B4J Question Reading from custom file descriptor

Hi,

I'm trying to write a checkpassword Java console app for Dovecot.

I found that in a bash script I should read data from file descriptor 3
(eg:
1
2
read-d$'\0'-r-u3USER
read-d$'\0'-r-u3PASS
)

I'm know on how to read from stdin (reader.Initialize (sys.getField('in")), but how to read from FD 3 ?

Thanks