How do you use the synchronized keyword to ensure atomicity and visibility of shared variables?

Note that synchronized code block shown earlier uses intrinsic lock to achieve atomicity. In addition to synchronized block shown earlier, Java also supports atomic classes (e.g. AtomicInteger) that provides non-blocking mechanism to perform atomic read-modify-write operation (e.g. counter++, counter--). Atomic classes use low level compare-and-swap CPU operations to achieve atomicity. The underlying variable used by Atomic classes is declared as volatile, which guarantees that any thread reading the value of Atomic class will always get most recent value of Atomic class.

Atomicity refers to operations that are indivisible and uninterruptible, ensuring that a sequence of operations completes entirely or not at all, preventing inconsistent or partial states. In Java, the 'synchronized' keyword is used to ensure atomicity and visibility of shared variables. When a method or block of code is marked as 'synchronized', a thread must acquire the associated lock before executing the code, ensuring exclusive access. This prevents race conditions by making sure that only one thread can modify the shared variable at a time, while also ensuring that changes made by one thread are visible to others, thereby maintaining data consistency.

The 'synchronized' keyword in Java ensures atomicity and visibility of shared variables by locking a method or block, allowing only one thread to execute it at a time, preventing race conditions. When a thread enters a synchronized block, it acquires a lock on the object, ensuring that changes to shared variables made within the block are visible to other threads. This happens because the lock enforces a memory barrier, flushing changes to the main memory when the lock is released and reloading from the main memory when the lock is acquired. Visibility means that changes made by one thread to shared variables are seen by other threads, preventing stale or inconsistent data reads.

The 'synchronized' keyword in Java ensures the atomicity and visibility of shared variables by providing an intrinsic lock. When a thread enters a synchronized method or block, it acquires the lock for the object or class, ensuring exclusive access to the critical section. This prevents other threads from entering any synchronized method/block on the same object/class until the lock is released, maintaining atomicity. It also ensures visibility, as the lock release operation (after a synchronized method/block execution) flushes changes to the main memory, making them visible to other threads. synchronized methods ensure that only one thread can execute this method at a time on the same object instance.

A real-world example of where synchronized block can greatly enhance system performance and scalability over synchronized method can be seen in the implementation of Singleton design pattern shown below: ```java public static MyClass getInstance() { if (instance == null) { synchronized (MyClass.class) { if (instance == null) { instance = new MyClass(); } } } return instance; } ``` This method uses double locking mechanism to ensure that only one instance of MyClass can be created. When this method will be invoked first time by a thread, it will acquire the lock. All additional invocation of this method will not acquire any lock.

Improper use of synchronized can cause deadlocks, where threads indefinitely wait for each other to release locks, freezing program. Deadlocks often arise when threads acquire multiple locks in inconsistent orders,leading to circular dependencies. To prevent deadlocks: 1) Ensure consistent lock acquisition order across threads to avoid circular waits. 2) Use tryLock with timeouts to prevent indefinite waiting. 3) Minimize lock scope, duration to reduce contention. 4) Regularly analyze, refactor code to minimize lock dependencies,optimize synchronized blocks. These practices help maintain thread safety while reducing deadlock risk. Careful lock management results in more robust multi-threaded applications, less prone to concurrency issues.