setCorePoolSize

setCorePoolSize is a method of ThreadPoolExecutor that sets the core pool size, the minimum number of threads that are kept alive in the pool even when idle. The value must be between 0 and maximumPoolSize; passing a value outside this range results in IllegalArgumentException.

In operation, the core pool size defines the baseline level of concurrency. If you increase corePoolSize, the pool will create additional core threads as needed to reach the new size, up to the maximumPoolSize, to help execute queued tasks more promptly. If you decrease corePoolSize, the pool will reduce the number of core threads by terminating excess idle core threads until the current core pool size matches the new value; active, non-idle threads are not interrupted.

Core threads are kept alive to handle incoming tasks, even when idle, unless a configuration option allows their timeout. Specifically, core threads are normally not terminated due to idleness, unless allowCoreThreadTimeOut is set to true. When allowCoreThreadTimeOut is enabled, core threads may time out and terminate after a period of inactivity, similar to non-core threads governed by keepAliveTime.

The keepAliveTime parameter governs how long threads beyond the core pool size may remain idle before being terminated; core threads follow that rule only if allowCoreThreadTimeOut is enabled. setCorePoolSize therefore provides a dynamic way to adjust the pool’s baseline level of concurrency in response to workload changes, influencing how many tasks can be handled in parallel and how aggressively idle threads are managed.

Usage notes: adjusting the core pool size can impact performance and resource usage. Changes take effect as the pool decides to create or terminate workers, and do not forcibly interrupt currently executing tasks.