JMM(Java Memory Model)について

What is a memory model?

At the processor level, a memory model defines necessary and sufficient conditions for knowing that writes to memory by other processors are visible to the current processor, and writes by the current processor are visible to other processors.

What is the JMM?

The Java Memory Model describes what behaviors are legal in multithreaded code, and how threads may interact through memory. 
It describes the relationship between variables in a program and the low-level details of storing and retrieving them to and from memory or registers in a real computer system. 
It does this in a way that can be implemented correctly using a wide variety of hardware and a wide variety of compiler optimizations.

What does synchronization do?

mutual exclusion -- only one thread can hold a monitor at once

ensures that memory writes by a thread before or during a synchronized block are made visible in a predictable manner to other threads which synchronize on the same monitor.

Happen before

When one action happens before another, the first is guaranteed to be ordered before and visible to the second

The rules of this ordering are as follows:

Each action in a thread happens before every action in that thread that comes later in the program's order.

An unlock on a monitor happens before every subsequent lock on that same monitor.

A write to a volatile field happens before every subsequent read of that same volatile.

A call to start() on a thread happens before any actions in the started thread.

All actions in a thread happen before any other thread successfully returns from a join() on that thread.

What does volatile do?

ensure that after fields are written, they are flushed out of the cache to main memory

before a volatile field is read,the cache must be invalidated so that the value in main memory,not the local processor cache,is the one seen.次はvolatileの使い方

です.

class VolatileExample {
      int x = 0;
      volatile boolean v = false;
      public void writer() {
        x = 42;
        v = true;
      }

      public void reader() {
        if (v == true) {
          //uses x - guaranteed to see 42.
        }
    }
}

xはvolatitle修飾されていないのに、なぜ可視性が保証されているのかと聞かれるかもしれません.次のように解釈されます.

The write to v in writer releases the write to x to memory, and the read of v acquires that value from memory. 
Volatile or not, anything that was visible to thread A when it writes to volatile field f becomes visible to thread B when it reads f.

弟は前の記事で、volatileは命令の前にLOCK命令(メモリバリア)を追加することに相当します.

メモリにvolatileを書いた後にwrite-barrierを挿入し、releaseワークメモリにデータをプライマリメモリに挿入した.

volatileを読む前にread-barrierを挿入し、ホストデータをワークメモリにリフレッシュします.

参照
Javaメモリモデル--JMM(Java Memory Model)JSR 133(Java Memory Model)FAQ Javaメモリモデルを深く理解する(一)--基礎