allowCoreThreadTimeOut

allowCoreThreadTimeOut is a method of the ThreadPoolExecutor class in Java that controls whether core threads are allowed to time out and terminate when idle. By default, core threads are kept alive regardless of the keepAliveTime setting.

When allowCoreThreadTimeOut is set to true, idle core threads may time out after the configured keepAliveTime and be terminated, just like non-core threads. This enables the pool to shrink its size even below the configured corePoolSize during periods of inactivity. If new tasks arrive while the pool has fewer active threads than needed, new threads will be created as required up to maximumPoolSize to handle the workload.

Key implications include a potential reduction in resource usage during low load, since idle threads can terminate, and a possible increase in task latency when bursts occur, as the pool may need to recreate threads. The keepAliveTime value continues to govern how long idle threads are allowed to wait before termination, and it applies to core threads only when allowCoreThreadTimeOut is true.

When to use this feature depends on workload characteristics. It is useful in environments with fluctuating traffic where conserving system resources is desirable. However, enabling it can introduce latency spikes during load rebounds, as threads must be re-created to handle incoming tasks.

Notes: This behavior is specific to ThreadPoolExecutor. After construction, you enable it by calling executor.allowCoreThreadTimeOut(true) and configuring keepAliveTime and core/max pool sizes accordingly. See also ThreadPoolExecutor, keepAliveTime, corePoolSize, maximumPoolSize.