When would you choose ReentrantLock instead of synchronized?

Chooses an explicit lock only when timed, interruptible, conditional, or advanced acquisition behavior justifies manual lifecycle.

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TL;DR

synchronized is concise and releases its monitor automatically when the block exits. ReentrantLock provides explicit lock and unlock operations with the same basic mutual-exclusion and visibility role. It is useful when code needs timed tryLock, interruptible acquisition, several Condition objects, or an explicit fairness policy. Because unlocking is manual, it belongs in a finally block immediately after successful acquisition.

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Lock API trade-offs, reliable unlock patterns, conditions, fairness, and preference for simpler intrinsic locks.

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Overview
Both can provide mutual exclusion and visibility; ReentrantLock adds control at the cost of manual ownership.
synchronized is concise and releases its monitor automatically when the block exits. ReentrantLock provides explicit lock and unlock operations with the same basic mutual-exclusion and visibility role. It is useful when code needs timed tryLock, interruptible acquisition, several Condition objects, or an explicit fairness policy. Because unlocking is manual, it belongs in a finally block immediately after successful acquisition.

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Forgetting to unlock in finally.
Choosing explicit locks only because they sound faster.
Enabling fairness without measuring its cost.

When would you choose ReentrantLock instead of synchronized?

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