Lock Visibility: Releasing a Lock Publishes Every Write

The synchronized keyword makes two separate guarantees. The familiar one is exclusion: one thread at a time. The second is visibility: when a thread leaves a synchronized block, everything it wrote inside is published, and a thread that then enters a block on the same lock sees all of it. The second is what makes locking a correctness tool rather than a queueing one.

The guarantee covers every field the thread wrote, including ones the block was never about. A lock is therefore a way to publish a whole set of changes at once, which volatile, the modifier that publishes each write to a single field, can do for one field and nothing else does for several.

A thread that builds an order, sets its total and marks it ready, all inside one lock, hands another thread a consistent order: the reader, taking the same lock, sees the total and the flag together, never one without the other.

So the two guarantees travel together. If you lock to write a field, you have to lock to read it too; otherwise you have paid for the exclusion and thrown away the visibility.