On ReentrantLock

• ReentrantLock is just an encapsulation of AQS: it has a Sync object which extends AQS
• Need to implement tryAcquire in AQS

vs synchronized

• Synchronized is based on JVM, RL is based on AQS in JDK
• RL supports lockInterruptibly, so that other waiting threads can stop waiting
• RL supports fair lock, synchronized is unfair lock
• synchronized uses notify()/notifyAll(), i.e, only one Condition for the whole monitor. No such limitation for ReentrantLock

State: 0 - nolock. Every acq +1, release -1

ReentrantReadWriteLock (RRWL) uses 16 bits to save write lock state, and 16 bits for read lock state