In Java thread synchronization, how does mutual exclusion occur inside an intrinsic monitor when multiple threads use synchronized blocks or synchronized methods on the same object?

Introduction / Context:
This question examines how Java implements thread synchronization using intrinsic monitors associated with every object. The synchronized keyword enforces mutual exclusion and visibility guarantees. Understanding how monitor locks work is essential for writing correct multithreaded Java programs that avoid race conditions and data corruption.

Given Data / Assumptions:

- Every Java object has an associated intrinsic monitor lock managed by the Java Virtual Machine.
- Synchronized methods and synchronized blocks use these monitor locks to coordinate thread access to critical sections.
- Multiple threads may attempt to enter the same synchronized region of code on the same object at the same time.
- We are focusing on the core mutual exclusion behavior, not on advanced concurrency utilities.

Concept / Approach:
When a thread enters a synchronized instance method or a synchronized block that uses a particular object as the monitor, it attempts to acquire the monitor lock of that object. If no other thread currently holds the lock, the thread acquires it and proceeds to execute the synchronized code. If another thread already holds that lock, the attempting thread is blocked and placed in the entry set for that monitor. The blocked thread remains suspended until the current owner exits the synchronized block or method and releases the lock, at which point one of the waiting threads is allowed to acquire the monitor and continue. This behavior enforces mutual exclusion inside the monitor, ensuring that only one thread at a time executes the critical section associated with that particular object lock.

Step-by-Step Solution:
Step 1: Consider a shared object called sharedResource, and two threads that both execute synchronized(sharedResource) { critical code }. Step 2: When the first thread reaches the synchronized block, it asks the virtual machine to acquire the monitor lock on sharedResource. If the lock is free, the thread becomes the owner and enters the block. Step 3: While the first thread is inside the synchronized block, a second thread reaches the same synchronized(sharedResource) block and attempts to acquire the same lock. Step 4: Because the lock is already owned by the first thread, the second thread is blocked and cannot enter the block. It waits in the monitor entry queue. Step 5: When the first thread exits the synchronized block, the virtual machine releases the monitor lock. At that moment it selects one of the waiting threads, which then acquires the lock and proceeds into the critical section. Step 6: This mechanism ensures that at most one thread is executing the critical section protected by that monitor at any time, which is the core idea of mutual exclusion.

Verification / Alternative check:
You can experiment with a simple Java program where several threads attempt to increment a shared counter inside a synchronized block using the same lock object. You will observe that the counter remains consistent, demonstrating mutual exclusion. Removing the synchronized keyword typically reveals race conditions, with incorrect counter values appearing. This practical test supports the explanation of how monitor based synchronization works.

Why Other Options Are Wrong:
Option B is wrong because the Java Virtual Machine does not allow all threads to execute synchronized code simultaneously on the same monitor lock; it relies on mutual exclusion, not post processing reordering. Option C is incorrect because synchronization is available for both static and instance methods, although static synchronized methods use the Class object as the monitor instead of an instance. Option D is wrong because there is exactly one intrinsic monitor per object; threads share that monitor, which is why they must block when it is already held by another thread.

Common Pitfalls:
Common pitfalls include synchronizing on different objects accidentally, which defeats mutual exclusion, and using overly broad synchronized regions that reduce concurrency and cause contention. Another frequent mistake is misunderstanding that synchronization also has a memory visibility effect, ensuring that changes made by one thread before releasing a lock become visible to another thread after acquiring the same lock. Properly choosing the lock object and scoping the synchronized region are essential skills for writing correct and efficient multithreaded Java code.

Final Answer:
Thread synchronization inside a monitor works by allowing only one thread at a time to acquire the monitor lock for a given object, so any other thread that tries to enter synchronized code on that same object blocks until the lock is released.