What guarantees does synchronized provide in Java?

Explains monitor mutual exclusion, visibility, reentrancy, lock identity, scope, and contention.

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A synchronized block acquires the monitor associated with a particular object, runs the block, and releases the monitor even if the block throws. Only one thread at a time can hold that same monitor, so protected code has mutual exclusion. Releasing the monitor happens-before a later acquisition of it, which provides visibility of prior writes. Java intrinsic locks are reentrant, so a thread that already owns the monitor can enter another synchronized section using it.

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Correct intrinsic-lock usage and understanding that synchronized protects a protocol only when all participants share it.

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synchronized provides mutual exclusion and memory-ordering guarantees around a specific object's monitor.
A synchronized block acquires the monitor associated with a particular object, runs the block, and releases the monitor even if the block throws. Only one thread at a time can hold that same monitor, so protected code has mutual exclusion. Releasing the monitor happens-before a later acquisition of it, which provides visibility of prior writes. Java intrinsic locks are reentrant, so a thread that already owns the monitor can enter another synchronized section using it.
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Lock identity is critical: two blocks synchronized on different objects do not protect each other. An instance synchronized method locks this, while a static synchronized method locks the object. I keep the critical section small but large enough to protect the full invariant, and I avoid calling slow or unknown external code while holding the lock. Synchronizing one accessor is insufficient if another path reads or writes the state without the same protocol.

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Synchronizing on different lock objects.
Holding a lock across slow I/O.
Protecting only some access paths.

What guarantees does synchronized provide in Java?

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