UDF to read a file name and use some of its contents to map fields in target.

I have a scenario where I need to take the month and year from the file name and map it to suitable fields in the target. Ex - filename_apr2014 from this i should be able to read the month and year and map it. How to do this? I tried the udf -

public String getFileName(Container container){

DynamicConfiguration conf = (DynamicConfiguration) container.getTransformationParameters().get(StreamTransformationConstants.DYNAMIC_CONFIGURATION);

DynamicConfiguration conf = (DynamicConfiguration) container.getTransformationParameters().get(StreamTransformationConstants.DYNAMIC_CONFIGURATION);

DynamicConfigurationKey key = DynamicConfigurationKey.create("http://sap.com/xi/XI/System/File","FileName");

String ourSourceFileName = conf.get(key);

return ourSourceFileName;

got this error while testing the mapping -

Runtime exception when processing target-field mapping /ns0:MT_<<>>_Output/I<<>>; root message: Exception:[java.lang.NullPointerException: while trying to invoke the method com.sap.aii.mapping.api.DynamicConfiguration.get(com.sap.aii.mapping.api.DynamicConfigurationKey) of a null object loaded from a local variable at slot 2] in class <<>> method getSourceFileName[com.sap.aii.mappingtool.tf7.rt.Context@5b04fc0f]

See error logs for details