Apache Nifi read flowfile content

Question

I would like to access the content of a flow file that should contain binary data but when I execute this code:

public void onTrigger(final ProcessContext context, final ProcessSession session) throws ProcessException {

       FlowFile ff = session.get(); 
       Logger.info("Flowfile size : " + ff.getSize());

       session.read(ff, new InputStreamCallback() {
       @Override
       public void process(InputStream in) throws IOException {
          Logger.info("InputStream size : " + in.available());
          …
       }
    }
}

I have this:

Flowfile size : 46662
InputStream size : 0

As a result, I don’t have any data to read.

Can someone guide me on how to read correctly the flow file content?