if im going to Osaka - from spore to haneda to itami by JAL. from haneda to itami do I need to exit the international airport?
Yes for JAL international->domestic transfers, you'll need to check-out your baggage from Haneda international and transfer to one of the 2 domestic terminals.
The good thing if you're taking JAL/ANA is their domestic check-in counters are located right outside the baggage claim belts within the international terminal - it'll save you the time and hassle of lugging your luggage to the domestic terminal. The free shuttle bus waiting area is also just one escalator away from the domestic check-in counters.
For your return journey from Osaka to Singapore however, your checked-in luggage should be all the way through to Singapore i.e. this time you don't have to check-out baggage and re-check-in again at Haneda domestic terminal.
From my experience, this applies to all JAL domestic->international transfers. I always wonder why it's like this.