What is the Java Memory Model?

The JMM is the concurrency contract between Java code and the JVM. It allows the compiler, JIT and CPU to reorder and cache operations, then defines which observations are legal. Without a happens-before edge, one thread is not guaranteed to see another thread's writes.

Happens-before edges

• Program order within a single thread
• Unlocking a monitor happens-before a later lock of the same monitor
• A write to a volatile field happens-before a later read of that field
• Thread.start happens-before the started thread's actions, and thread actions happen-before join returning
• Transitivity: if A happens-before B and B happens-before C, then A happens-before C

public class Worker implements Runnable {
    private volatile boolean running = true;

    public void run() {
        while (running) {
            doWork();
        }
    }

    public void stop() {
        running = false;
    }

    private void doWork() {
    }
}

Do not confuse the JMM with runtime memory areas. Heap, stack, metaspace and the program counter are storage regions. The JMM is about the rules for visibility and ordering across threads. final fields also have special safe publication guarantees: a properly constructed object is visible with its final state intact.