Stworzyłem opakowanie do klas jExcelApi w celu łatwego eksportowania list obiektów do Excela. Aby zminimalizować tworzenie obiektów, formaty komórek są tworzone jako pola statyczne i wykorzystywane ponownie w kolejnych wywołaniach eksportu. Ale mam problemy z formatem daty - pierwsze połączenie działa dobrze, ale we wszystkich kolejnych eksportach komórki daty mają format liczbowy zamiast formatu daty. Jeśli utworzę nowy obiekt dla formatu daty zamiast używania pola statycznego, wszystko jest w porządku. Czy jest jakiś powód, dla którego użycie tego samego obiektu formatu dla różnych arkuszy lub skoroszytów kończy się niepowodzeniem?
Oto kod z uproszczoną obsługą wyjątków, pominięto inne typy danych i prawdopodobnie brakuje niektórych importów:

ExcelCellGenerator.java:

import jxl.write.WritableCell;

public interface ExcelCellGenerator<T> {
 WritableCell getCell(int col, int row, T arg);
}

ExcelCellGeneratorFactory.java:

import jxl.write.DateFormat;
import jxl.write.DateTime;
import jxl.write.Label;
import jxl.write.NumberFormat;
import jxl.write.NumberFormats;
import jxl.write.WritableCell;
import jxl.write.WritableCellFormat;
import ExcelExporter.DateTimeExtractor;

final class ExcelCellGeneratorFactory {
 private ExcelCellGeneratorFactory() {}

 private static final WritableCellFormat DATE_FORMAT = new WritableCellFormat ( new DateFormat ("dd MMM yyyy hh:mm:ss")); // reusing this field fails

 static public <T> ExcelCellGenerator<T> createDateCellGenerator(final DateTimeExtractor<T> extractor) {
  return new ExcelCellGenerator<T>() {
   public WritableCell getCell(int col, int row, T arg) {
    return new DateTime(col, row, extractor.extract(arg), DATE_FORMAT);
                // if there is new WritableCellFormat(new DateFormat(...)) instead of DATE_FORMAT, works fine
   }

  };
 }
}

ExcelExporter.java:

import jxl.Workbook;
import jxl.write.DateFormat;
import jxl.write.DateTime;
import jxl.write.Label;
import jxl.write.NumberFormat;
import jxl.write.WritableCellFormat;
import jxl.write.WritableSheet;
import jxl.write.WritableWorkbook;

public class ExcelExporter<T> {
 // describe a column in Excel sheet
 private static class ColumnDescription<T> {
  public ColumnDescription() {}

        // column title
        private String title;
        // a way to generate a value given an object to export
        private ExcelCellGenerator<T> generator;
 }

    // all columns for current sheet
 private List<ColumnDescription<T>> columnDescList = new ArrayList<ColumnDescription<T>>();

    // export given list to Excel (after configuring exporter using addColumn function
    // in row number rowStart starting with column colStart there will be column titles
    // and below, in each row, extracted values from each rowList element
 public byte[] exportList(int rowStart, int colStart, List<? extends T> rowList) {
  final ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
  WritableWorkbook workbook;
  try {
   workbook = Workbook.createWorkbook(outputStream);
  } catch (IOException e) {
   e.printStackTrace();
  }
  final WritableSheet sheet = workbook.createSheet("Arkusz1", 0);

  int currRow = rowStart;
  try {
   int currCol = colStart;
   for (ColumnDescription<T> columnDesc : columnDescList) {
    final Label label = new Label(currCol, currRow, columnDesc.title);
    sheet.addCell(label);
    currCol++;
   }
   currRow++;

   for (T object : rowList) {
    currCol = colStart;
    for (ColumnDescription<T> columnDesc : columnDescList) {
     sheet.addCell(columnDesc.generator.getCell(currCol, currRow, object));
     currCol++;
    }
    currRow++;
   }

   workbook.write();
  } catch (Exception e) {
   e.printStackTrace();
  } finally {
   try {
    workbook.close();
   } catch (Exception e) {
    e.printStackTrace();
   }
  }

  return outputStream.toByteArray();
 }

    // configure a Date column
    public ExcelExporter<T> addColumn(String title, DateTimeExtractor<T> extractor) {
  final ColumnDescription<T> desc = new ColumnDescription<T>();
  desc.title = title;
  desc.generator = ExcelCellGeneratorFactory.createDateCellGenerator(extractor);
  columnDescList.add(desc);
  return this;
 }

    // and test that shows the problem
    public static void main(String []args) {
        final ExcelExporter<Date> exporter = new ExcelExporter<Date>();
        exporter.addColumn("Data", new DateTimeExtractor<Date>() {
            public Date extract(Date date) {
                return date;
        }});

        // this file looks OK
        FileOutputStream ostream = new FileOutputStream("C:\\tmp\\test1.xls");
        try {
            ostream.write(exporter.exportList(0, 0, Collections.singletonList(new Date())));
        } finally {
            ostream.close();
        }

        // but in this file date is shown in cell with numeric format
        final ExcelExporter<Date> exporter2 = new ExcelExporter<Date>();
        exporter2.addColumn("Data", new DateTimeExtractor<Date>() {
            public Date extract(Date date) {
                return date;
        }});

        ostream = new FileOutputStream("C:\\tmp\\test2.xls");
        try {
            ostream.write(exporter2.exportList(0, 0, Collections.singletonList(new Date())));
        } finally {
            ostream.close();
        }
    }
}