5250 suppress stdout?

I have an RPGLE *SRVPGM that calls a Java method. A third-party package in
that Java method writes to stdout.

In my unit tests from RDi, the stdout goes to a spooled file. In the
actual 5250 programs, stdout flashes up on the screen. I need to suppress
that.

As I read the docs, QIBM_USE_DESCRIPTOR_STDIO should do what I want, but it
has no effect. I'm setting QIBM_USE_DESCRIPTOR_STDIO=N before starting the
JVM.

Ref:
"If the environment variable QIBM_USE_DESCRIPTOR_STDIO is not set or is set
to any other value, file descriptors are not used for STDIN, STDOUT, or
STDERR. Instead, STDOUT and STDERR are routed to a spooled file that is
owned by the current job, and use of STDIN results in an IO exception."
https://www.ibm.com/support/knowledgecenter/ssw_ibm_i_73/rzaha/invocapi.htm


TIA