Java Concurrent Locks

Apart from the monitor-based synchronisation, Java provides various locks in java.util.concurrent.locks for more flexible and fine-grained locking schemes.

Much like the monitor-based synchronisation, there’s 3 components:

• Locking implementations
  • ReentrantLock
  • ReentrantReadWriteLock
  • StampedLock (More performant Read/Write lock, doesn’t implement Lock)
• Lock Interface (Acquiring the lock)
• Condition Interface (Signalling Methods)

Architecture

All StdLib lock implementations are built on AbstractQueuedSynchronizer (AQS), which provides the core synchronisation infrastructure:

Sync Queue (Entry Queue):

• Holds threads waiting to acquire the lock
• Analogous to Entry Set in java-monitor
• Doubly-linked list managed by AQS
• FIFO ordered, but fairness setting determines if new threads can “barge” ahead

Condition Queues:

• Holds threads waiting on specific conditions via await()
• Analogous to Wait Set in java-monitor
• Multiple singly-linked lists, one per Condition object
• When signalled, threads move from condition queue → sync queue