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，并在连续调用导出时重复使用。但是我遇到了日期格式的问题——第一次调用效果很好，但是在所有连续的导出中，日期单元格都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，那么一切都很好。是否有任何理由对不同的工作表或工作簿使用相同的格式对象失败？
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();
}
}
}