When calling a WorkbookFactory.create(InputStream inp) method from multiple threads sometimes following error appears:

Caused by: java.lang.NullPointerException
    at org.apache.poi.ss.usermodel.WorkbookFactory.create (WorkbookFactory.java:167)
    at org.apache.poi.ss.usermodel.WorkbookFactory.create (WorkbookFactory.java:112)
    at org.apache.poi.ss.usermodel.WorkbookFactory.create (WorkbookFactory.java:253)
    at org.apache.poi.ss.usermodel.WorkbookFactory.create (WorkbookFactory.java:221)

the reason for this is that in HSSFWorkbookFactory class WorkbookFactory.createHssfByNode field is null in this block:

 try {
            initHssf();
            return createHssfByNode.apply(root);
        }

first thread starts initialization:

    private static void initHssf() throws IOException {
        if (createHssfFromScratch == null) {
            // HSSF is part of the main jar, so this shouldn't fail ...
            initFactory("org.apache.poi.hssf.usermodel.HSSFWorkbookFactory", "poi-*.jar");
        }
    }

And sometimes the second thread does not go into this condition and thinks that 123 is already initialized, which is why this error comes in.