Synchronizing an object shared across threads, but not accessed concurrently

Question

Let\'s say I have a shared object with field data. Multiple threads will share a reference to this object in order to access the field. The threads never access

Answer 1

Regarding the second part of your question: if you do not use volatile on your variable X, it is possible that a given thread will always use a locally cached version of the value of the variable. Your use of variable Y as a lock will work very well as a means to insure that the two threads do not write concurrently to X but can't guarantee that one of the threads won't be looking at stale data.

From the JLS: "A write to a volatile variable v synchronizes-with all subsequent reads of v by any thread". The way I read this is that the spec offers no guarantees about the reads to other variables besides v.