6.27 BlockingQueue

1. What It Is

A thread-safe blocking queue.

Characteristics:

• First in, first out: Since it is a queue, it is naturally first in, first out.
• Blocking: When inserting an element, if the queue is full, the insertion can block and wait until the queue is no longer full. When removing an element, if the queue is empty, the removal can block and wait until the queue is no longer empty.
• Bounded or unbounded: This refers to the capacity of the queue. An unbounded queue is effectively bounded by Integer.MAX_VALUE.
• Thread-safe.

2. Use Cases

Producer-consumer.

3. How to Use It

Method / Handling	Throws Exception	Returns Special Value	Blocks Indefinitely	Times Out
Insert	add(e)	offer(e)	put(e)	offer(e,time,unit)
Remove	remove()	poll()	take()	poll(time,unit)
Examine	element()	peek()	Not available	Not available

4. Detailed Comparison of Different BlockingQueue Implementations

	ArrayBlockingQueue	LinkedBlockingQueue	PriorityBlockingQueue	SynchronousQueue
Data structure	Array	Singly linked list	Array (binary heap)	Singly linked list
How blocking is implemented	Lock + Condition	Lock + Condition	Lock + Condition	CAS + LockSupport
Bounded / unbounded	Bounded	Bounded	Unbounded	Unbounded (does not store elements)
Throughput (using LinkedBlockingQueue as the baseline)	Lower than LinkedBlockingQueue (read-read, read-write, and write-write block each other)	/ (read-read and write-write block each other; read-write do not block each other)	Unbounded (read-read, read-write, and write-write block each other)	Higher than LinkedBlockingQueue (a read and a write must match before they can proceed)

• ArrayBlockingQueue.md
• LinkedBlockingQueue.md
• PriorityBlockingQueue.md
• SynchronousQueue.md