Here are Sun’s rules for thread creation in simple terms:

1. If the number of threads is less than the corePoolSize, create a new Thread to run a new task.
2. If the number of threads is equal (or greater than) the corePoolSize, put the task into the queue.
3. If the queue is full, and the number of threads is less than the maxPoolSize, create a new thread to run tasks in.
4. If the queue is full, and the number of threads is greater than or equal to maxPoolSize, reject the task.

Full article

Origin answer