public void processCas(CAS aCAS) throws ResourceProcessException {
    String modelFileName = null;

    JCas jcas;
    try {
      jcas = aCAS.getJCas();
    } catch (CASException e) {
      throw new ResourceProcessException(e);
    }

    // retrieve the filename of the input file from the CAS
    FSIterator it = jcas.getAnnotationIndex(SourceDocumentInformation.type).iterator();
    File outFile = null;
    if (it.hasNext()) {
      SourceDocumentInformation fileLoc = (SourceDocumentInformation) it.next();
      File inFile;
      try {
        inFile = new File(new URL(fileLoc.getUri()).getPath());
        String outFileName = inFile.getName();
        if (fileLoc.getOffsetInSource() > 0) {
          outFileName += ("_" + fileLoc.getOffsetInSource());
        }
        outFileName += ".xmi";
        outFile = new File(mOutputDir, outFileName);
        modelFileName = mOutputDir.getAbsolutePath() + "/" + inFile.getName() + ".ecore";
      } catch (MalformedURLException e1) {
        // invalid URL, use default processing below
      }
    }
    if (outFile == null) {
      outFile = new File(mOutputDir, "doc" + mDocNum++ + ".xmi");     
    }
    // serialize XCAS and write to output file
    try {
      writeXmi(jcas.getCas(), outFile, modelFileName);
    } catch (IOException e) {
      throw new ResourceProcessException(e);
    } catch (SAXException e) {
      throw new ResourceProcessException(e);
    }
  }