What's Spring's default queue size with a ThreadPoolTaskExecutor?

Setting maxPoolSize implicitly allows for tasks to get dropped. However, the default queue capacity is Integer.MAX_VALUE, which, for practical purposes, is infinity.

Something to watch out for is that ThreadPoolTaskExecutor uses a ThreadPoolExecutor underneath, which has a somewhat unusual approach to queueing, described in the docs:

If corePoolSize or more threads are running, the Executor always prefers queuing a request rather than adding a new thread.

This means that maxPoolSize is only relevant when the queue is full, otherwise the number of threads will never grow beyond corePoolSize. As an example, if we submit tasks that never complete to the thread pool:

• the first corePoolSize submissions will start a new thread each;
• after that, all submissions go to the queue;
• if the queue is finite and its capacity is exhausted, each submission starts a new thread, up to maxPoolSize;
• when both the pool and the queue are full, new submissions are rejected.