Rebellion through sex seems pretty customary, but rebellion by keeping a journal?

I think that the journal is important for Winston to keep his sanity. He has to express the myriad of frustrations that have been boiling inside him. Winston is the first to acknowledge the distinct possibility that nobody will read his journal. The act of writing it is a rebellion even if it is symbolic and destined to be burned in obsolescence. The journal becomes a catalyst for Winston's greater rebellion. It gives him context about his tortured life that enables him to continue searching for some kind of emotional and intellectual redemption.