Is Visibility Problem in Java caused by JVM or Hardware?

Previously I think the Visibility Problem is cause by CPU Cache for performance.

But I saw this article: http://www.ibm.com/developerworks/java/library/j-5things15/i开发者_Go百科ndex.html

In the paragraph 3. Volatile variables, it tells that Thread holds the cache, sounds like the cache is caused by JVM.

What's the answer? JVM or Hardware?

JVM gives you some weak guarantees. Compiler and Hardware cause you problems. :-)

When a thread reads a variable, it is not necessarily getting the latest value from memory. The processor might return a cached value. Additionally, even though the programmer authored code where a variable is first written and later read, the compiler might reorder the statements as long as it does not change the program semantics. It is quite common for processors and compilers to do this for performance optimization. As a result, a thread might not see the values it expects to see. This can result in hard to fix bugs in concurrent programs.

Most programmers are familiar with the fact that entering a synchronized block means obtaining a lock on a monitor that ensures that no other thread can enter the synchronized block. Less familiar but equally important are the facts that

(1) Acquiring a lock and entering a synchronized block forces the thread to refresh data from memory. (2) Upon exiting the synchronized block, data written is flushed to memory. http://www.javacodegeeks.com/2011/02/java-memory-model-quick-overview-and.html

See also JSR 133 (Java Memory Model and Thread Specification Revision) http://jcp.org/en/jsr/detail?id=133 It was released with JDK 1.5.