shutdownNow

shutdownNow is a method in the Java standard library, defined by the ExecutorService interface in the java.util.concurrent package. It was introduced to provide a means of attempting an immediate shutdown of an executor, as an alternative to the graceful shutdown provided by shutdown().

When invoked, shutdownNow tries to stop all actively executing tasks and halts the processing of waiting tasks. It does not guarantee that running tasks will stop promptly; it will attempt to interrupt their threads, and tasks may ignore the interruption or respond in custom ways. It does not guarantee that all tasks will terminate immediately.

The method returns a List<Runnable> containing the tasks that were submitted to the executor but had not yet begun execution. Tasks that have already started or completed will not be in the returned list.

shutdownNow does not guarantee cancellation of tasks that are currently running in a non-cooperative way; it relies on cooperative cancellation. If the security manager denies interruption, a SecurityException may be thrown. After calling shutdownNow, the executor is considered shut down for new submissions, though already running tasks may continue to completion if they resist interruption.

Compared with shutdown(), which transitions the executor to a state where no new tasks are accepted but running tasks are allowed to complete, shutdownNow is intended for rapid termination when a graceful shutdown is not possible. Developers should handle InterruptedException in tasks and check the returned list to determine what remains pending.