使用 Apache POI 将工作表附加到 Excel 文件 - NullPointerException 引起的 POIXMLException

这是我的代码：

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.util.ArrayList;

import org.apache.poi.openxml4j.exceptions.InvalidFormatException;
import org.apache.poi.xssf.usermodel.*;

public class testingsyncclass {
    public static void main(String[] args) throws IOException, InvalidFormatException{
        writeParametersTableToFile("C:/ENTERPRISE_TESTING", "XLSX_TEST", "SHEETNAME");
    }

    public static void writeParametersTableToFile(String fileLocation, String fileName, String tableLocator) throws InvalidFormatException{
        String[] columnHeaders = {"What", "is", "a", "computer", "?"};
        ArrayList<Object[]> xlsxFileData = new ArrayList<Object[]>();
        for(int i = 0; i < 10; i++){
            Object[] o = {"CoffeeCup", "AnglerJS", "Emerald", "C♭", "C--"};
            xlsxFileData.add(o);
        }
        try{
            if(fileName.contains(".xlsx")){
                fileName = fileName.replaceAll(".xlsx", "");
            }
            File file = new File(fileLocation);
            if(file.mkdirs() || file.exists()){
                File excelFile = new File(fileLocation  + "/" + fileName + ".xlsx");
                XSSFWorkbook workbook;
                XSSFSheet sheet;
                if(excelFile.exists()){
                    workbook = new XSSFWorkbook(excelFile);
                } else {
                    workbook = new XSSFWorkbook();
                }
                if(workbook.getSheet(tableLocator)==null){
                    sheet = workbook.createSheet(tableLocator);
                } else {
                    sheet = workbook.createSheet("NEW_TEST_SHEET");
                }
                XSSFRow firstRow = sheet.createRow(0);

                //write headers
                for(int i = 0; i < columnHeaders.length; i++){
                    XSSFCell cell = firstRow.createCell(i);
                    cell.setCellValue(columnHeaders[i]);
                }
                //write data
                for(int i = 0; i < xlsxFileData.size(); i++){
                    XSSFRow row = sheet.createRow(i+1);
                    for(int j = 0; j < xlsxFileData.get(i).length; j++){
                        XSSFCell cell = row.createCell(j);
                        String value = (String) xlsxFileData.get(i)[j];
                        cell.setCellValue(value);
                    }
                }
                FileOutputStream fileOut = new FileOutputStream(fileLocation  + "/" + fileName + ".xlsx");

                //write this workbook to an Outputstream.
                System.out.println(fileOut + " (fileout)");
                System.out.println(workbook + " (workbook)");
                workbook.write(fileOut);
                fileOut.flush();
                fileOut.close();
                workbook.close();

            } else {
                System.err.println(fileLocation + " was an invalid directory.");
            }
        } catch (IOException e){
            System.err.println("Something went wrong!");
            e.printStackTrace();
        }
    }
}

创建文件工作正常 - 文件按预期创建。但是，如果文件存在，我希望此方法创建一个工作表并将其添加到工作簿中。使用我目前拥有的代码，我得到以下异常：

Exception in thread "main" org.apache.poi.POIXMLException: java.lang.NullPointerException
at org.apache.poi.POIXMLDocument.getProperties(POIXMLDocument.java:168)
at org.apache.poi.POIXMLDocument.write(POIXMLDocument.java:246)
at testframework.testingsyncclass.writeParametersTableToFile(testingsyncclass.java:67)
at testframework.testingsyncclass.main(testingsyncclass.java:17)
Caused by: java.lang.NullPointerException

抛出此异常的行workbook.write(fileOut)仍然是 workbook 和 fileOut 都不为空。输出文件在第二次运行时也会损坏。

我究竟做错了什么？