Java Memory Model Deep Dive

[ghost]

Q: Explain happens-before relationship with real multithreading examples.

[shivrajcodez]

Guarantees visibility + ordering between threads in Java Memory Model.

1. Thread start

int x = 10;
new Thread(() -> {
System.out.println(x); // always sees 10
}).start();

➡️ start() happens-before thread execution

2. Thread join

int x = 0;
Thread t = new Thread(() -> x = 5);
t.start();
t.join();
System.out.println(x); // always 5

➡️ thread completion happens-before join()

3. Volatile

volatile boolean flag = false;

Thread 1: flag = true;
Thread 2: if(flag) { /* guaranteed visible */ }

➡️ write → read has happens-before

4. Synchronization

synchronized(lock) { x = 10; } // Thread 1
synchronized(lock) { print(x); } // Thread 2 → sees 10

➡️ unlock happens-before next lock

5. Atomic (CAS)

AtomicInteger a = new AtomicInteger(0);
a.incrementAndGet(); // visible to all threads

➡️ uses memory barriers internally

TL;DR:
If A happens-before B → B sees all changes from A