I see what you're getting at. If the delivery mode is persistent then the
message must be persisted so that the message can be delivered reliably, such
as not being lost in the event of server restart. If the delivery mode is
non-persistent then the message need not be persisted and a lower level of
reliability is permitted. However there is nothing in the JMS spec that says
that non-persistent messages cannot be swapped to disk when the JVM becomes
short of memory.
However with GlassFish MQ the situation is simple. Non-persistent messages
are never written to disk. If a MQ broker contains so many messages that the
JVM is running low on memory it will take action to avoid running completely
out of memory. The exact behaviour is configurable, but can include
throttling back producers, moving messages to a dead message queue, or
discarding messages.
Nigel
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