synchronized vs ReentrantLock: Which Java Lock Wins in Performance and Flexibility?

Lock Implementation Methods

synchronized : synchronized is a Java keyword implemented by the JVM. It provides implicit locking without explicit calls, using object‑header flags or monitor locks.

ReentrantLock : ReentrantLock is an explicit lock from java.util.concurrent, relying on OS atomic operations and using lock() and unlock() typically with try/finally blocks.

Performance Comparison

JDK 1.5 and earlier : synchronized had a coarse implementation, resulting in poor performance under contention.

JDK 1.6 and later : Introduced spin locks, lock elision, and lock coarsening, narrowing the performance gap; both mechanisms perform similarly in uncontended scenarios.

Features and Extensibility

synchronized : Simple, non‑fair, automatic release, low risk of deadlock, but lacks fine‑grained control and advanced features.

ReentrantLock :

Interruptible lock waiting : lockInterruptibly() allows a thread to respond to interrupts while waiting for the lock.

Fair vs non‑fair locks : Constructor can specify fairness; synchronized is always non‑fair.

Condition support : Provides Condition with newCondition() for multiple wait queues, unlike synchronized which relies on wait() and notify().

Reentrancy : The same thread can acquire the lock multiple times without deadlocking.

Lock Release Mechanism

synchronized : The JVM automatically releases the lock when exiting the synchronized block or method.

ReentrantLock : Must explicitly call unlock(), usually within a try/finally block to ensure release even on exceptions.

Applicable Scenarios

synchronized : Suitable for simple synchronization needs, low concurrency, and when advanced features like interruptibility or fairness are unnecessary.

ReentrantLock : Ideal for complex synchronization requiring interruptible waits, fairness, condition variables, or high‑concurrency environments.