Can multiple threads see writes on a direct mapped ByteBuffer in Java?

原始问题：

 

http://stackoverflow.com/questions/7002510/can-multiple-threads-see-writes-on-a-direct-mapped-bytebuffer-in-java

 

具体的回复：

 

Memory mapping with the JVM is just a thin wrapper around CreateFileMapping (Windows) or mmap (posix). As such, you have direct access to the buffer cache of the OS. This means that these buffers are what the OS considers the file to contain (and the OS will eventually synch the file to reflect this). So there is no need to call force() to sync between processes. The processes are already synched (via the OS - even read/write accesses the same pages). Forcing just synchs between the OS and the drive controller (there can be some delay between the drive controller and the physical platters, but you don't have hardware support to do anything about that). Regardless, memory mapped files are an accepted form of shared memory between threads and/or processes. The only difference between this shared memory and, say, a named block of virtual memory in Windows is the eventual synchronization to disk (in fact mmap does the virtual mem without a file thing by mapping /dev/null). Reading writing memory from multiple processes/threads does still need some synch, as processors are able to do out-of-order execution (not sure how much this interacts with JVMs, but you can't make presumptions), but writing a byte from one thread will have the same guarantees as writing to any byte in the heap normally. Once yo've written to it, every thread, and every process, will see the update (even through an open/read operation). For more info, look up mmap in posix (or CreateFileMapping for Windows, which was built almost the same way.