java - jExcelApi - 单元格日期格式重用不起作用

Question

我为 jExcelApi 类创建了一个包装器，以便轻松地将对象列表导出到 Excel。为了最大限度地减少对象创建，单元格格式被创建为静态字段，并在连续的导出调用中重复使用。但我对日期格式有问题 - 第一次调用效果很好，但在所有连续导出的日期单元格中，日期单元格都具有数字格式而不是日期格式。如果我为日期格式创建一个新对象而不是使用静态字段，一切都很好。对不同的工作表或工作簿使用相同的格式对象会失败吗？
这是代码，简化了异常处理，省略了其他数据类型，并且可能缺少一些导入:

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();
        }
    }
}

Answer 1

Telcontar 的回答很有帮助，因为它说明这是一个功能，而不是错误，但还不够，因为它没有提供任何指向常见问题解答或文档的链接。所以我做了一些研究，发现了一个 FAQ上面写着:

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.

所以答案是 - 格式不能在不同的工作表中重复使用，因为它们不是为以这种方式重复使用而设计的。