ScheduledExecutorService

ScheduledExecutorService is a subinterface of java.util.concurrent.ExecutorService that adds scheduling capabilities for delayed and periodic task execution. It provides a uniform API for running tasks after a delay or at fixed intervals, integrating timing with asynchronous execution. The typical implementation is ScheduledThreadPoolExecutor, often created via the Executors factory methods.

Key features and methods include: schedule(Runnable, long delay, TimeUnit unit) and schedule(Callable<V>, long delay, TimeUnit unit) for one-time delayed tasks, both returning a ScheduledFuture representing the pending result. For recurring work, scheduleAtFixedRate(Runnable, long initialDelay, long period, TimeUnit unit) schedules executions based on a fixed cadence between start times, while scheduleWithFixedDelay(Runnable, long initialDelay, long delay, TimeUnit unit) schedules the next execution after the previous one completes, using a fixed delay.

Return values such as ScheduledFuture allow cancellation of pending tasks and monitoring of completion. If a task has not begun, cancellation prevents it from starting; cancellation of a running task is subject to the executor’s policies.

Lifecycle and usage considerations: ScheduledExecutorService inherits shutdown and shutdownNow from ExecutorService. After shutdown, new tasks are rejected, though previously submitted tasks may continue until termination. It is well-suited for timers, periodic maintenance, and background tasks that require non-blocking, time-based scheduling. Exact timing is not guaranteed; scheduling depends on thread availability, system load, and OS scheduling. Typical usage patterns involve configuring an appropriate pool size (single-threaded or multi-threaded) and selecting between fixed-rate or fixed-delay scheduling based on the desired cadence of executions.