strit | Package strit introduces a new type | Functional Programming library

In order to figure out what visibility guarantees are provided, you need to understand the Java Memory Model a little better, and more specifically, what happens-before means in the context of the JMM. The JMM describes things that happen as actions, for example, normal reads and writes, volatile reads and writes, lock, unlock, etc.

There are a handful of rules in the JMM that establish when one action happens-before another action. The rules relevant in your case are the following:

The single thread rule: in a given thread, action A happens-before action B if A precedes B in program order.

The monitor lock rule (synchronized): An unlock of given monitor happens-before a subsequent lock on the same monitor.

It's important to know that happens-before is transitive, i.e. if hb(a, b) and hb(b, c), then hb(a, c).

In your example, one thread releases the monitor when exiting syncronizedMethodA(), and another thread subsequently acquires the monitor when entering syncronizedMethodB(). That's one happens-before relation. And since HB is transitive, actions performed in syncronizedMethodA() become visible for any thread that subsequently enters syncronizedMethodB().

On the other hand, no happens-before relation exists between the release of the monitor in syncronizedMethodA() and subsequent actions performed in notSynchronizedMethod() by another thread. Therefore, there are no guarantees that the writes in syncronizedMethodA() are made visible to another thread's reads in notSynchronizedMethod().