Java memory model - what exactly is flushed to memory on crossing memory barrier?

Question

I think I understand the happened-before relationship for single variables. If I write a volatile field, all subsequent reads of this field will contain this new value. Writing a volatile crosses the memory barrier and flushes new value to main memory.

I still am not clear on what happens in all other cases - for instance Thread.start(), synchronized or new locks in java.util.concurrent. What does it mean that they also cross memory barrier? What data is flushed from local cache to main memory? In other words, what is the scope of the crossing?

Is everything always flushed? Now back to volatile, does it flush more than just the single volatile field?

Answer 1

When a memory barrier is crossed JVM synchronizes all locally (in context of the current thread) cached variables with the main memory. Besides that, it removes any locally cached data that is marked as dirty in the main memory.

Regarding the volatile - yes, it also synchronizes everything locally cached with the main memory, not only the single volatile field (since 1.5)

http://www.javamex.com/tutorials/synchronization_volatile_java_5.shtml