jExcelApi - cell date format reusing doesn't work

问题

I created a wrapper to jExcelApi classes to easily export lists of objects to Excel. To minimize object creation cell formats are created as static fields and reused in consecutive calls to export. But I have problems with Date format - the first call works good, but in all consecutive exports date cells have numeric format instead of date format. If I create a new object for date format instead of using static field, everything is fine. Is there any reason that using same format object for different sheets or workbooks fails?
Here is the code, with exception handling simplified, other data types omitted and probably some imports missing:

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();
        }
    }
}

回答1:

Telcontar's answer was helpful as stating it's a feature, not a bug, but was not enough as not giving any link to FAQ or doc. So I did some research and found out a FAQ that says:

also, it's important that you Do Not declare your cell formats as static. As a cell format is added to a sheet, it gets assigned an internal index number.

So the answer is - formats cannot be reused in different sheets because they are not designed to be reused this way.

回答2:

In jxl format objects can't be reused in multiple workbooks. i don't know why.

回答3:

It is actually worse than that. The fonts and formats implicitly rely on "the workbook". The question of which workbook illuminates the problem. They appear to need to be reassigned after creating subsequent workbooks.

    final WritableWorkbook workbook = Workbook.createWorkbook(response
            .getOutputStream());

    // We have to assign this every time we create a new workbook.
    bodyText = new WritableCellFormat(WritableWorkbook.ARIAL_10_PT);
    ...

The API should be changed so that the constructors require as an argument the workbook they relate to, or the constructors should be private and the fonts and formats should be obtained from the workbook.

    WritableCellFormat bodyText = new WritableCellFormat(workbook,
            WritableWorkbook.ARIAL_10_PT);

or

    WritableCellFormat bodyText = workbook.getCellFormat(
            WritableWorkbook.ARIAL_10_PT);

回答4:

you can try SmartXLS,the cell format can be reused anywhere you want.

来源：https://stackoverflow.com/questions/1460673/jexcelapi-cell-date-format-reusing-doesnt-work