We propose a Java library BiXJ for bidirectional XML transformation. A bidirectional transformation generates target XML documents from source XML documents in forward transformations, and updates source documents in backward transformations by reflecting back modifications on target documents. The benefit of using BiXJ is that users can get the corresponding backward transformation automatically just by writing one forward transformation. BiXJ has addressed several limitations of the existing bidirectional transformation languages, and can be used for general purpose XML processing. For example, bidirectional XPath expressions written in BiXJ can be used to locate and extract data from XML documents. To validate the usability and expressiveness of BiXJ, we have bidirectionalized some typical examples of XQuery and XSLT with this library. The results of these experiments are promising.