Java.Multithreading.if you say monitor is the mutex, tell me in terms of mutex how sync happens ?

💡 First: A Monitor in Java = A Mutex + Condition Variable

Java’s monitor is not just a lock — it’s a synchronization mechanism that includes:

• a mutex (for mutual exclusion)
• zero or more condition queues (for waiting/notification: wait(), notify())

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🧵 Let’s Translate Java to Low-Level Terms

Java Code:

synchronized (obj) {
    // critical section
}

Under the Hood (conceptually):

lock(obj->mutex);     // monitorenter
   // critical section
unlock(obj->mutex);   // monitorexit

monitorenter is like pthread_mutex_lock()

monitorexit is like pthread_mutex_unlock()

Java uses reentrant mutexes, so the same thread can lock multiple times without deadlocking.

🔐 Key Concepts in Terms of Mutex

Java Concept	C / OS-Level Equivalent
synchronized(obj)	pthread_mutex_lock(&obj->mutex)
monitorenter	Acquire mutex
monitorexit	Release mutex
wait()	pthread_cond_wait() (releases mutex, waits)
notify()	pthread_cond_signal()
notifyAll()	pthread_cond_broadcast()

📦 How Scoped Synchronization Works (Step by Step)

public void doSomething() {
    System.out.println("Before sync");

    synchronized(this) {
        System.out.println("In critical section");
    }

    System.out.println("After sync");
}

In terms of mutex:

1. Before sync is unsynchronized code
2. synchronized(this):
   • JVM calls monitorenter → acquires the mutex on this
   • Only one thread can pass this point at a time
3. Critical section runs
4. Exiting block → monitorexit → releases the mutex
5. After sync is again unsynchronized

So even though we’re locking on the object, we’re only holding the mutex for the duration of the block.

🛠 Real Mutex Behavior

Imagine 3 threads:

Thread-1: synchronized(obj) { ... }
Thread-2: synchronized(obj) { ... }
Thread-3: synchronized(obj) { ... }

This happens:

• Thread-1 acquires obj‘s mutex
• Thread-2 and Thread-3 block (they spin or sleep waiting)
• When Thread-1 exits the block, mutex is released
• One of the others (e.g., Thread-2) wakes up and acquires the mutex
• Repeat

➡️ This is classic mutex behavior, just hidden inside the JVM.

🔁 Visual Summary

+------------------+       +------------------+
|   Java Object    |       |   Java Monitor   |
|------------------|       |------------------|
|   Fields, etc.   | <---> |   Mutex (lock)   |
|                  |       |   Condition(s)   |
+------------------+       +------------------+

synchronized(obj)  →  monitorenter(obj) → mutex.lock()
exit block         →  monitorexit(obj)  → mutex.unlock()

✅ Final Word

Yes — Java’s monitor is a high-level abstraction of a reentrant mutex + condition variable.
So when you write synchronized(obj), you’re doing exactly what you’d do in C with pthreads:

pthread_mutex_lock(&obj->mutex);
// do work
pthread_mutex_unlock(&obj->mutex);

Java just hides that plumbing — and makes it object-based and type-safe.