java.lang.NullPointerException when appending information to an existing Excel file

问题

I'm trying to append information to an existing Excel file, but I keep receiving java.lang.NullPointerException. Please refer to the code and exception message below.

Code:

private WritableSheet appendingSheet;
private static File report;

public void AppendToDoc (String path) throws IOException, WriteException, BiffException {

    this.inputFile = path;
    report = new File(inputFile);

    Workbook appendingWorkbook = Workbook.getWorkbook(new File(inputFile));
    WritableWorkbook copy = Workbook.createWorkbook(new File("output.xls"),appendingWorkbook);
    appendingSheet = copy.getSheet("Sheet 1");      
}

public void WriteToDoc (int option, String testName, String execDate, String time, boolean status) throws RowsExceededException, WriteException, IOException, BiffException{        


    int startingRow = NumOfRows(this.inputFile) + 1; //I have a function which finds the number of existing rows in the existing document. It works. 
    Label label = new Label(1, startingRow, "hello", times);    
    appendingSheet.addCell(label);

}

Calling code:

AppendToDoc("C:/Users/smith/ExcelTestFile.xls");
WriteToDoc(2, "This is a test", "executed", timeStamp, true);

Exception message:

Exception in thread "main" java.lang.NullPointerException at jxl.write.biff.Styles.getFormat(Styles.java:214) at jxl.write.biff.CellValue.addCellFormat(CellValue.java:468) at jxl.write.biff.CellValue.setCellDetails(CellValue.java:282) at jxl.write.biff.LabelRecord.setCellDetails(LabelRecord.java:216) at jxl.write.biff.WritableSheetImpl.addCell(WritableSheetImpl.java:1199) at ExcelOperations.WriteToDoc(ExcelOperations.java:149) at ChequeImage.main(ChequeImage.java:174)

NOTE: The exception seems to be happening at the line "appendingSheet.addCell(label);".

Thanks for the help.

EDIT 1: The below image shows the contents of appendingSheet right before "appendingSheet.addCell(label)" is executed. I'm not exactly sure what it represents, but it certianly doesn't seem to be null.

回答1:

I can't say I'm familiar with Biff, but POI seems to work in a similar fashion. It looks as though you have created a workbook:

WritableWorkbook copy = Workbook.createWorkbook(new File("output.xls"),appendingWorkbook);

But then you don't create the sheet that you reference with:

appendingSheet = copy.getSheet("Sheet 1");

I'm guessing you're getting an NPE because "copy.getSheet("Sheet 1");" is returning null. Unlike when you simply open a workbook in excel, the API will likely not create that default sheet for you. I'm betting you'll have to create the sheet object before you're able to add cells.

回答2:

So...I've managed to solve the problem.

I can't really explain why, but after I redefined "copy" and "appendingWorkbook" (not appendingSheet) as global variables instead of local to AppendToDoc(); as well as added the line copy.write(), the program worked...

Thanks everyone for the help. I really appreciate it.

回答3:

I have face the following Exception while writing the data into an XLS file using JXL API.

Exception in thread "main" java.lang.NullPointerException
    at jxl.write.biff.Styles.getFormat(Styles.java:214)
    at jxl.write.biff.CellValue.addCellFormat(CellValue.java:468)
    at jxl.write.biff.CellValue.setCellDetails(CellValue.java:282)
    at jxl.write.biff.LabelRecord.setCellDetails(LabelRecord.java:216)
    at jxl.write.biff.WritableSheetImpl.addCell(WritableSheetImpl.java:1199)
    at com.java.report.XLS_JXLReport.main(XLS_JXLReport.java:31)

I have observed the following error is occurring due to WritableFont is empty. Source code from where the exception is raised is:

Styles.java:214
    // Do the same with the statically shared fonts
    if (format.getFont() == WritableWorkbook.ARIAL_10_PT) // Line 214
    {
      format.setFont(getArial10Pt());
    }

To avoid this error make sure to provide the Font Name.

Sample code using Java Excel API » 2.6.12

public class XLS_JXLReport {
    static String filePath = "C:/Yash/";
    public static void main(String[] args) throws IOException, JXLException {
        //Creates a writable workbook with the given file name
        jxl.write.WritableWorkbook workbook = jxl.Workbook.createWorkbook(new File(filePath+"CopyCell.xls"));
        WritableSheet sheet = workbook.createSheet("My Sheet", 0);
        
        // Create cell font and format
        WritableFont cellFont = new jxl.write.WritableFont(WritableFont.TIMES, 16);
            cellFont.setColour(jxl.format.Colour.BLUE);
        WritableCellFormat cellFormat = new WritableCellFormat(cellFont);
            cellFormat.setBackground(jxl.format.Colour.GRAY_25);
        
        //cellFormat = null; // To get the NullPointerException at Styles.java:214. Uncomment this line of code.
        
        // Create the label, specifying content and format
        Label label = new Label(1, 2, "ABCD", cellFormat);
        sheet.addCell(label);
        
        //Create copies of cell in a loop
        WritableCell copiedCell = null;
        for (int i = 0 ; i < 4 ; i ++) {
            copiedCell = label.copyTo(1, 4 + i); //Deep copy cell
            sheet.addCell(copiedCell);
        }
        
        closeWorkbook(workbook);
    }
    public static void closeWorkbook(WritableWorkbook workbook) throws IOException, JXLException {
        if (workbook == null)
            return;
        if (workbook.getNumberOfSheets() == 0) {
            workbook.createSheet("No data", 0); // otherwise pointer error
        }
        //Writes out the data held in this workbook in Excel format
        workbook.write(); 
        //Close and free allocated memory 
        workbook.close(); 
    }
}

---

JXL API^wiki allows users to read, write, create, and modify sheets in an Excel(.xls) workbook at runtime. It doesn't support .xlsx format.

• JXL API supports Excel documents with versions Excel 95, 97, 2000, XP, and 2003. These documents hold the extension .xls
• For .xlsx use Apache POI.

来源：https://stackoverflow.com/questions/17979749/java-lang-nullpointerexception-when-appending-information-to-an-existing-excel-f