Sample Apache NiFi custom processor written in Groovy.

GroovyProcessor.groovy

class GroovyProcessor implements Processor {
    def REL_SUCCESS = new Relationship.Builder().name("success")
        .description("FlowFiles that were successfully processed").build()
    def ProcessorLog log

    @Override
    void initialize(ProcessorInitializationContext context) {
        log = context.getLogger()
    }

    @Override
    Set<Relationship> getRelationships() {
        return [REL_SUCCESS] as Set
    }

    @Override
    void onTrigger(ProcessContext context, ProcessSessionFactory sessionFactory) throws ProcessException {
        try {
            def session = sessionFactory.createSession()
            def flowFile = session.get()
            if (!flowFile) return
            def selectedColumns = ''
            flowFile = session.write(flowFile,
                    { inputStream, outputStream ->
                        String line
                        final BufferedReader inReader = new BufferedReader(new InputStreamReader(inputStream, 'UTF-8'))
                        line = inReader.readLine()
                        String[] header = line?.split(',')
                        selectedColumns = "${header[1]},${header[2]}"                  
                        while (line = inReader.readLine()) {
                            String[] cols = line.split(',')
                            outputStream.write("${cols[2].capitalize()} ${cols[3].capitalize()}\n".getBytes('UTF-8'))
                        }
                    } as StreamCallback)

            flowFile = session.putAttribute(flowFile, "selected.columns", selectedColumns)
            flowFile = session.putAttribute(flowFile, "filename", "split_cols_invoke.txt")

            // transfer
            session.transfer(flowFile, REL_SUCCESS)
            session.commit()
        }
        catch (e) {
            throw new ProcessException(e)
        }
    }

    @Override
    Collection<ValidationResult> validate(ValidationContext context) { return null }

    @Override
    PropertyDescriptor getPropertyDescriptor(String name) { return null }

    @Override
    void onPropertyModified(PropertyDescriptor descriptor, String oldValue, String newValue) { }

    @Override
    List<PropertyDescriptor> getPropertyDescriptors() { return null }

    @Override
    String getIdentifier() { return null }
}

processor = new GroovyProcessor()