调试时空指针（java.io.File）异常消失
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null pointer (java.io.File) exception dissappears while debugging
提问by July
I have an application which throws this error (happens only with xlsx-files):
我有一个抛出此错误的应用程序（仅发生在 xlsx 文件中）：
java.lang.NullPointerException
at java.io.File.<init>(File.java:222)
at de.mpicbg.tds.core.ExcelLayout.openWorkbook(ExcelLayout.java:75)
The method 'openWorkbook' looks like this:
'openWorkbook' 方法如下所示：
private void openWorkbook() throws IOException {
File excelFile = new File(fileName);
timestamp = excelFile.lastModified();
// open excel file
if (fileName.endsWith(".xlsx")) {
InputStream excelStream = new BufferedInputStream(new FileInputStream(excelFile));
this.workbook = new XSSFWorkbook((excelStream));
} else {
this.workbook = new HSSFWorkbook(new POIFSFileSystem(new FileInputStream(excelFile)));
}
}
If I execute everything in debug mode, everything goes smoothly and the error message does not appear. I don't have any explanation for this behaviour nor any idea how to fix it. Can anybody help?
如果我在调试模式下执行所有内容，一切都会顺利进行，并且不会出现错误消息。我对这种行为没有任何解释，也不知道如何解决它。有人可以帮忙吗？
回答by Peter Lawrey
The error message says your fileNameis null
错误消息说你fileName是null
If you cannot reproduce this when debugging you can add a log message at the start of you method.
如果在调试时无法重现这一点，您可以在方法开始时添加一条日志消息。
System.out.println("The fileName is `" + fileName+"`");
Instead of using a field which may or may not be set, I suggest you use a parameter.
我建议您使用参数，而不是使用可能设置或可能未设置的字段。
private void openWorkbook(String fileName) throws IOException {
assert fileName != null;
