No block will despawn unless it's a leaf decaying. There's also this unused block called "locked chest" that is never used in actual minecraft. It just has a missing texture...but that disappears eventually after you place it.
There's no vanilla way to lock a chest. But you can use a bukkit plugin to lock them. Or you could make it really hard for the person to get to the chest by making a safe room out of bedrock and a redstone lock.
I'm confused on what people hate so much about the new launcher...is it the console that appears? I find that to be very useful for troubleshooting things or writing scripts with chat.
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