¿Cómo descargo periódicamente las filas en Excel usando jxl mientras hago el cierre en el último?

Below is my code. I first create headers in init method. Then in pushData I fill the rows. The problem is once I do write and flush in init method nothing else comes in my excel sheet.

The rows that I would be writing to excel could be huge. The idea of using flush is not free the memory periodically.

Please tell me what mistake I am doing here. How do i achieve what I intent?l

import java.io.IOException;
import java.io.OutputStream;
import java.util.ArrayList;
import java.util.List;
import java.util.Map;

import jxl.Workbook;
import jxl.WorkbookSettings;
import jxl.write.Label;
import jxl.write.WritableSheet;
import jxl.write.WritableWorkbook;
import jxl.write.WriteException;
import jxl.write.biff.RowsExceededException;




public class ExportTranscriptDetailsToExcel implements IExportToFormat {
    private static final ILogger logger = ILabsPlatform.getLogger(ExportTranscriptDetailsToExcel.class.getName());
    OutputStream outputStream;
    List<String> labels;
    WritableWorkbook workbook;
    WritableSheet sheet0;

    public ExportTranscriptDetailsToExcel() {
        this.outputStream = null;
        workbook = null;
        sheet0 = null;
    }

    @Override
    public void init(String sheetName, List<String> labels, OutputStream outputStream) throws IOException,
            RowsExceededException, WriteException {
        this.outputStream = outputStream;
        this.labels = labels;

        WorkbookSettings workbookSettings = new WorkbookSettings();
        workbookSettings.setEncoding("Cp1252");
        workbook = Workbook.createWorkbook(outputStream, workbookSettings);

        sheet0 = workbook.createSheet(sheetName, 0);

        for (int i = 0; i < labels.size(); ++i) {
            Label label = new Label(i, 0, labels.get(i));
            sheet0.addCell(label);
        }
        workbook.write();
        outputStream.flush();
    }

    @Override
    public void pushDataRows(Object listOfResultRow) {
        if ((listOfResultRow == null)) {
            return;
        }
        String fieldName = null;
        String fieldValue = null;

        @SuppressWarnings("unchecked")
        ArrayList<Map<String, Object>> interactionMap = (ArrayList<Map<String, Object>>) listOfResultRow;
        try {
            int i = 1;// the data rows starts from row1
            for (Map<String, Object> element : interactionMap) {
                for (int j = 0; j < labels.size(); j++) {
                    fieldName = labels.get(j);
                    Object ob = element.get(fieldName);
                    if (ob != null) {
                        fieldValue = ob.toString();
                    }
                    if (fieldValue == null) {
                        fieldValue = "-";
                    }
                    System.out.println("***********************fieldName:" + fieldName);
                    System.out.println("***********************fieldValue:" + fieldValue);
                    Label label1 = new Label(j, i, fieldValue);
                    fieldValue = null;
                    sheet0.addCell(label1);
                }
                i++;
            }
        } catch (Exception e) {
            System.out.println(e.getMessage());
        }
        try {
            workbook.write();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
        try {
            outputStream.flush();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }

    @Override
    public void done() {
        try {
            workbook.close();
        } catch (WriteException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        } catch (IOException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }

    }
}