What is the producer/consumer pattern and what does the queue actually provide?

Producers and consumers never touch each other's data; the only shared object is the queue, and BlockingQueue handles the synchronisation, the blocking and the memory visibility (a put happens-before the corresponding take). That reduces a concurrency problem to a data structure choice.

The bound is the design decision. Bounded means a fast producer is slowed down by a slow consumer, which is backpressure, and the system degrades gracefully. Unbounded means the mismatch accumulates in memory until the process dies.

The poison pill is the idiomatic shutdown: put a sentinel per consumer so each one sees exactly one and exits, rather than interrupting mid-work. A thread pool is this pattern wearing a nicer API.