Understanding Java Memory Model, Volatile, Atomicity, Visibility, and Ordering

The Java Memory Model (JMM) specifies that all variables reside in main memory and each thread has its own working memory, requiring reads and writes to go through these caches, which can cause visibility and ordering problems in multithreaded programs.

Examples illustrate dirty reads, non‑atomic increments, and instruction reordering, showing why the volatile keyword alone cannot guarantee atomicity.

Three core concepts of concurrent programming—atomicity, visibility, and ordering—are defined, with Java guaranteeing atomicity only for simple reads/writes of primitive types, while synchronized, Lock, and atomic classes provide stronger guarantees.

The volatile keyword ensures that writes are immediately visible to other threads and prevents certain reordering, but it does not make compound actions atomic; proper use cases and limitations are discussed.

Implementation details of volatile are described, including memory barriers, cache invalidation, and how the JVM emits lock‑prefix instructions to enforce ordering.

Typical scenarios such as status flags, thread termination, and double‑checked locking in singleton patterns are presented, with code examples wrapped in ... blocks.

Code snippets demonstrate the problems and solutions:

i = 10++;

x = 10; // statement 1
 y = x; // statement 2
 x++; // statement 3
 x = x + 1; // statement 4

i = 9;

// Thread 1
int i = 0;
while(!stop){
    doSomething();
}
// Thread 2
stop = true;

// Example with volatile flag
volatile boolean flag = false;
while(!flag){
    doSomething();
}
// In another thread
flag = true;

class Singleton {
    private volatile static Singleton instance = null;
    private Singleton() {}
    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

public class Test {
    public volatile int inc = 0;
    public void increase() { inc++; }
    public static void main(String[] args) {
        final Test test = new Test();
        for (int i = 0; i < 10; i++) {
            new Thread(){
                public void run() {
                    for (int j = 0; j < 1000; j++) test.increase();
                }
            }.start();
        }
        while (Thread.activeCount() > 1) Thread.yield();
        System.out.println(test.inc);
    }
}