Does Java volatile prevent caching or enforce write-through caching?

Question

I'm trying to understand Java's volatile keyword with respect to writing to a volatile atomic variable in a multithreaded program with CPU caches.

I've read several tutorials and the Java Language Specification, particularly section 17.4.5 on "happens-before ordering". My understanding is that when a thread writes a new value to a volatile variable, the updated value must be visible to other threads reading that variable. To me, these semantics can be implemented in one of two ways:

1. Threads can cache a volatile variable in the CPU cache, but a write to the variable in the cache must be flushed immediately to main memory. In other words, the cache is write-through.

2. Threads can never cache a volatile variable and must read and write such variables in main memory.

Approach 1 is mentioned in this tutorial (http://tutorials.jenkov.com) that says:

By declaring the counter variable volatile all writes to the counter variable will be written back to main memory immediately.

Approach 2 is mentioned in a Stackoverflow question "Volatile variable in Java" and also this tutorial that says:

The value of this variable will never be cached thread-locally: all reads and writes will go straight to "main memory"

Which one is the correct approach used in Java?

Related Stackoverflow questions which do not answer my question:

Volatile variable in Java

Does Java volatile read flush writes, and does volatile write update reads

Java volatile and cache coherence

Answer 1

The guarantees are only what you see in the language specification. In theory, writing a volatile variable might force a cache flush to main memory, or it might not, perhaps with a subsequent read forcing the cache flush or somehow causing transfer of the data between caches without a cache flush. This vagueness is deliberate, as it permits potential future optimizations that might not be possible if the mechanics of volatile variables were spelled out in more detail.

In practice, with current hardware, it probably means that, absent a coherent cache, writing a volatile variable forces a cache flush to main memory. With a coherent cache, of course, such a flush isn't needed.