Hi, I’m new on Motoko development, coming from the world of Java.
I wonder what is the best way to write a debug log and how to consult all messages if necessary ?
Debug.print("Is it possible, from the on-chain canister, to view this line later ?");
Thanks 
You might consider using something like canistergeek for that:
When developing locally, this will be printed to the replica console (the stuff you see if you run dfx start without a --background flag). On the live deployment, there’s currently no way to see your canister’s logs. For that, other tools (like the one mentioned by Paul) are necessary.
Ok ! Thanks for your answers