I get the error:
java.lang.NoSuchMethodError: org.apache.xmlbeans.XmlOptions.setSaveAggressiveNamespaces()Lorg/apache/xmlbeans/XmlOptions;
at org.apache.poi.POIXMLDocumentPart.<clinit>(POIXMLDocumentPart.java:56)
from the line 56:
public static Workbook wb = new XSSFWorkbook();
I just had to re-link my library, but all i did was move all the files and reset the build path to where i moved the files to, and removed the old one.