Locks: ReentrantLock, ReadWriteLock, StampedLock

ReentrantLock offers tryLock, timeout, interruptibility, fairness, and multiple Conditions that synchronized cannot provide, but requires manual unlock in a finally block.

LockSupport.park/unpark are permit-based (no lock needed, unpark before park is NOT lost), while wait/notify are monitor-based (lock required, notify before wait IS lost); AQS uses park/unpark internally.

StampedLock adds an optimistic read mode to read-write locking: tryOptimisticRead() returns a stamp without acquiring any lock, and validate(stamp) confirms no write occurred, giving zero-lock overhead for read-heavy workloads.

ReentrantReadWriteLock allows multiple concurrent readers or one exclusive writer, dramatically improving throughput for read-heavy workloads but adding overhead for write-heavy ones due to the shared reader-count CAS.

Striped lock partitioning uses an array of N locks where each key hashes to a stripe, reducing contention N-fold while enabling parallel access to different stripes.

Fair locks serve threads in FIFO order to prevent starvation but pay a wake-up cost that lowers throughput; unfair locks let threads barge in for higher throughput at the risk of long waits.

Downgrade safely by taking the read lock before releasing the write lock; never try to upgrade read to write because two readers both upgrading deadlock, which is why the JVM does not support it.