java - ArrayBlockingQueue uses a single lock for insertion and removal but LinkedBlockingQueue uses 2 separate locks

Question

I was going through the source code of ArrayBlockingQueue and LinkedBlockingQueue. LinkedBlockingQueue has a putLock and a takeLock for insertion and removal respectively but ArrayBlockingQueue uses only 1 lock. I believe LinkedBlockingQueue was implemented based on the design described in Simple, Fast, and Practical Non-Blocking and Blocking Concurrent Queue Algorithms. In this paper, they mention that they keep a dummy node so that enqueuers never have to access head and dequeuers never have to access tail which avoids deadlock scenarios. I was wondering why ArrayBlockingQueue doesn't borrow the same idea and use 2 locks instead.

Answer 1

ArrayBlockingQueue has to avoid overwriting entries so that it needs to know where the start and the end is. A LinkedBlockQueue doesn't need to know this as it lets the GC worry about cleaning up Nodes in the queue.