synchronized vs ReentrantLock in Java

Feature	synchronized	ReentrantLock
Basic Concept	Uses intrinsic lock (monitor) on objects.	Uses an explicit lock from java.util.concurrent.locks.
Lock Acquisition	Acquired implicitly when entering a synchronized block or method.	Acquired explicitly via lock() method.
Release of Lock	Automatically released when the thread exits the synchronized block or method.	Must be explicitly released via unlock().
Reentrancy	Supports reentrancy (same thread can acquire the same lock multiple times).	Supports reentrancy just like synchronized.
Fairness	Unfair by default (no control over thread access order).	Can be fair or unfair (configurable with ReentrantLock(true)).
Interruptibility	Cannot respond to interrupts while waiting for the lock.	Supports interruptible locking via lockInterruptibly().
Try Locking	Not supported. A thread will block indefinitely if the lock is not available.	Supports tryLock() to attempt locking without blocking or with timeout.
Condition Variables	Uses wait() / notify() / notifyAll() methods on the intrinsic lock.	Supports multiple Condition objects for finer-grained wait/notify control.
Timeout Support	Not supported. If the lock is held by another thread, it will wait indefinitely.	Supports timeout locking with tryLock(long timeout, TimeUnit unit).
Performance Overhead	Low for simple scenarios with little contention.	Higher overhead but provides greater control over locking behavior.
Fair Locking Option	Not supported (always unfair).	Fair locking can be enabled with ReentrantLock(true).
Use in Non-blocking Operations	Not possible.	Possible with tryLock() (non-blocking).
Flexibility and Control	Limited to synchronized methods or blocks.	Greater flexibility: lock multiple sections, lock only part of a method, or use multiple conditions.
Suitability for Deadlock Avoidance	Requires external logic to prevent deadlocks (acquire locks in the same order).	Easier to prevent deadlocks using tryLock() and timeouts.
Memory Usage	No additional memory overhead. Uses the object’s monitor.	Requires additional memory for lock objects and lock metadata.
Readability and Simplicity	Easier to read and maintain (especially for small, simple use cases).	More complex code with explicit lock management.
Error Handling	No need to manage lock release in a finally block. The lock is automatically released.	Requires explicit unlock() in finally blocks to avoid deadlocks or memory leaks.
Thread Starvation	Prone to thread starvation in high contention scenarios.	Can prevent starvation using fair lock mode.
Recommended Use Case	Best for simple synchronization needs where you don’t need advanced control.	Recommended for complex concurrency scenarios needing fine-grained locking, fairness, tryLock, or interruptibility.

---

When to Use synchronized vs ReentrantLock

Use synchronized	Use ReentrantLock
When you need simple, block-level or method-level synchronization.	When you need advanced control over locking behavior (e.g., tryLock, fairness, or interruptibility).
When you want automatic lock release (less error-prone).	When you need multiple locks or condition variables.
When performance matters in low-contention scenarios (lower overhead).	When dealing with high contention and you need fair scheduling to prevent starvation.
When you don't need non-blocking operations or timeouts.	When you want non-blocking operations using tryLock() or timeout-based locking.
When the code needs to be simple and easy to read.	When code complexity is acceptable for greater flexibility.

---

Summary

Both synchronized and ReentrantLock have their own strengths and use cases. Use synchronized for simpler, lower-level concurrency needs, and ReentrantLock when you need more control, fairness, or advanced features like non-blocking locking and condition variables.

In general: - synchronized is easier to use and less error-prone. - ReentrantLock is more powerful and flexible, but with more overhead and complexity.