicy;
import org.apache.poi.xssf.usermodel.XSSFSheet;
import org.apache.poi.xssf.usermodel.XSSFWorkbook;
import java.io.FileInputStream;
import java.io.FileOutputStream;

public class App2 throws Exception{
    public static void main(String[] args){
        XSSFWorkbook workbook = new XSSFWorkbook(new FileInputStream("input.xlsx"));
        XSSFSheet sheet = workbook.getSheet("Sheet1");
        sheet.copyRows(0, 2, 3, new CellCopyPolicy());
        FileOutputStream out = new FileOutputStream("output.xlsx");
        workbook.write(out);
        out.close();
    }
}

Referencing Qwerty's answer, if the destRow isnot null, sheet.shiftRows() will change the destRow's reference to the next row; so we should always create a new row:

if (destRow != null) {
  sheet.shiftRows(destination, sheet.getLastRowNum(), 1);
}
destRow = sheet.createRow(destination);

Referencing Qwerty's answer, you can avoid to inflate XL size by re-using cellStyle. And when the type is CELL_TYPE_BLANK, getStringCellValue returns "" instead of null.

private static void copyRow(Sheet worksheet, int sourceRowNum, int destinationRowNum) {
  // Get the source / new row
  Row newRow = worksheet.getRow(destinationRowNum);
  Row sourceRow = worksheet.getRow(sourceRowNum);

  // If the row exist in destination, push down all rows by 1 else create a new row
  if (newRow != null) {
    worksheet.shiftRows(destinationRowNum, worksheet.getLastRowNum(), 1);
  } else {
    newRow = worksheet.createRow(destinationRowNum);
  }

  // Loop through source columns to add to new row
  for (int i = 0; i < sourceRow.getLastCellNum(); i++) {
    // Grab a copy of the old/new cell
    Cell oldCell = sourceRow.getCell(i);
    Cell newCell = newRow.createCell(i);

    // If the old cell is null jump to next cell
    if (oldCell == null) {
      newCell = null;
      continue;
    }

    // Use old cell style
    newCell.setCellStyle(oldCell.getCellStyle());

    // If there is a cell comment, copy
    if (newCell.getCellComment() != null) {
      newCell.setCellComment(oldCell.getCellComment());
    }

    // If there is a cell hyperlink, copy
    if (oldCell.getHyperlink() != null) {
      newCell.setHyperlink(oldCell.getHyperlink());
    }

    // Set the cell data type
    newCell.setCellType(oldCell.getCellType());

    // Set the cell data value
    switch (oldCell.getCellType()) {
    case Cell.CELL_TYPE_BLANK:
      break;
    case Cell.CELL_TYPE_BOOLEAN:
      newCell.setCellValue(oldCell.getBooleanCellValue());
      break;
    case Cell.CELL_TYPE_ERROR:
      newCell.setCellErrorValue(oldCell.getErrorCellValue());
      break;
    case Cell.CELL_TYPE_FORMULA:
      newCell.setCellFormula(oldCell.getCellFormula());
      break;
    case Cell.CELL_TYPE_NUMERIC:
      newCell.setCellValue(oldCell.getNumericCellValue());
      break;
    case Cell.CELL_TYPE_STRING:
      newCell.setCellValue(oldCell.getRichStringCellValue());
      break;
    }
  }
}