Gerando Relatório em Excel sem uso de API

import java.io.IOException;
import java.io.OutputStream;
import java.sql.ResultSet;
import java.sql.SQLException;
import javax.swing.table.AbstractTableModel;
import org.apache.poi.hssf.usermodel.HSSFCell;
import org.apache.poi.hssf.usermodel.HSSFCellStyle;
import org.apache.poi.hssf.usermodel.HSSFRow;
import org.apache.poi.hssf.usermodel.HSSFSheet;
import org.apache.poi.hssf.usermodel.HSSFWorkbook;
import org.apache.poi.hssf.util.HSSFColor;

/**
 *
 * @author Renato
 */
public final class ExcelUtil {

    public static void export(AbstractTableModel tableModel, OutputStream out, String sheetName) throws IOException {
        try {
            HSSFWorkbook workbook = new HSSFWorkbook();
            HSSFSheet sheet = workbook.createSheet(sheetName);

            HSSFRow row = sheet.createRow(0);
            for (int i = 0; i < tableModel.getColumnCount(); i++) {
                HSSFCell cell = row.createCell(i);
                cell.setCellValue(tableModel.getColumnName(i));
                HSSFCellStyle cellStyle = workbook.createCellStyle();
                cellStyle.setFillForegroundColor(HSSFColor.LIGHT_CORNFLOWER_BLUE.index);
                cellStyle.setFillPattern(HSSFCellStyle.SOLID_FOREGROUND);
                cell.setCellStyle(cellStyle);
            }

            for (int i = 0; i < tableModel.getRowCount(); i++) {
                row = sheet.createRow(i + 1);
                for (int j = 0; j < tableModel.getColumnCount(); j++) {
                    Object value = tableModel.getValueAt(i, j);
                    HSSFCell cell = row.createCell(j);
                    if (value.getClass().equals(Number.class)) {
                        cell.setCellType(HSSFCell.CELL_TYPE_NUMERIC);
                        cell.setCellValue((Double) value);
                    } else if (value.getClass().equals(String.class)) {
                        cell.setCellType(HSSFCell.CELL_TYPE_STRING);
                        cell.setCellValue((String) value);
                    }
                }
            }

            workbook.write(out);
        } finally {
            out.flush();
            out.close();
        }
    }

    //Esse método eu não testei ainda
    public static void export(ResultSet resultSet, OutputStream out, String sheetName) throws IOException, SQLException {
        try {
            HSSFWorkbook workbook = new HSSFWorkbook();
            HSSFSheet sheet = workbook.createSheet(sheetName);

            HSSFRow row = sheet.createRow(0);
            for (int i = 1; i <= resultSet.getMetaData().getColumnCount(); i++) {
                HSSFCell cell = row.createCell(i);
                cell.setCellValue(resultSet.getMetaData().getColumnName(i));
                HSSFCellStyle cellStyle = workbook.createCellStyle();
                cellStyle.setFillForegroundColor(HSSFColor.LIGHT_CORNFLOWER_BLUE.index);
                cellStyle.setFillPattern(HSSFCellStyle.SOLID_FOREGROUND);
                cell.setCellStyle(cellStyle);
            }

            while (resultSet.next()) {
                row = sheet.createRow(resultSet.getRow());
                for (int j = 1; j <= resultSet.getMetaData().getColumnCount(); j++) {
                    Object value = resultSet.getObject(resultSet.getMetaData().getColumnName(j));
                    HSSFCell cell = row.createCell(j - 1);
                    if (value.getClass().equals(Number.class)) {
                        cell.setCellType(HSSFCell.CELL_TYPE_NUMERIC);
                        cell.setCellValue((Double) value);
                    } else if (value.getClass().equals(String.class)) {
                        cell.setCellType(HSSFCell.CELL_TYPE_STRING);
                        cell.setCellValue((String) value);
                    }
                }
            }

            workbook.write(out);
        } finally {
            out.flush();
            out.close();
        }
    }

}