CAS consumer not working as expected - uima

Question

I have a CAS consumer AE which is expected to iterates over CAS objects in a pipeline, serialize them and add the serialized CASs to an xml file.
public class DataWriter extends JCasConsumer_ImplBase {
private File outputDirectory;
public static final String PARAM_OUTPUT_DIRECTORY = "outputDir";
#ConfigurationParameter(name=PARAM_OUTPUT_DIRECTORY, defaultValue=".")
private String outputDir;
CasToInlineXml cas2xml;
public void initialize(UimaContext context) throws ResourceInitializationException {
super.initialize(context);
ConfigurationParameterInitializer.initialize(this, context);
outputDirectory = new File(outputDir);
if (!outputDirectory.exists()) {
outputDirectory.mkdirs();
}
}
#Override
public void process(JCas jCas) throws AnalysisEngineProcessException {
String file = fileCollectionReader.fileName;
File outFile = new File(outputDirectory, file + ".xmi");
FileOutputStream out = null;
try {
out = new FileOutputStream(outFile);
String xmlAnnotations = cas2xml.generateXML(jCas.getCas());
out.write(xmlAnnotations.getBytes("UTF-8"));
/* XmiCasSerializer ser = new XmiCasSerializer(jCas.getCas().getTypeSystem());
XMLSerializer xmlSer = new XMLSerializer(out, false);
ser.serialize(jCas.getCas(), xmlSer.getContentHandler());*/
if (out != null) {
out.close();
}
}
catch (IOException e) {
throw new AnalysisEngineProcessException(e);
}
catch (CASException e) {
throw new AnalysisEngineProcessException(e);
}
}
I am using it inside a pipeline after all my annotators, but it couldn't read CAS objects (I am getting NullPointerException at jCas.getCas()). It looks like I don't seem to understand the proper usage of CAS consumer. I appreciate any suggestions.